CA2192440A1 - Micromechanical memory sensor - Google Patents

Abstract

A micromechanical memory sensor includes a latch member assembly (20, 30) mechanically latching upon detection of a threshold value of a variable condition (ambient temperature, acceleration, pressure). The mechanical latching is detected by circuitry of a readout mechanism (21, 22). The sensor further includes a resetting mechanism such as a thermal resistor (24), proof mass (52, 101) or electrostatic combing drive (170, 175) for electrically unlatching the latch member assembly whereby the sensor is latched purely mechanically in its operative states and is electrically reset for repeat use.

Description

WO 95134904 2 1 q 2 4 4 0 r~ a ~ 1 MTrl?l ;INTCAI, MEMORY SENSOR
Ba~k~u~,d of the Invention This invention relates to a micr~---h~n;r~l memory sensor. More particularly, the invention is directed to a mi~L, --h~nic~l device which serves as a mechanical memory latch or sensor, the activation of which is triggered by a change of conditions, e.g., temperature, acceleration and/or pressure. Contents of the memory latch can be conveniently detected at any time after latching. The device is electronically resettable so that it can be conveniently reused.
While the invention is particularly directed to the art of micr~ h~n;cal memory sensors, and will be thus described with specific reference thereto, it will be appreciated that the invention may have usefulness in other fields and applications.
Micr~-~-hAn;cal memory sensors are used or have potential use in sensing a variety of different variables or conditions. These variables or conditions include t~eL~Lu.~, acceleration, ~es~uL~, force...etc.
For example, a mi~ n;cal memory sensor adaptable for use in sensing temperature exLL purely ~n;~lly and being electronically resettable would be advantageous for applications wherein field testing is con~ ed on products and no power supplies are available WOgS/34904 2 1 92440 P~ "~

in the field. However, there are no known mi~" ~h~n;cal temperature sensors of this type.
Conventional electronic temperature sensors require a power supply when monitoring temperatures.
S However, in most instances where the temperature extreme to which a product has been exposed is the desired information, the field monitoring of temperature is not possible with conventional techniques since a power supply may oftentimes be unavailable.
A bistable snapping microactuator having a power supply, or battery, has also been disclosed. H.
Matobo, T. Ishikawa, C. Kim, R. Muller, A Bistable SnaDDinq Microactivator, I.E.E.E., January 1994, pp. 45-50. The microactuator includes a flexible cantilever which buckles when a temperature extreme, induced by a current, is ~PtPrtP~. While this device is ultimately triggered by a temperature change, i.e., resistive dissipation, acceptable operation is only achieved through the use of driving voltages and current pulses applied in a particular timing sequence. This microactuatOr is not triggered purely r- ' ~n;rAl ly.
As a further example, certain mi~ h~nica memory sensors adapted for use as latch accelerometers are known and provide an inP~pPn~ive way of sensing moderate and high-g accelerations by using a mi~- ~hAnir~1 memory sensor. A latch accelerometer is w09sl34904 r~ /J~a ~ _3_ a switch which latches if accelerated by a predetPrmi n~
acceleration in a particular direction and remains closed after the acceleration ceases. The primary advantage of latch accelerometers over the conventional acceleration sensing devices is that latch accelerometers do not require complicated sensing electronics. The sensed acceleration can be read out long after the accelerating event. Acceleration latches operate without a power supply and can be made to operate at g levels ranging from only a few g's to several thousand g's and to sense the duration for which the acceleration lasts.
U.S. Patent No. 4,891,255 to Ciarlo discloses an acceleration latch which uses bulk micromachining of (110) oriented silicon wafers to make two cantilever beams having proof masses, or plates, attached thereto that interlock at a set threshold acceleration. FIGURES
21(a) and (b) show such a latching accelerometer. The cantilever beams C are typically several millimeters in length. The fabrication of the cantilever beams c and the proof masses P is fairly complicated since corner ~ , tion and silicon bulk mi~Ll rhin;ng of (110) wafers are used. (110) bulk mi~Ll -h;nlng is not readily compatible with IC processing.
The cantilever beams C of the Ciarlo patent must undergo large deflections before latching at their proof masses C. Further, since the horizontal cantilever W09~34904 P~l/u~ a beam c must force deflection of the vertical cantilever C, which involves the sliding of the two large surfaces, the frictional force between the two proof masses P can be significant and can result in uncertainties in the acceleration sensed. Moreover, the cantilever beams C
are not delatchable, thus not resettable.
Another main disadvantage of the latch of the Ciarlo patent is the complicated readout schemes that must be used. Since the cantilever beams C are made by etching through a silicon wafer, the two cantilever beams C cannot be electrically isolated, making a simple continuity test between the two cantilever beams C
impossible. The readout schemes of the Ciarlo patent use either capacitive or optical techniques. In either of these schemes the accelerometer wafer must be sandwiched between two other wafers containing capacitive plates or light emitting diodes to sense the position of the cantilevers. This makes the fabrication process much more complicated and expensive. Also, bulk mi~L~ ehin;ng results in large sized devices.
A direct implementation of the latching -- ' In; G- of the Ciarlo patent using surface mi~ rh;n;ng is possible and may solve the problem of sensing the latch. However, the device would still su~fer from other noted problems related to excessive wogsl34904 2 1 9 2 4 4 ~ PCT~S9S/0733S

length of beams C with the proof masses P attached at ends thereto and would still not be resettable.

Summarv of the Invention An object of the present invention is to S provide a mi~L, --h~n;c~l memory sensor comprising a latch which senses a change in a variable or condition.
An additional object of the present invention is the provision of a mi~L --h~nical memory sensor capable of purely mechanically latching in response to sensing a threshold value of a condition or variable.
A still further object of the present invention is the provision of a mi~L -~h~nic~l memory sensor comprising a memory latch, the contents of which can be detected at any time after latching.
A still further object of the present invention is the provision of a mi~L -h~nical memory sensor that is resettable.
In one aspect of the invention, the mi~L --h~nical memory sensor records temperature 20 ~LL~ CC eYperienced beyond a preset value, without the use of any electrical power. That is, the sensor is purely mechanically induced. Additionally, the sensor is resettable, small, and in~Yp~ncive.
In a further aspect of the invention, the mi~ ~~h~n;cal memory sensor records acceleration W095l34904 2 1 92~4~ P llu~

extremes experienced beyond a preset value without the use of any electric power. The sensor is purely -- AnirAlly induced. Additionally, the sensor is r~cet~Ahl~, small, and ~n~r~n~ive.
In a still further aspect of the invention, the microchemical memory sensor records pressure extremes experienced beyond a preset valve without the use of electric power. The sensor is purely -hAn;~Ally induced. Additionally, the sensor is resettable, small, and inp~r~n~ive.
Further advantages and scope of the applicability of the present invention will become apparent from the detailed description provided below.
It should be understood, however, that the detailed description and specific examples, while indicating preferred ~ho~i Ls of the invention, are given by way of illustration only, since various changes and modificatiOns within the spirit and scope of the invention will become apparent to those skilled in the art. ~

rescri~tion of the Lrawinas The present invention exists in the cu..~LLu~Lion, arrangement, and combination, of the various parts of the device, whereby the objects contemplated are attained as hereinafter more fully set w09s~4904 2 1 9 2 4 4 ~ r l~u~ s forth, specifically pointed ou,t in the claims, and illustrated in the accompanying drawings in which:
FIGURES l(a~-(d) are a diagrammatic representation of the latching process of an exemplary 5 Pmho~;t -tt of the present invention;
FIGURES 2(a)-(d) are a diagrammatic representatiOn of the resetting process of the Pmho~ i r L
of FIGURES l(a)-(d);
FIGURE 3 is a side cross-sectional view of a mi~L~ ch~n;c~l memory sensor of the present invention;
FIGURES 4(a)-(p) show the fabrication steps for the sensor of FIGURE 3;
FIGURE 5 is a cross-sectional view of an alternative Pmho~;--nt of the sensor of FIGURE 3;
FIGURE 6 is a cross-sectional view of an alternative ~mhQ~tr L of the sensor of FIGURE 3;
FIGURE 7 is a cross-sectional view of an alterative ~mho~; L of the sensor of the present invention;
FIGURES 8(a)-(g) show the fabrication steps of the sensor of FIGURE 7;
FIGURE 9 is a cross-sectional view of an alternative Pmho~ nt of the sensor of FIGURE 7;
FIGURE 10 is a cross-sectional view of an 25 alternative Prho~;r~~t of the sensor of FIGURE 7;

WO9~/34904 2 ~ 9 2 4 4 0 P~11U~Y~U/~

FIG~RFS ll(a)-(c) are top views of a further Pr,ho~;r L of the mi~L, -ch~n jc~l memory sensor of the present invention for sensing acceleration;
FIGURES 12(a)-(c) show the fabrication steps of the sensor of FIGURES ll(a)-(c) using polysilicon surface micro-~ rhining;
FIG~RES 13(a)-(c) show the fabrication steps of the sensor of PIGURES ll(a)-(c) using nickel surface mi~L~ rh;n;ng;
FIGURE 14 is a top view of a further embodiment of the mi~L~ ~h~n; cal memory sensor of the present inventiOn for sensing acceleration in one direction;
FIGURE 15 is a top view of a further embodiment of the mi~L: ~h~n;~l memory sensor of the present invention for sensing acceleration in two directions;
FIGURE 16 is a top view of a further Prhofl; L
of the mi~L~ -~h~n; cal memory sensor of the present invention for sensing acceleration;
FIGURE 17 is a top view of a further Prho~;r-nt of the mi~ h~n;cal memory sensor of the present invention for sensing acceleration;
FIGURE 18(a)-18(b) are stylized representations of in-plane latch direction and out-of-plane latch direction, respectivelyi W09534904 2 1 9 2 ~ 4 ~ PCT~595/07335 _g_ FIGURE 19 is a top view of a further embodiment of the mi~L -Ah~n;cal memory sensor of the present invention for sensing out of plane acceleration;
FIGURE 20 is a top view of the micromechanical memory sensor of FIGURE 19 incorporating a resetting n;C~; and, FIGURES 21(a)-(b) show an acceleration latch of the prior art in an unlatched state and latched state, respectively.

Detailed Descri~tion of the Preferred F~hodiments The present invention is directed to a mi~L, Ah~n;cal memory sensor having a variety of potential uses including, in one aspect of the invention, sensing temperature ~xLL~ - to which the sensor is exposed, in a further aspect of the invention, sensing acceleration extremes to which the sensor is subjected and, in a still further aspect of the invention, sensing ~ XLL~ A to which the sensor is subjected. The sensor comprises a latch which is triggered by the detection of a predetarmined threshold, or extreme, in a selected condition, i.e., temperature, acceleration, ~s~uL~...etc.
Referring now more particularly to the drawings wherein the showings are for purposes of illustrating the preferred o~ho~;- ts of the invention only and not for W09~34904 2 1 9 2 4 4 0 rc~ JJ~

purposes of limiting same, EIGURES l(a)-(d) illustrate a principal concept of the present invention.
Specifically, a sensor latch L includes a sensing r-~h~nj~m S which senses an external force, or variable, F and r~-h~n;~lly latches under a resetting r--h~n;c~ R
when the force F exceeds a predet~rm;n~A extreme value for which the latch L is calibrated. While the t ~n;crc S and R are generally shown as beams longitnAin~lly disposed in the same axis, it is appreciated that other suitable types of m?~h~n;~C and arrangements therefor, preferred ones of which will be described hereafter, may be used. Further, the force F
imposed on the - ~n;cm S may be the result of a temperature change, accelerator change, ~ US~UL~ change, or the like. Likewise, the actual ~, t of r~-h~n;cm S may result from utilization of principles involving the bimetallic effect, mass movement, diaphragm characteristics, or the like.
Notably, the latching is accomplished entirely mechanically. That is, no power supply is needed in order to sense the extremes. This feature is particularly useful where it is desired to gather information respecting extreme conditions to which products, prototypes, or other devices are exposed during field use or testing. Typically, power supplies are not readily available durlng field use or testing. For W095l34904 2 1 9 2 4 4 0 r~ Ja example, when tires are tested and it is important to detect a temperature extreme to which the tested tires are exposed, pl~l L of a power supply on the tire to do so during use is impractical. Accordingly, a sensor of the presént tp~rhinl is useful.
According to the present invention, once an extreme condition has been detected and the sensor has latched, as shown in FIGURE 1td), the sensor remains latched. This feature provides a memory of the extreme condition sensed.
Additionally, the present invention includes readout n--h~ni~-cl or test ports, by which it is ~PtPrm i nPd whether the sensor is latched. A convenient reading scheme, i.e., conductivity test or the like, obviates the need for visual inspection and complicated reading electronics. If a plurality of sensors are fabricated on one substrate, simple multiplexing circuitry is used to selectively ~etP~r;nP whether sensors are latched. An illustration of the advantages of a simple reading scheme resides in field testing products wherein the sensor can be conveniently read either in the field or in a test laboratory subsequent to testing or use.
As shown in FIGURES 2(a)-(d), the present mi~L~ ~h~nir~l memory sensor is resettable. The re8etting ~ ni :m R is preferably microactuated to W095/34904 ~ S 2 1 ~ ? ~ 5 indure the sensing mechanism s to unlatch. In the illustrated method, the ~--h~n; cm R is induced to bend to the extent that the ---h~n;sm S tends to slip off r--h~ni c~ R to return to its original position.
MPrh~n;cm R may be microactuated thermally (h;- llically)l piezoelectrically, or electrostatically.
Resettability allows the sensor to be reused.
However, the structure of the sensor according to the present invention is simple and economical. Aroor~;ngly, it is recognized that the sensor may also be disposable with or without the resetting feature included.
In FIGURES l(a)-2(d), a general Pmho~;~-nt and concept of the invention are illustrated. The description hereafter sets forth specific examples of the present invention. First, various omho~;r-ntS
prP~ in~ntly bulk micrn--rhinpd will be described (FIGURES 3-10). Next, pro~nm;n~ntly surface mi~ll rh;n~d embodiments will be treated (FIGURES ll(a)-20).
Referring now to FIGURE 3, one preferred : '-';r L for sensing temperature, the mi~L, ~-h~n;ca memory sensor 10 is comprised of a resetting beam 20, test ports 21, a sensing beam 30, and support structures 40 and 50. The beams 20 and 30 are both ~;cposod along the same longitudinal axis. However, beam 30 is more W09~34904 2 1 9 2 4 ~ O PCT~Sg~/07335 flexible than beam 20. Further, the beams 20 and 30 overlap in that the sensing beam 30 is disposed in opposed relation to a first surface 25 of p + silicon portion 26 of the resetting beam 20.
S Resetting beam 20 in~ Pc a metal layer 22.
The metal layer 22 is preferably gold. However, any metal compatible with the fabrication process is recognized as being suitable. The resetting beam 20 further includes a polysilicon heating resistor 24 and the p + silicon portion 26. The metal layer 22, the heating resistor 24, and the p + silicon portion 26 are respectively divided by two (2) silicon nitride (Si3N4) layers 2B and 29.
In beam 20, the p + silicon portion 26 extends beyond the terminal end of the metal layer 22, heating resistor 24, and silicon nitride (si~N4) layers 28 and 29.
The extension of the p + silicon portion 26 has a first surface 25, as noted above, and a second surface 27.
Sensing beam 30 ;nclt~Pc a metal layer 32. As with the metal layer 22, metal layer 32 is preferably gold but could be of any suitable substance compatible with the fabrication process. Sensing beam 30 further includes an n-type polysilicon layer 34 and silicon nitride (Si3NJ) dividing layers 3B and 39.
Test ports 21 are connected to portion 26 on beam 20 and layer 32 on the beam 30. These test ports WOss/34904 2 1 9 2 4 4 0 P~l/ s are of any known type which are compatible with conductivity tests, as will be appreciated by those skilled in the art.
The support structures 40 and 50 are formed of S silicon substrate and have portions 60 comprising layers of silicon nitride 62, 66 and polysilicon 64. Those skilled in the art will appreciate that, while silicon substrate is preferred for convenience, alternative materials having similar properties may be used without avoiding the scope of the invention.
Moreover, in operation, the sensor 10, of FIGURE 3, utilizes the bimetallic effect which results from metal layers 22 and 32 and silicon layers 24 and 34 respectively having different thermal coefficients of expansion. As illustrated in FIGURE 3, both of the beams 20 and 30 are h;- Al 1 ;~A. Therefore, both beams 20 and 30 bend when a change in temperature occurs.
More specifically, referring generally to FIG~RES l-~a)-(d) wherein r-~h~nicrl R uu~L~u--ds to beam 20 and ~~AhAn;c~ S corresponds to beam 30, when the ambient temperature increases, both of the beams 20, 30 begin to bend. Since the sensing beam 30 is more flexible than the rpcot~;nrJ beam 20, as a result of differing gP~ L~ic dimensions such as length, ~h;rknPc5 and width, it bends a greater amount than the beam 20.
In the process, the sensing beam 30 contacts the WO95/34904 2 1 9 2 4 4 0 PCT~S9~/07335 ~ -15-resetting beam 20. Consequently, an additional bending moment is induced in the resetting beam 20 due to the force supplied by the contacting sensing beam 30 as shown in FIGURE l(b).
As the ambient temperature increases above a preset temperature, the horizontal deflections of the beams 20, 30 surpass their initial overlap. This causes the sensing beam 30 to slip off the resetting beam 20, as shown in FIGURE l(c). Since the sensing beam 30 is more flexible than the resetting beam 20, it will have a larger vertical deflection than beam 20 after the slip occurs, also shown in FIGURE l(c).
Finally, as the ambient temperature returns to room temperature, the beams 20, 30 will move back to their original places without any vertical deflection.
However, the sensing beam 30 will become latched n~n~th the resetting beam 20 in a latched arrangement, as depicted in FIGURE l(d), and engage the resetting beam 20. Therefore, the sensor 10 has recorded the fact that the temperature extreme it was designed to sense has been ~re~ . The temperature extreme is actually the point at which the sensing beam 30 slips off the resetting beam 20.
It is recognized that while a change in temperature creates a bending moment in the respective beams 20, 30, resulting in a vertical deflection, the W095/34904 2 ~ 9 2 ~ 4 0 r~ 5 /~s vertical deflection likewise results in a horizontal deflection since the beam length will essentially remain constant during a temperature increase. The effects of thermal expansion on beam length is minimal in comparison to the horizontal deflection caused by the vertical deflection.
A simple conductivity test can be done to determine if the beams 20, 30 are latched. Test ports, or readout m~h~n; ~mc~ 21 shown in FIGURE 3, are placed on the sensor at a convenient location. As noted above, if a plurality of sensors are fabricated on a single substrate, then simple multiplexing circuitry is used to selectively detect whether sensors are latched.
Specifically, if the sensing beam 30 is latched underneath the resetting beam 2C, the metal layer 32 of sensing beam 30 is in contact with the p + silicon portion 26 of resetting beam 20 resulting in a closed circuit. This contact is ohmic, and, there~ore, will result in a potential difference proportional to the amount of current flowing therethrough. The ohmic contact is detected through manipulation of the test ports 21 or related multiplexed circuitry.
However, if the sensing beam 30 is not latched underneath the resetting beam 20, but is just touching it, as would occur for a slight temperature increase from room temperature which is less then the preset value, an 21 9244û
W09sl34904 PCT~S95/07335 open circuit results. Polysilicon layer 34 of beam 30 touches the surface 25 of p + silicon portion 26. The respective test ports 21 are consequently separated by a nrnron~ ;ve path. As a result, a user easily distinguishes between the two different types of contacts through manipulation of the test portions, or related circuitry, and, consequently, whether latching has occurred.
As will be appreciated by those skilled in the art, if the sensor 10 of FIGURE 3 is in the state as shown in FIGURE l(a), a conductivity test will similarly indicate that an open circuit is present. Detection of such an open circuit represents the fact that no temperature extreme was sensed and that the sensor 10 is not latched.
As described above, the sensor 10 latches (FIGURE l(d~) when the ambient t~ ~UL~ exceeds a prPdetPrmin~d value. It is recognized that the ability to reset the sensor 10 is advantageous. However, it is also readily appreciated that the mi~L -~hAnical memory sensor 10 may be designed to be ~icpos~hlP and, thus, not resettable.
The resetting scheme will now be explained with general reference to FIGURES 2(a)-(d) wherein - -h~niQ~ R
CULL~a~ldS to beam 20 and r- ' ~nicm 5 corresponds to beam 30. As noted above, a heating resistor 24 is W09~34904 2 1 9244~ ~"1 ~ /aaS

disposed on the resetting beam 20. When an electrical current is induced in and passed through the heating resistOr 24, the heat generated is dissipated onto the resetting beam 20. The heat generated by the heating resistOr 24 has little affect on the sensing beam 30 since thermal conductivity between the resetting beam 20 and the sensing beam 30 is minimal. In any event, however, thermal conductivity will not cause a malfunction in the resetting scheme as mentioned below.
Therefore, the resetting beam 20 will begin to bend Vertically and will therefore create a bending moment in the sensing beam 30, as shown in FIGURE 2(b).
Eventually, the power dissipated by the heating resistcr 24 will be large enough such that the horizontal deflection of the two beams 20, 30 will be greater than their initial overlap. This will cause the resetting beam 20 to slip off the sensing beam 30 as shown in FIGURE 2(c). The sensing beam 30 will consequently spring to its original position as no heat is dissipated onto it. once the current in the heating resistor 24 is open circuited as a result of losing contact with the sensing beam 30, the resetting beam 20 will no longer QXperience a temperature rise. Accordingly, the resetting beam 20 will bend to its original position, as shown in FIGURE 2(d), returning the sensor 10 as a whole to its original position.

WO95/34904 }~~ /aa~
~ --19--Resetting has been described utilizing the bimetallic effect. However, an alternative thermal arrangement or an arrangement using piezoelectric material and electrodes could also be used. Moreover, electrostatic resetting may be accomplished using an aLL~ny~ L adaptable from that described in connection with FIGURE 20.
While the memory sensor has been described to sense high ~xL,, - , it is recognized that low ~XL
may be detected as well. More particularly, in an alternative Pmhn~im-nt, the sensor is prelatched so that the sensing beam 30 is latched under the resetting beam 20, is shown in FIGURE 2(a). As the value of the temperature decreases, the beam 30 will deflect upwards and will tend to slip off beam 20. once a low extreme is reached, the beams will become completely unlatched. A
simple conductivity test can then be performed to detect whether the sensor is unlatched.
Referring now to FIGURES 4(a)-(p), wherein reference numerals are increased by two hundred and designate like elements, the fabrication of the device of FIGURE 3 begins with a double-side polished (lO0) oriented silicon wafer having a thin film 262 of silicon nitride on top and bottom surfaces (FIGURE 4(a)). The silicon nitride 262 is then patterned using photolithography techniques and reactive ion etching Wogsl34904 2 1 9 2 4 4 0 PCT~S95/0733s (FIGURE 4(b)). A silicon dioxide layer 211 is then grown, patterned, and used as a mask for p + diffusion 226 (FIGURES 4(c)-(e)). After the p ~ diffusion, the silicon dioxide is removed tFIGURE 4(f)) and silicon nitride 219 i5 then deposited and patterned into portions 229 and 239 (FIGURES 4(g)-(h)). A silicon dioxide layer 217 is then grown where the silicon nitride was removed (FIGURE 4(i)). Polysilicon 244 is then deposited and doped (FIGURE 4(j)). Next a layer of silicon nitride 248 is deposited (FIGURE 4(k)). Both the silicon nitride and polysilicon are patterned to form portions 224, 228, 234, and 238 and an oxidation is performed for insulation ~uL~oses (PIGURES 4(1)-(m)). Next the metallic layer (e.g., Cr/Au) is sputtered on and patterned into portions 222 and 232 (FIGURE 2(n)). Bulk etching from the backside and release of the sacrificial silicon dioxide layer are then performed (FIGURES 4(o)-(p)). Note that portions 260 comprising layers of silicon nitride and silicon are formed as a result of the process.
Referring now to FIGURE 5, a still further alternative ~rho~ir ~ of the memory sensor for detecting acceleration ~X~LI . is shown. The sensor is virtually identical to the sensor 10 of FIGURE 3, in both construction and fabrication, except that a proof mass 52 is fabricated on the bottom of sensing beam 30, as will be appreciated by those skilled in the art. AcCeleration =~
W095/34904 2 ~ 9244 0 ~ ' /J~

~XLL, - in the vertical direction are detected, not by manipulation of the bimetallic effect as in the ~ho~; L described in connection with FIGURE 3, but by ~niplllAtional of mass movement and inertia. When acceleration increases, movement of the proof mass 52 in a predetPrminPd direction causes the beam 30 to bend and, consequently, latch under beam 20 upon detection of an extreme.
Similar to that of the Pmho~i- L of FIGURE 3, a simple conductivity test is conducted using test ports 21 to ~P~PrminP whether the sensor is latched and heating resistor 24 (or, alternatively, other thermal piezoelectric or electrostatic techniques) is used to reset the device.
FIGURE 6 illustrates mi~L, --h~nical sensor similar in construction and fabrication to those of FIGURES 3 and 5, except that such sensor detects pressure. Specifically, a sensing beam 30 and a resetting beam 20 are ~;~posP~ on a diaphragm 54 constructed of p+ silicon similar to portion 26 in FIGURE

3. As PLeS~ULe in the vertical direction causes the .
diaphragm to buckle, or depress, downwardly, the beam 30 latches beneath the beam 20 upon detection of a pr~Ptprm;npd ~L~S UL~ extreme.
As will be appreciated by those skilled in the art, a simple conductivity test may be accomplished using W09~34904 ~ 9 2 4 4 ~ PCT~S9~/07335 test ports 21 to determine latching and the device may be reset thermally, bime~lli~~lly, piezoelectrically, or electrostatically Further, pressure in an opposite vertical direction may be sensed if the sensor is initially latehed.
FIGURE 7 illustrates a further Pmho~ nt of the present invention. As with all figures, like numbers ~uLL~a~ulld to like ~LLU~LUL~1 ~1 ta although specific compositions of like layers may vary. As shown, the sensor 10 of FIGURE 7 is similar to that of FIGURE 3 except that p + silicon portion 26 is not included and does not extend beyond the r~ ning layers of beam 20.
Instead, overlap is created between beams 20 and 30 in the sensor 10 by the metal extension 36 of the beam 30.
Additionally, portions 60 vary in composition compared to the embodiment of FIGURE 3, sio2 layer 37 is disposed between supports 40, 50 and beams 20, 30, respectively, and portion z3 is ~icpoSP~ on the lower ~rm;n~l surface of beam 20. Portion 23 is useful for readout as will be hereafter described.
It is appreciated that the sensors 10 of FIGURES 3 and 7 have only subtle distinctions in operation from one another due to differences in configurations. For example, the FIGURE 3 sensor 10 ~n~ c first surface 25, which is contacted by the beam 30 upon an increase in temperature, and a second surface woss/349o4 2 ~ 9 2 4 4 0 PCT~S95107335 .

27 under which the beam 30 is ultimately latched. On the other hand, the FIGURE 7 sensor 10 includes an extension 36 which latches under the beam 20 and contacts portion 23 upon detection of a threshold temperature.
To determine latching, test ports 21 are utilized to conduct a simple conductivity test. In this ~mho~;r-~t, test ports 21 are connected to metal layers 22 and 32. If latched, extension 36 contacts portion 23 and a closed circuit results, a conductive path running through layer 24. If not latched, an open circuit results.
The sensor is reset on the device of FIGURE 3 as described in connection with FIGURE 2, using heating resistor 24 (or, alternatively, other thermal, piezoelectric or electrostatic techniques).
Additionally, low temperature extremes are sensed if the sensor is initially latched.
Now referring to FIGURES 8(a)-(g), wherein the reference numerals have been increased by four hundred and designate like elements, the fabrication of the device in FIGURE 7 begins with a double-side polished (100) oriented silicon wafer with thin films of silicon dioxide 437 and silicon nitride 449 (FIGURE 8(a)). The first step consists of patterning the silicon nitride on the frontside to form portions 429 and 439 using photolithography and reactive ion etching techniques Wogs/34904 2 1 9 2 4 4 0 r l~U~ S

(FIGURE ~(b)). Next polysillcon 424, 423, 434 and silicon nitride 428, 438 are deposited and patterned on the frontside and b~rk~i~p (FIGURE 8(c)). A
photolithography step is then performed to leave a photoresist sacrificial layer 417 (FIGURE 8(d)). The metallic layers 422, 432, and 436 are then sputtered on and patterned (FIGURE 8(e)). Note that after the metal is patterned all of the photoresist 417 is removed. Bulk etching and release by removal of the silicon dioxide layer are then performed (FIGURES 8(f)-(g)). Note that portions 460 are formed in the fabrication process.
FIGURES g and lo represent alternative QmhO~; Ls of the sensor 10 of FIGURE 7 and illustrate an accelerator latch and ~L~ latch similar to those of FIGURES 5 and 6, respectively. Their operation is likewise substantially similar to that described in rnnnPr~;on with those FIGURES. The fabrication process associated with the p~ho~; ts of FIGURE 9 and lO is similar to the process described in connection with FIGURES 8(a)-(g), as will be appreciated by those skilled in the art. In fact, to obtain the sensor of FIGURE 9, the same process is used with the exception of the formation of mass 52.
During the fabrication of the device in FIGURES
3, 5, 6, 7, 9 and lO, residual stresses are induced in the thin films. These stresses relieve themselves after W O 95134904 2 ~ 9 2 4 4 ~ pC~r/US95107335 the release step. As a result, the beams will bend up if the residual stress is tensile and down if it is compressive. This residual stress is utilized to tailor the sensitivity of the device. For example, if the beams exhibit an initial deflection in the downward direction, for an equal small temperature (or, accelerator or P~S~UL~) increase it would exhibit a greater horizontal tip deflection then if the beams were flat. That is, higher stress on the beam results in increased initial deflection. Therefore, the stress can be used to increase sensitivity.
FIGURE ll(a) shows an overall view of another preferred embodiment of the mi~L~ - Anical memory sensor, i.e., an accelerator latch 100, fabricated using surface micrn~~rh i n i ng . While the structural configuration of the latch 100 visually differs from that of FIGURES 3, 5, 6, 7, 9, and 10, the basic concepts described in connection with FIGURES l(a) -2 (d) apply equally. That is, the sensor is mechanically latched upon detection of an extreme of some external force, conveniently tested for latching using a simple conductivity test, and electrically reset.
As shown, the acceleration latch 100 comprises a rectangular plate, or proof mass, 101 formed of silicon or nickel supported by four folded beams 102. The folded beams 102 help to relieve the stress in the latch 100.

W09~34904 2 1 9 2 4 4 0 PCT~S95/07335 When the plate lol is subject to an acceleration, the extended portion 103, or male latching member, of the plate 101 pushes against the two fan shaped structures 104a and 104b and hence respectively push the two cantilevers 105a and 105b away from one another as illustrated in FIGURE ll(b). The combination of the structures 104a-b and 105a-b act as a female latching member corr~p~n~;ng to the male latching member The fan shaped ends 104a and 104b are contoured to provide only a line contact with the extended portion 103 to minimize sliding friction. If the acceleration exceeds a certain threshold value, the extended portion 103 and hence the plate 101 latch on to the fan shaped ends 104a and 104b of the cantilever beams 105a and 105b and stay latched, as shown in FIGURE ll(c).
The acceleration latch 100 SenSQS accelerations in the range of several hundred g's to several t~o~ n~
g's and has folded beams 102 of length 200 to 400 ~m, a plate lOl of 200 to 400 ~m side and cantilevers 105a and 105b of 100 to 200 ~m long. These ~ rC result in an acceleration latch lO0 of less than one m; 11 ~r '~
square in size.
For smaller g's, the lengths of the cantilevers 105a and 105b can be increased and also the mass of the plate lOl can be increased by electroless plating of WOgs/34904 2 1 9244 0 P~

metals, i.e., nickel on top of the polysilicon plate. To sense larger g's (several ~hmlc~n~) the stiffness of the cantilevers lO5a and 105b can be increased.
The duration of contact required for latching between extended portion 103 and fan-shaped structures 104 can be increased to make the device 100 insensitive to shocks of smaller durations. The same can also be achieved by making the extended portion 103 of the plate 101 move a greater distance before it starts pushing the fan shaped structures 104a and 104b near the end of the cantilevers 1osa and 105b. Controlling these different features, accelerations ranging from few g's to several thousands of g can be sensed.
The latch can be verified by testing for electrical continuity between the pads 106 and 107a-d which serve as test ports or readout r- ' ~n;c~c, This is possible since the cantilevers 105a-b and the plate 101 are initially electrically isolated. This is a simple procedure as compared to capacitive or optical sensing.
The latch 100 of FIGURE ll(a)-(c) (and FIGURES
14-20 described hereafter) is constructed of silicon based material. Those skilled in the art will rP~ogn;~e the convenience of using such material in the preferred miuL ch i n; ng techniques.
The device in FIGURE ll(a) (and FIGURES 14-20 hereafter described) is cu..~L-u~ed using surface W095/349~4 2 1 9 2 4 4 0 ~ ",~

micrr-~-hining of tl00) silicon wafers, a process compatible with IC processing techniques. The ~~~hAn;~Al c -nts of the sensor 100 are made by patterning a polysilicon layer of desired thickness (typically 2-5 microns). The polysilicon layer is deposited on a layer of sacrificial oxide of desired thickness which is deposited on the silicon wafer. Only one patterning step is sufficient. Other materials, such as nickel, can also be used in place o~ polysilicon.
Specifically, with reference to FIGURES 12(a)-(c) and 13(a)-(c), the surface micro~9rh;nP~ acceleration latches can be fabricated using either polysilicon or nickel surface mi~ rh;n;ng processes. As regarding FIGURES 12(a)-(c), the polysilicon surface micrn~rh;n;rg technique begins with a silicon wafer with thin films of silicon dioxide 810 and polysilicon 820 (FIGURE 12(a)).
The polysilicon is then patterned using photolithography and reactive ion etching techniques (FIGURE 12(b)). The acceleration latch 100 including proof mass 101 is then releasQd in hydrofluoric acid, leaving sll~p~n~ plates 101 and associated beams (FIGURE 12(c)).
Now, referring to FIGURES 13(a)-(c), the nickel surface micrn-~rh;n;ng technique begins with a silicon wafer With films of silicon dioxide and polysilicon 940 (FIGURE 13(a)). Next, a photolithography step, depositi~g photoresist 930 is performed and nickel is plated (FIGURE 13(b)). The photoresist is then removed and the sacrificial polysilicon layer is removed in a silicon etchant (e.g., potassium hydroxide), leaviny s~lcpPn~Pd plates 101 and associated beams (FIGURE 13(c)).
In a further Prho~ir~nt, as shown in FIGURE 14, the sensor 100 is rendered immune to accelerations in directions other than a selected direction of interest.
Stops 108a-d prevent the motion of the plate in directions other than the sense direction. The silicon substrate and stop 109 prevent the motion of the plate 101 perpendicular to the plane, or surface, of the plate 101. Stop 109 requires 2-polysilicon surface mi~L, -h i n; n.-J for fabrication thereof.
Moreover, the sensor 100 is modified in a still further _mho~;r-nt to sense accelerations in two directions, as shown in FIGURE 15. The sensor 100 in FIGURE 15 is of an identical configuration of the sensor 100 of FIGURE ll(a)-(c) but for the inclusion of an additional latching r- ~ An;cr comprising components 103'-106' to allow bi-directional latching. It is appreciated that the ~ -ntS 103'-106' operate in an identical manner to previously illustrate -ntS 103-106.
The latching arrangement illustrated in FIGURE
16 represents a further Prho~ir L of the invention. As shown, the plate 101 deflects the resilient cantilever 111 until the cantilever 111 passes over protrusion 110 W095/34904 2 ~ 9 2 4 4 0 PCT~S95/07335 ~

to latch upon the detection of a prede~prm;nefl acceleration.
FIGURE 17 shows still a further Pmho~ir-rt of a latching accelerometer according to the present inventiOn. As shown, two plates, or proof masses, 120 and 140 are used to avoid any frictional contact between the PYtPn~Pfl portion, or male latching member, 125 of plate 120 and the fan shaped end 130 of the cantilever 135. Plate 140 is used to pull the fan shaped end 130 away when both plate 120 and 140 are subjected to the preset acceleration. The natural frequencies of the two s~rPn~P~ plates 120 and 140 are chosen such that latching takes place without the extended portion 125 of plate 120 pushing against the fan shaped end 130.
The acceleration latches described in FIGURES
ll(a)-17 are in-plane latching devices. That is, the latching takes place in the plane of the silicon wafer and the proof mass 101, as shown in FIGURE 18(a). Out-of-plane acceleration latches latch in the direction perpPn~;cn1~r to the silicon wafer and the proof mass 101, as shown in FIGURE 18(b). Several devices, inr1n~;ng both in-plane and out-of-plane types, can be inr]n~Pfl on the same chip to sense acceleration in X, Y, and Z directions. However, (110) bulk micromachined devices, such as the Ciarlo device noted above, can in~uL~oL~te only in-plane acceleration sensing (in X and WO9Sl34904 ~ /JJ~
~ -31- 2 1 92440 Y direction) on the same chip. An out-of-plane latch 100 similar to the in-plane latches is shown in FIGURE 19.
More particularly, the latching cantilever 150 overlaps the proof mass 101, which consists of the first polysilicon layer and/or a metallic layer, as shown.
When the proof mass 101 is subjected to an acceleration in the out-of plane direction, perpendicular to the surface of the proof mass 101, a force is generated on the latching cantilever 150, which is anchored to the substrate, causing it to deflect in the out-of-plane direction. This vertical cantilever tip 155 deflection will also result in a horizontal/in-plane deflection.
Once the in-plane deflection is greater than the overlap, the cantilever beam 150 slips off the proof mass 101 and latches nn~Prn~ath.
The out-of-plane latch is conveniently fabricated using 2-polysilicon surface mi~LI rhinirg techni~ues. Moreover, while the existing (110) bulk micrr--rh;nr~ latch of FIGURE 21 is not resettable, meaning that it cannot be delatched and reused, a resetting -- An;rm, ag will be described with reference to FIGURE 20 is further conveniently microfabricated on the surface micrr--rh;nr~ acceleration latches of the present invention. It is appreciated that a similar resetting r ' An;rm can be likewise incorporated into the in-plane latching devices. It is further appreciated W095~34904 2 I q 2 4 4 0 PCT~S9~0733~

that alternative resetting schemes incorporating thermal, bimetallic and piezoelectric principles will become readily apparent to those skilled in the art upon a reading hereof.
~IGURE 20 shows a top view of an out-of-plane latch acceleration sensor according to the present invention which incorporates a resetting r- '~nicm 170.
The resetting - '~n;qrl 170 is comprised of an electrostatic comb-drive 175 as shown in FIGURE 20. To reset the device 100, the electrostatic comb-drive 175 is implemented. A potential di~ference is placed on the ele~LLu~dtic comb-drive 175 to enable the proof mass 101 to be pulled away from the latched cantilever 150. This pulling away is accomplished with relative ease. ~hen the proof mass 101 is pulled away from the cantilever 150 a distance greater than that of the overlap, the latched cantilever 150 can be delatched and thus restored back to its original position so that the sensor can be reused.
Additionally, g-second devices may be fabricated using the surface mi~L~ rh;n~ accelerometers described herein. A g-second device is different from a conventional accelerometer as it responds to a combination of the acceleration magnitude and the time duration over which the acceleration is sustained. An alternate way of considering this device is as a velocity latch since the device effectively responds to the area WO9s/34904 2 1 9 2 4 4 0 PCT~S95107335 under the acceleration/time curve. viscous damping is used to achieve this feature. By proper selection of the device ~;r--~~lonc through r--'el;rg and effective use of viscous damping, it is possible to achieve g-second requirements for time durations of up to several tens of seconds.
Any of the sensors described in accordance with this invention in FIGURES 1-20 are useful as a single mi~L, ' ~nic~l sensor and, when used in conjunction with a plurality of other sensors, may be used as a sensing system. More particularly, two modes of operation may be accomplished according to the present invention: boolean and quasicontinuous. The boolean operational mode, using one sensor 10, answers the true/false question: Was the preset extreme PY~PP~P~l? On the other hand, the quasicontinuous operational mode, which utilizes a plurality of sensors, indicates the range of ~L~ to which the sensing system was exposed, not just whether a single extreme has been PYcee~Pd. A system used in the quasicontinuous mode indicates the actual e~LL~ -- that the system was exposed to by using an array of sensors 10 that accomplish the boolean function individually, as described above. Each device in the array detects a different extreme in specific in~L~ Ls.
For example, four boolean type sensors 10 that sense e~LLI -- in increments of 10~C: 100~C, 110~C, WOgs/34904 21 q2440 r~

120~C, and 130~C can be used. If the maximum temperature extreme that this array was exposed to was 125~C, then the 100~C, 110~C, and 120~C sensors will indicate that their designed temperature extreme has been P~re~rlr~rl.
However, the 130~C sensor will not indicate the 125~C
te...~r-r~tur~ extreme. Therefore, the quasicontinuous mi~" --hAn;cal memory sensor system will indicate that an exposed temperature extreme between 120~C and 130~C
has oc~uLred. Further examples respecting acceleration and ~s~u~e will not be specifically described.
However, those skilled in the art will appreciate that CU1~ e~onding quasicontinuous systems for acceleration and ~1 ~SYU1~ are readily apparent upon a reading hereof.
A further significant advantage of the present invention is that not only can a plurality of sensors be fabricated on a single substrate, but a plurality of types of sensors can be fabricated on a single substrate.
So, for example, a temperature sensor, acceleration sensor, and pressure sensor may be fabricated on the same substrate to produce a multi-purpose device.
Practical application of the present invention extends beyond sensing technology as described. The invention may also find use as an electrical switch in certain applications.
The above description merely provides a ~icclosl~re of particular r~mhorli - Ls of the invention and W095/34904 PCT~S95/07335 _352 192440 is not intended for the purpose of limiting the same thereto. As such, the invention is not limited to only the above described ~mho~;m~nts~ Rather, it is recognized that one s~illed in the art could conceive alternative embodiments that fall within the scope of the invention.

Claims (31)

Having thus described the invention, we claim:

1. A micromechanical sensor comprising:
a mechanical latch induced upon detection of a threshold value of a variable condition;
a readout mechanism for detecting whether the latch member is latched; and, a resetting mechanism electrically unlatching the latch member whereby the sensor latched purely mechanically is electrically reset for repeat use.

2. The sensor of claim 1 wherein the variable condition is ambient temperature.

3. The sensor of claim 1 wherein the variable condition is acceleration.

4. The sensor of claim 1 wherein the variable condition is pressure.

5. A micromechanical memory sensor comprising:

a latch member mechanically latching upon detection of a predetermined temperature extreme; and, a readout mechanism facilitating detection of whether the latch member is latched.

6. A micromechanical memory sensor comprising:
a latch member mechanically latching upon detection of a predetermined pressure extreme; and, a readout mechanism facilitating detection of whether the latch member is latched.

7. A micromechanical memory sensor comprising:
a latch member mechanically latching upon detection of a predetermined acceleration extreme;
a readout mechanism facilitating detection of whether the latch member is latched; and a resetting mechanism electrically unlatching the latch member whereby the sensor latched purely mechanically is electrically reset for repeat use.

8. A micromechanical memory sensor comprising:
a first beam supported at a first end thereof by a substrate and having a second end; and, a second beam supported at a first end thereof by a substrate and having a second end having flexibility greater than the first beam, the first and second beam being disposed in a first arrangement so that the second end of the second beam engages a first surface of the first beam at the second end of the first beam, an increase in ambient temperature facilitating a first deflection in the first beam and a second deflection, greater than the first deflection, in the second beam in accordance with the difference in the flexibility of the first and second beams, and, a decrease in the ambient temperature facilitating movement of the first and second beams to a second arrangement, the second beam engaging a second surface of the first beam in the second arrangement so that the second beam is latched on the first beam.

9. The sensor of claim 8 wherein the first .
beam comprises a heating resistor which, when having an electric current supplied thereto, facilitates a third deflection in the first beam, the third deflection being greater than the first deflection, to disengage the second surface from the second beam to cause the electric current to terminate and cause the first and second beams to return to the first arrangement.

10. A micromechanical memory sensor comprising:
a first beam formed of a first material having a first thermal coefficient of expansion and a second material having a second thermal coefficient of expansion, the second coefficient being different than the first coefficient, the first and second materials being disposed in layers having a terminal end and formed on a substrate, the substrate extending beyond the terminal end; and, a second beam having flexibility greater than the first beam disposed in a first arrangement such that the second beam opposes a first surface of the substrate, the second beam being formed of the first material and the second material, an increase in ambient temperature facilitating a first deflection in the first beam and a second deflection, greater than the first deflection, in the second beam in accordance with the difference in the first and second coefficients and the flexibility of the first and second beams, and, a decrease in the ambient temperature facilitating movement of the first and second beams to a second arrangement, the second beam engaging a second surface of the substrate in the second arrangement so that the second beam is latched on the first beam.

11. The sensor of claim 10 wherein the first beam comprises a heating resistor which, when having an electric current supplied thereto, facilitates a third deflection in the first beam, the third deflection being greater than the first deflection, to disengage the second surface from the second beam to cause the electric current to terminate and cause the first and second beams to return to the first arrangement.

12. A micromechanical memory sensor comprising:
a first beam having a first length disposed along a longitudinal axis, the first beam being formed of a first material having a first thermal coefficient of expansion and a second material having a second thermal coefficient of expansion, the second coefficient being different than the first coefficient, the first and second materials being disposed in layers having a terminal end and formed on a substrate, the substrate extending beyond the terminal end; and, a second beam having a second length greater than the first length disposed along the longitudinal axis so that in a first arrangement the second beam engages a first surface of the substrate, the second beam being formed of the first material and the second material, an increase in ambient temperature facilitating a first deflection in the first beam and a second deflection, greater than the first deflection, in the second beam in accordance with the difference in the first and second coefficients and the first and second length of the first and second beams, a decrease in the ambient temperature facilitating movement of the first and second beams to a second arrangement, the second beam engaging a second surface of the substrate in the second arrangement so that the second beam is latched on the first beam, and a supply of electric current to the second material of the first beam facilitating a third deflection in the first beam, the third deflection being greater than the first deflection, to disengage the second surface from the second beam to cause electric current to terminate and cause the first and second beams to return to the first arrangement.

13. A micromechanical sensing system comprising:
a plurality of micromechanical memory sensors, each sensor comprising:
a first beam having a first length disposed along a longitudinal axis, the first beam being formed of a first material having a first thermal coefficient of expansion and a second material having a second thermal coefficient of expansion, the second coefficient being different than the first coefficient, the first and second materials being disposed in layers having a terminal end and formed on a substrate, the substrate extending beyond the terminal end; and, a second beam having a second length greater than the first length disposed along the longitudinal axis so that in a first arrangement the second beam opposes a first surface of the substrate, the second beam being formed of the first material and the second material, an increase in ambient temperature to a predetermined temperature facilitating a first deflection in the first beam and a second deflection, greater than the first deflection, in the second beam in accordance with the difference in the first and second coefficients and the first and second lengths of the first and second beams, the predetermined temperature being different for each sensor, and a decrease in the ambient temperature facilitating movement of the first and second beams to a second arrangement, the second beam engaging a second surface of the substrate in the second arrangement so that the second beam is latched on the first beam.

14. The system of claim 13 wherein the first beam of each sensor comprises a heating resistor which, when having an electric current supplied thereto, facilitates a third deflection in the first beam, the third deflection being greater than the first deflection, to disengage the second surface from the second beam to cause the electric current to terminate and cause the first and second beams to return to the first arrangement.

15. A micromechanical memory sensor comprising:
a first beam supported at a first end and having a second free end;
a second beam supported at a first end and having a second free end, the second end of the second beam overlapping the second end of the first beam, and the second end of the second beam having a proof mass extending therefrom in a first direction, movement of the proof mass in the first direction caused by an acceleration to which the sensor is subjected bending the second beam to latch under the first beam if a predetermined level of acceleration is reached;
a readout mechanism to detect whether the sensor is latched; and, a resetting mechanism to electrically unlatch the first and second beams.

16 A micromechanical memory sensor comprising:
a rectangular plate having four corners and sides;
a male latching member extending from a first side of the plate;
folded beams, each connected at one end to one of the four corners to support the plate and at a second end to electrical pads;
a female latching member in opposed relation to the male latching member;
acceleration of the plate in a first direction toward the female latching member causing a deflection of the folded beams to facilitate movement of the male latching member toward the female latching member and subsequent engagement thereof to allow movement of the male latching member in the first direction and prevent the movement of the male latching member in a second direction opposite the first direction.

17. The sensor of claim 16 further comprising a first stop opposing a second side of the plate, a second stop opposing a third side of the plate, and a third stop opposing a fourth side of the plate to allow movement of the plate in only the first direction.

18. The sensor of claim 16 wherein the female latching member comprises a second rectangular plate having an extension corresponding to the male latching member, the second plate and extension being deflected upon the acceleration in the first direction.

19. The sensor of claim 16 further comprising:
a second male latching member extending from a parallel second side of the plate;
a second female latching member in opposed relation to the second male latching member;
acceleration of the plate in the second direction toward the second female latching member causing a second deflection of the folded beams to facilitate movement of the second male latching member toward the second female latching member and subsequent engagement thereof to allow movement of the second male latching member in the second direction and prevent movement of the male latching member in the first direction.

20. The sensor of claim 16 further comprising a resetting mechanism.

21. The sensor of claim 20 wherein the resetting mechanism comprises an electrostatic comb drive.

22. The sensor of claim 17 further comprising an electrical resetting mechanism.

23. The sensor of claim 18 further comprising an electrical resetting mechanism.

24. The sensor of claim 19 further comprising an electrical resetting mechanism.

25. A micromechanical memory sensor comprising:
a rectangular plate having four corners and sides;
folded beams, each connected at one end to one of the four corners to support the plate and at a second end to electrical pads;
a protrusion extending from one side of the plate;
a resilient cantilever perpendicular to the one side in alignment with the protrusion, a first end of the cantilever overlapping a first side of the protrusion and being disposed a predetermined distance from the protrusion in the first direction;
acceleration of the plate in a first direction perpendicular to the cantilever causing a deflection of the folded beams to facilitate movement of the protrusion in the first direction to engage and deflect the cantilever so that the cantilever engages a second side of the protrusion to allow movement in the first direction and prevent movement in a second direction opposite the first direction.

26. The sensor of claim 25 further comprising a resetting mechanism.

27. A micromechanical sensor comprising:
a rectangular plate having four corners, sides, a first surface, and a second surface;
folded beams, each connected at one end to one of the four corners to support the plate and at a second end to electrical pads;
a cantilever having an end, the cantilever being positioned perpendicular to a first side of the plate and the end overlapping the plate so that the cantilever opposes the first surface;
acceleration of the plate in a first direction perpendicular to the surfaces of the plate causing a deflection of the folded beams to facilitate movement of the plate in the first direction to engage and deflect the cantilever so that the cantilever engages the second surface of the plate.

28. The sensor of claim 27 further comprising a resetting mechanism.

29. The sensor of claim 27 wherein the resetting mechanism comprises an electrostatic comb-drive.

30. A micromechanical memory sensor comprising:
a diaphragm;
. a first beam on the diaphragm;
a second beam on the diaphragm and overlapping the first beam, a depression of the diaphragm caused by a pressure to which the sensor is subjected inducing relative movement between the first and second beam so that the second beam latches under the first beam if a predetermined level of pressure is reached; and, a readout mechanism to detect whether the sensor is latched.

31. The sensor of claim 30 further comprising a resetting mechanism to electrically unlatch the first and second beams.