Motion detection using capacitor having different work function materials

An apparatus for detecting mechanical displacement in a micro-electromechanical system includes a capacitor having first and second plates spaced from one another, the first and second plates having different work functions and being electrically connected with each other. The capacitor plates are movable with respect to one another such that a spacing between the plates changes in response to a force. A current through the capacitor represents a rate of change in the spacing between the plates at a given time.

FIELD OF THE INVENTION

The present invention relates to methods and apparatuses for detecting motion.

BACKGROUND OF THE INVENTION

In modern video game controllers, for example, the conventional way to detect motion is by measuring capacitance. One such motion detector10based on capacitance measurements is shown inFIG. 1. The motion detector10includes two metal plates12,14separated by a small distance D. One of the plates is fixed and the other is free to move relative to the other in the direction illustrated by the arrow in response to some force. Some form of spring, such as bellow or diaphragm16, is disposed between the two metal plates12,14to restore the plates to their static relationship after a force induced motion of the two plates relative to one another. If there is a motion, the distance between the two metal plates12,14will change from D to D′. The slight difference between D′ and D will result in a change in the capacitance between the two metal plates. By measuring the capacitance during the course of motion, the acceleration, the velocity and the distance of motion can be calculated. The motion sensors in Nintendo's Wii® game controller are based on this principle.

The capacitance of the capacitor10is C=x*y*∈/D, where x and y are the two dimensions of the metal plates12,14, ∈ is the permittivity of the dielectric between the plates12.14, and D is the distance between plates. With these types of prior art motion detectors, the change in capacitance is very small and hard to measure in real time. An alternative motion sensor that is faster, cheaper, and/or more accurate is desired.

SUMMARY OF THE INVENTION

An apparatus for detecting mechanical displacement in a micro-electromechanical system includes a capacitor having first and second plates spaced from one another, the first and second plates having different work functions and being electrically connected with each other. The capacitor plates are movable with respect to one another such that a spacing between the plates changes in response to a force. A current through the capacitor represents a rate of change in the spacing between the plates at a given time.

The above and other features of the present invention will be better understood from the following detailed description of the preferred embodiments of the invention that is provided in connection with the accompanying drawings.

DETAILED DESCRIPTION

A method and apparatus for detecting mechanical displacement in micro-electromechanical (MEMS) devices is provided. The displacement determination is based on the detection of current generated by modulating the distance between two plates of a capacitor where the capacitor plates are made of materials with dissimilar work functions. By “work function” it is meant the minimum amount of energy required to remove an electron from the surface of a conducting or semi-conducting material. If the plates are electrically connected to one another, the work function difference between the materials forming the two capacitor plates generates a built-in electrical field. The electric field across the capacitor is modulated with the distance between the plates. That is, at equilibrium there will be an electric field built-in between the plates and no current flowing in the system. If the distance between these two plates changes, the field will change and the current will flow. By measuring the magnitude of the current, the change in distance between the metal plates can be calculated. The current through the capacitor represents the rate of change in spacing between the capacitor plates, i.e., plate velocity. The acceleration can also be calculated for a MEMS motion sensor. The sensor disclosed herein can have far ranging applications, such as in video game controllers and joy sticks, accelerometers, gyroscopes, safety devices (such as air bag deployment) and others.

The capacitor plates can be formed between metals with dissimilar work functions, metal/semiconductor structures with dissimilar work functions, semiconductor materials with dissimilar work functions, between semiconductor materials with different dopants, or combinations of these materials or structures.FIGS. 2(a) to2(d) illustrate various embodiments of capacitors having capacitor plates having different work functions.FIG. 2(a) illustrates a capacitor100A including a first capacitor plate110A and second capacitor plate120A. The plates are spaced a distance D from one another. The dielectric between the capacitor plates110A,120A is assumed to be air or vacuum. Each capacitor plate110A,120A is formed from a metal layer112, such as Al, W, Ti, Au or other metals suitable for integration in a specific MEMS process flow and a semiconductor layer formed thereon. The capacitor plate110A includes layer114of an N+ doped semiconductor material (e.g., silicon) and the capacitor plate120A includes layer116of a P+ doped semiconductor material (e.g., silicon). The capacitor plate's work function is determined by the work function of the material facing into the “gap” between the plates110A,120A (i.e., by the N+/P+ layers114,116). The metal layers112can be provided to provide a specific work function (if facing the gap between the plates) or to provide a low resistance connection as shown inFIG. 2(a).

The capacitor plates110A,120B are physically connected to one another via an elastic element shown as elastic element140. If there is no external force, the capacitor plates stay at a static position. If there is relative movement between the plates, the elastic element140helps restore the plates to their static position. The elastic element140may be a spring, an elastic sheet, elastic diaphragm or the like. One of the electrode plates may be a stationary electrode plate (i.e., fixed) and the other electrode plate may be considered a movable electrode plate positioned parallel to the stationary plate and free to move in a direction orthogonal to the major surface of the plates.

The capacitor plates110A,120B are electrically connected to one another. This electrical connection is shown as connection130in the drawings ofFIGS. 2(a) to2(d). When two electrically connected materials with dissimilar work functions are brought in close proximity to each other, the Fermi levels of these materials line up due to the absence of electric current between the two materials. The work function difference between them creates a charged capacitor and the electric field between these features. The charge on the capacitor is equal to Q=−ΔΦ*C/e, where e is the electron charge sign, ΔΦ is the work function difference (in eV) between the materials and C is the capacitance of the structure. The electric field between the features (e.g. uniform field in a parallel plate capacitor) is E=−ΔΦ*/De, where D is the distance between the plates at a given time.

If there is movement of the plates relative to one another, such that the spacing between the plates changes, an electrical current is generated. The magnitude of the current can be determined (for a parallel plate capacitor) as follows:

I=−ΔΦ*x*y*∈/e[1/D−1/D′]/Δt=A*ΔD/[Δt(D*D′)], where A is the “structure” constant and is equal to the area of the plates (x*y) times ∈/e; ∈ is the dielectric constant (e.g., 1 for a vacuum); D-D′ is ΔD; and Δt is the change in time. This expression shows that the detection current is proportional to the relative velocity of the two electrodes.

The relative movement in the capacitor plates will cause an AC/transient current. Assuming the distance between the two plates is not very large, the current will be proportional to the velocity of the plate (ΔD/Δt). Continuously detecting velocity provides a change in velocity (Δv) over a given time period (Δt). The first derivative of velocity/time (Δv/Δt) is acceleration. As such, the capacitor can be used as a motion detector for an accelerometer.

It should be noted that it is not necessary to know the physical properties of the elastic element that attaches the first and second capacitor plates in order to determine velocity and acceleration, but knowing the physical properties of the elastic element can help provide information on the sensitivity of the motion detector since ΔD is dependent on the properties of the elastic element. In embodiments, the maximum value of ΔD may be between around 1 μm to a few microns.

FIG. 2(b) illustrates an embodiment of a capacitor100B where the two capacitor plates are semiconductor materials having different dopant kinds, e.g., N+ and P+. In the illustrated embodiment, the capacitor plate110B is a semiconductor material doped with n-type dopants and the capacitor plate120B is a semiconductor material doped with p-type dopants.

FIG. 2(c) illustrates an embodiment of a capacitor100C where the two capacitor plates are formed from different metal materials having different work functions. Capacitor plate110C is formed from a first metal and capacitor plate120C is formed from a second metal.

FIG. 2(d) illustrates an embodiment of a capacitor100D wherein the two capacitor plates are formed from different semiconductor materials having different work functions. Capacitor plate110C is formed from a first semiconductor material (e.g., n- or p-doped silicon) and capacitor plate120C is formed from a second semiconductor material (e.g., n- or p-doped germanium). Different dopants, different doping concentrations, different semiconductor base materials or combinations thereof can be used to provide materials having different work functions. By way of example, the expected work function difference between the N+ and P+ Si is approximately 1.1 eV. Certain metals can produce work function differences greater than about 2 eV.

FIG. 3illustrates an embodiment of a motion detector having capacitor sensors aligned along three different axes, including a vertical axis, a lateral axis and a longitudinal axis. This orientation can be used to form a 3-dimensional accelerometer sensing acceleration in the direction of each of the three axes.

FIG. 4illustrates a capacitor100formed as discussed above, i.e., a capacitor with two capacitor plates of different work functions and movable with respect to one another in response to a force, connected to a current sensing circuit. More specifically, the capacitor100is connected in parallel with resistor R1for generating a first current I1. This current I1is then mirrored and amplified via a JFET amplification stage comprising a JFET M1, resistors R2, R3and R4and capacitor C1. Resistor R4and the JFET M1form an amplifier. Resistors R2, R4bias the JFET into a desired state. Capacitor C decouples the DC bias on the gate of the JFET from the MEMS element. Resistor R1provides a voltage drop due to the MEMS-generated current. This voltage drop is sensed by the amplifier.

FIG. 5illustrates an embodiment of a sensor with differential sensing. This embodiment employs two capacitors100,200. The first capacitor100is formed in the manner described above. That is, capacitor100has two capacitor plates of different work functions spaced apart from one another and movable relative to each other in response to a force. The second capacitor200is identical to capacitor100only the two capacitor plates are fixed relative to one another so as to provide a reference current. Each capacitor is connected to the source of an NMOS transistor M2or M3, which are biased in the on state by voltage VBIAS. Resistors R6and R5are connected to the drains of the transistors M2and M3, respectively. Reference current I3is generated by the reference stage (i.e., R5, M3, capacitor200) and transient current I4is generated in the detection stage (i.e., R6, M2, capacitor100). Voltage drops occur across resistors R5and R6representative of the currents I3and I4, respectively. The resulting voltages at nodes A and B are compared and amplified by the differential amplifier

FIG. 6illustrates an embodiment of a motion sensor including a plurality of capacitors arranged in an interdigitated assembly300. The interdigitated assembly includes first and second comb electrode sections310,320that are movable relative to one another in the direction of the arrow illustrated next to the assembly300. For example, the first section310can be fixed whereas the second section320is free to move relative to the first section310, or vice versa. The first section310includes a plurality of capacitor plate arms312extending from a stem or trunk section311. These capacitor plate arms312are formed from a material having a first work function. The second section320includes a plurality of capacitor plate arms322extending from a step or trunk section321. Each capacitor plate arm322includes a first side323formed from a material having the same work function as the first work function of the capacitor plate arms312and a second side324formed from a material having a different (second) work function than the first work function. Each section324and the capacitor plate arm312that it faces forms a respective capacitor. So, the interdigitated structure includes a plurality of capacitors with each capacitor having a first plate with a first work function and a second plate with a second work function. As the two sections310,320move relative to one another, the spacing (D) between the capacitor plate sections324and the capacitor plate arm312of each capacitor changes equally. A current is generated through each branch of the capacitor. Electrical contacts can be made to trunk section311and to trunk section312to sense the generated current. The sum of these currents can be detected and averaged across the total number of capacitors contributing to the detected current to provide a better indication of the actual displacement. This larger current and changes therein are easier to detect than the smaller current through a single capacitor, making for a more accurate detector. It should be noted that capacitors are also formed between arms312and sections323of arms322. However, these capacitor plates have the same work function and thus produce zero current.

InFIG. 6, it should be understood that the one of the sections310,320can be part of or connected to a fixed support member attached to a substrate. The other of the sections310,320is part of or connected to a deflection member attached to the support member, such as by an elastic element as discussed above, so as to deflect relative to the support member. Any number of kinds of support members and deflection members can be provided for positioning the capacitor described herein relative to one another. These support members are known in the art of capacitive motion detection and are used to support and allow deflection relative to one another of the plates of capacitors10described above in the Background section.

As is conventional, the capacitor structures disclosed herein can be incorporated into a sense element chip. While not shown, a device incorporating the sense element chip may include an interface electronics chip, a substrate, a ceramic chip carrier, a cover enclosing these structures or other structures as will be familiar to those in the art of capacitive motion detection.