FIELD

The present invention relates to an accelerometer of a type having no proof or inertial mass and no moving parts or parts under stress such as piezo or strain gauge accelerometers.

BACKGROUND

Accelerometers find use in widely diverse applications including automobile air bags and suspension systems, computer hard disc drivers, smart detonation systems for bombs and missiles and machine vibration monitors. Silicon micromachined acceleration sensors are beginning to replace mechanical acceleration switches. Present accelerometers are all based upon the classical Newtonian relationship of force, F, mass, m, and acceleration, a, in which F = ma. Thus, for a cantilevered beam, the force due to acceleration causes the beam to deflect. This deflection is sensed either by sensing the change in piezo resistors or by a change in capacitance. Such systems are not stable over wide temperature ranges and have a response which peaks due to insufficient mechanical damping.

One form of accelerometer made by bulk micromachining consists of a membrane or diaphragm of silicon formed by chemical etching having a large mass of silicon at the centre and tethers of thin film piezo- resistors, whose resistance is sensitive to strain and deformation, suspending the mass. Acceleration causes the large silicon mass to move, deforming the diaphragm and changing the resistance of the piezo-resistors . Such bulk micromachined devices are large by integrated circuit standards and consistent with semiconductor circuit fabrication techmgues.

Another system made by surface micromachining is based on a differential capacitor. Surface micromachining creates much smaller, more intricate and precisely patterned structures than those made by bulk micromachining. It involves the same process that is used to make integrated circuits, namely, depositing and etching multiple thin films and layers of silicon and silicon-oxide to form complex mechanical structures. In this case a central beam is affixed in an "H" configuration with the spaced apart parallel arms of the "H" supporting respective ends of the cross beam.

A plate affixed perpendicular to the beam forms a moving capacitor plate that is positioned between two fixed plates, thus, forming two capacitors sharing a common moving plate. When the unit is subjected to an accelerating force the beam and hence moving plate moves closer to one of the fixed plates and away from the other fixed plate. The effect is to reduce one of the capacitors and increase the other by an amount proportional to the acceleration. The device requires proper orientation with the cross beam parallel to the direction of acceleration. However, surface micromachining is used to create a much smaller device adapted to the same techniques used to make integrated circuits. The moving capacitor plate accelerometer suffers from high noise and exhibits drift at low acceleration measurements.

It is an object of the present invention to provide an improved accelerometer. It is a further object of the invention to provide an accelerometer having no proof mass and a corresponding increased ruggedness

SUMMARY OF THE INVENTION

According to the invention there is provided an accelerometer having a substrate with a cavity therein, a

heater wire and a pair of sensor wires extending across the cavity with the heater wires equidistant from the heater and on either side thereof.

Preferably the substrate is silicon and the cavity, heater wire and sensor wires are made by micromachining. Micromachining is the technology without which practical accelerometers of the kind described herein cannot be produced.

The means for passing electrical current through the heater so as to develop a symmetrical temperature gradient extending outwardly from the heater to the two temperature sensors .

The heater wire and sensors may be polysilicon. Alternatively they may be a thin' film of metal . A suitable metal may be selected from the group consisting of nickel, chromium, gold or platinum.

Advantageously, there may be provided means for measuring the differential resistance of the temperature sensors.

Means may also be provided to convert the differential temperature measurement to an acceleration value.

An auxiliary heater wire may be formed on each side of the heater wire and symmetrically disposed with respect to the heater wire.

BRIEF DESCRIPTION WITH REFERENCE TO THE DRAWINGS

The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as other features and advantages thereof, will be best understood by reference to

the detailed description which follows, read in conjunction with the accompanying drawings, wherein:

Fig. 1 is a perspective view of a preferred embodiment of the accelerometer;

Fig. 2 is a graph showing the normal temperature gradient and the temperature gradient shift due to acceleration;

Fig. 3 to 6 are sectional views showing the steps in fabricating the device of Fig. 1;

Fig. 7 is a plan view of the device;

Fig. 8 is a schematic diagram of the circuitry used with the accelerometer; and

Fig. 9 is a top or plan view of the device incorporating two auxiliary heaters for self-testing.

Fig. 10 is a top view of a thermal accelerometer effective to measure acceleration along two perpendicular directions .

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

Referring to Figure 1 the accelerometer is formed on a silicon substrate 10 in which a cavity 20 is formed underneath a central heater 23 and sensors 22 and 24 positioned at equal distances from the heater 23 on either side thereof. Wire bonding pads 28 and 34 are formed on either end of the heater 23 to provide electrical contact thereto. Wire bonding pads 26 and 32 are formed on either end of sensor 22 and wire bonding pads 30 and 36 are formed on either end of sensor 24 also to provide electrical contact. Electrical current is passed through the heater 23 which heats the air around it. The

temperature gradient established is shown by the solid lines 34 and 36 in Figure 2. Resistive temperature sensors 22 and 24 are used to measure the temperature of the surrounding air. The distance between the heater 23 and each sensor is about 100 microns while heater 23 is 10 to 15 microns wide. The length of the heater 23 and sensors 22 and 24 is approximately 500 microns. However, other dimensions may be selected depending upon the desired specifications of operation.

With the sensors 22 and 24 equidistant from the heater 23 the differential temperature between the sensors 22 and 24 will be zero. If the substrate 10 is subjected to an accelerating force in a direction perpendicular to the heater 23 but along the surface of the substrate, the temperatures distribution of the air will shift as shown in the dotted lines 36 and 38. In this case sensor 22 will experience an increase in temperature whereas sensor 24 will detect a reduced temperature, giving a net non-zero differential temperature measurement between the sensors 22 and 24 of a magnitude which is proportional to acceleration. For the device to operate properly, it is placed in a sealed chamber so that the temperature gradient will not be disturbed by external air current or flow.

Referring to Figure 3, the silicon wafer 12 is n-type. Thermal oxidation at 1,100 °C produces a layer of silicon dioxide 14 0.5 μm thick. On top of the layer of silicon dioxide 14 a layer of polysilicon 16 0.8 μm is deposited. The polysilicon layer 16 is lightly doped to increase its electrical conductivity. Following this doping, another oxidation step is used to develop a 0.5 μm thick layer of silicon dioxide 18 on top of the polysilicon 16 reducing the thickness of the polysilicon layer to 0.5 μm. Standard photolithographic techniques are used to pattern the silicon dioxide layer 18 over the polysilicon 16 as seen in Figure 4. The silicon dioxide 18 is used a?.

an etch mask for the removal of exposed polysilicon using ethylenediamine-pyrocatechol-water (EDP) mixture as an etchant at 85 °C. Oxide 14 underneath the polysilicon 16 protects the silicon substrate 10 during etching. The resulting structure after etching shown in Figure 4 defines three polysilicon bridges that are used as the heater 23 and the two sensors 22 and 24.

Another oxidation step produces oxide on the side walls of the polysilicon 16 to protect it from a later silicon etch. The oxide layers above and below the polysilicon layer 16 are patterned to create openings for the bonding pads and formation of the cavity in the silicon substrate as shown in Figure 5.

An aluminum nickel seed layer is sputtered onto the wafer and photoresist is patterned so the exposed bonding pad area can be selectively plated with gold. The photoresist and seed layer are removed and the wafer is etched in EDP to create a deep cavity 20 underneath. During EDP etching of the silicon substrate 10, the polysilicon bridges 22, 23, and 24 are protected by the oxide layers and the polysilicon 16 underneath the bonding pads 26, 28, 30, 32, 34, and 36 is protected by gold plated pads 44.

The fabrication process is compatible with CMOS and bipolar processes. This allows the accelerometer to be integrated with signal conditioning circuits.

Referring to Figure 7, the final device consists of three elongated strips each of which consists of a layer of polysilicon sandwiched between layers of oxide 22, 23, 24 coupled to their respective bonding pads and suspended over a cavity 20. The space around the heater 23 and temperature sensors 22 and 24 is filled with either a fluid or a gas. Although polysilicon nan been described

as the material of which the heater 23 and sensors 22 and 24 are made, thin film metal resistors such as nickel, chromium, gold or platinum can be used.

Heater 23 is used in a bridge circuit formed by resistors Rl, R2, R _L , and R„ shown in Figure 8. The junctions of the bridge are sampled by lines 46 and 48 and fed into the input of a differential amplifier 50 which provides an output on line 52. When acceleration is applied, the balance of the bridge is disturbed causing a differential voltage to be applied to the amplifier 50. The amplifier 50 converts the differential signal to a single-ended voltage at its output on line 52. With Rl = R2, and no acceleration, V _a =0. When acceleration is applied this balance is disturbed and the differential voltage V _a is amplified and converted into a single-ended signal by the differential amplifier 50.

Although micromachining was described as the technology used to produce this accelerometer, there are other low cost manufacturing technologies that can also be used. In order to maximize the differential temperature change of the sensors 22 and 24 the direction of acceleration is perpendicular to the heater 23 and along the surface of the substrate 10, Acceleration perpendicular to the surface of the substrate will cause a change in the temperature gradient but will affect each sensor in the same way.

Referring to Figure 9, a self-testing capability can be implemented by the addition of two auxiliary heaters 23A and 23B, one on each side of main heater 23. In normal operation, all three heaters 23, 23A, arid 23B are powered to produce a symmetrical temperature gradient which is disturbed only by acceleration. By switching off one of the auxiliary heaters 23A or 23B, this symmetrical temperature gradient is disturbed. For example, when

auxiliary heater 23A is switched off, the point of symmetry of the temperature gradient will shift from the center of heater 23 towards sensor 24 without application of an acceleration. This change will produce an output to indicate that the accelerometer's function is intact. Switching off heater 23B has a similar effect but in the opposite direction. Instead of cutting off the current completely, a more elaborate testing can be done by controlling the amount of auxiliary heater current reduction to anywhere between 0 and 100%.

Obviously, two or three accelerometers oriented at right angles to each other could be used to sense acceleration in two dimensions or three dimensions, respectively, rather than having to orient the accelerometer in the direction of the acceleration. By using a small heater plate in place of the long heater wire, and increasing the number of temperature sensors from two to four, as illustrated in Figure 10, this same device can be used to measure acceleration in two axes simultaneously.

A thermal accelerometer capable of detecting acceleration in 2 axes can be produced using the same principle as above. Illustrated in Figure 10 is the top view of a 2-axes thermal accelerometer 61. It consists of a suspended structure 63 over an etched cavity 65. At the center of this suspended structure is a small heater plate 60 which generates a symmetrical temperature gradient in both the X and the Y axes. Four temperature sensors, 62, 64, 66 and 68 are placed at equal distance from the heater plate. When acceleration is applied to the device in the X direction, differential temperature is registrated only by 62 and 66, producing an output. Similarly when acceleration is applied to the device in the Y direction, differential temperature is registrated only by 68 and 64

Accordingly, while this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to this description. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.