GB2385946A

GB2385946A - Micro electro mechanical system

Info

Publication number: GB2385946A
Application number: GB0204701A
Authority: GB
Inventors: Jan Szczepan Przyblyla
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2002-02-28
Filing date: 2002-02-28
Publication date: 2003-09-03
Also published as: GB0204701D0

Abstract

A micro electro mechanical system (MEMS) includes a positional drive with a control system which is dependent on the electrical resonance of a resonance circuit and which incorporates a capacitance formed by at least two components of the MEMS. The MEMS may include a silicon substrate layer 401, a bonded silicon on insulator layer 402 and a mirror layer 403 elastically suspended by insulating layers of silicon dioxide 404. Polysilicon comb fingers (or electrode fingers) 406 may be movable in response to an applied electric field, and movement of the fingers varies the capacitance between them. The system may also include a circuit having a rectifier to provide a drive voltage for the MEMS with a ripple voltage to excite the resonant circuit. A signal processing circuit may extract the ripple voltage and provide an error signal dependent on the position of a moveable component of the MEMS which is then multiplied by an RF voltage to provide a drive voltage for the moveable component.

Description

Micro Electro Mechanical System The invention relates to a micro electro mechanical system (MEMS) and a method of precision positioning of a component in such a system.

The use of MEMS is well-established in a wide variety of applications in electronics and optoelectronics. One common approach in the design of drive mechanisms for MEMS is to use a comb structure. In this approach, the comb structure is moved electro-statically, which movement changes the capacitance of the comb structure. This change in capacitance is measurable and correlates directly to the movement of the structure. Using an appropriate feedback loop permits the change in capacitance as an aid in the precision positioning of the structure.

US5291335 discloses a compact and high precision lens position detector, which detects the position of the lens holder by a change in electrical capacitance. The lens holder is driven by either an electromagnetic motor or using a solid state device such as a piezo-electric element. The control mechanism applied to the lens movement consists of detecting changes in the capacitance between a conducting guide bar supporting the lens holder and an electrode using known AC methods such as a bridge method or a resonant method. In the resonant method, a resonant circuit is formed by said capacitance and a known inductance. The capacitance is determined from the resonance frequency in the known manner using the standard equation f=1I (2nv'LC).

This approach together with other known approaches suffers from the problem that the size of the movements is relatively small and the structures are also small. Consequently, the absolute values of the capacitance are also small and the change of the capacitance is very small. In an exemplary application, where a MEMS is used to drive a tuneable mirror, the positioning of the mirror needs to be accurate to within 1nm. The capacitance of a comb structure used to drive the mirror will typically be of the order of0. 75pF. A Inm change in position of the comb structure will change the capacitance by 0.02fF. The measurement of such small changes in capacitance is challenging and normally requires sophisticated and hence expensive equipment.

US5291335 is silent on the precise techniques used to measure such small changes in capacitance.

The present invention seeks to provide a method of measuring small capacitances in a device such as a MEMS and a MEMS adapted to use such a method.

According to the invention, there is provided a micro-electro-mechanical system (MEMS) comprising a positional drive adapted to precision position a component of the MEMS, wherein the positional drive is provided with a control system dependent on the electrical resonance of a resonance circuit incorporating a capacitance formed by at least two components of the MEMS.

In a preferred embodiment, the system comprises a further circuit having a rectifier, which rectifier is adapted to provide a drive voltage for the MEMS, which drive voltage has a ripple voltage, which ripple voltage excites the resonance circuit, changes in the ripple voltage being detectable by the control system.

Preferably, the resonant frequency of the circuit is close to the ripple frequency, thereby providing gain at the ripple frequency. Preferably, the ripple frequency is sufficiently greater than the mechanical resonant frequency of MEMS that the ripple voltage does not substantially positionally affect the MEMS.

Preferably, the control system further comprises a signal processing circuit adapted to extract the ripple voltage, rectify and smooth the ripple voltage, which ripple voltage is then added to the position set voltage, thereby providing an error signal dependent on the position of the MEMS component. Preferably, the error signal is multiplied by an RF voltage, thereby providing a drive voltage for the positionable MEMS component. Preferably, the resonant circuit is provided with an LC filter adapted to smooth the drive voltage for the MEMS, the resonant frequency of which LC filter is determined by the mass and stiffness of the MEMS components. Preferably, the capacitance comprises a comb structure.

An exemplary embodiment of the invention will now be described with reference to the drawings in which:

Fig. 1 shows an overall schematic of the system; Fig. 2 shows a schematic of the comb drive; Fig. 3 shows the variation of comb capacitance and error ripple voltage with comb position; Fig. 4 shows a schematic of the MEMS; Fig. 5 shows an alternative waveform generator.

In an exemplary embodiment, the invention relates to a precision control system for a MEMS tuneable optical filter, which filter comprises a positionable mirror. The control system utilises a low voltage high frequency drive (e. g. 12V, 2MHz) to excite a resonant tank circuit, the voltage across which when rectified supplies the required dc voltage to drive the mirror membrane. A schematic is shown in Figure 1.

The system comprises an external drive having a series resonant circuit (L2C2), the voltage across which is rectified using a full wave bridge rectifier to generate the high voltage drive voltage. The mirror system itself comprises a plurality of components, two of which form a capacitance Cl, which together with an external resonance LI forms a resonant circuit. By appropriately selecting the resonance frequency of this circuit so that it is excited by the ripple on the rectified drive voltage, the mirror position is proportional to the ripple frequency amplitude. The ripple voltage can then be processed and amplified. This processed ripple voltage is then added to a position set voltage, which is equal to the voltage of the low voltage high frequency drive, and this signal is compared to the original to generate an error signal. This error signal can then be used as a control for the mirror drive.

The rectified dc drive voltage has superimposed upon it an ac ripple voltage, which in the case of a full wave rectifier, will have a ripple frequency twice that of the input signal from the power supply unit. Generally, smoothing circuits are used to smooth ripple as it is generally undesirable. However, in the invention the resulting ripple voltage is used to excite a second resonant circuit of which the mirror forms part.

The principle of operation is based upon the concept of using the capacitance of the mirror structure together with an external inductor to form a high Q resonant circuit.

The circuit is driven by an external voltage source having a resonant frequency close to the resonant frequency of the mirror structure resonant circuit. If the operating point is correctly chosen, then this has the effect that a small change in capacitance due to movement of the mirror results in a magnification of the high frequency drive voltage. The use of a secondary resonant circuit produces the dc voltage required to move the mirror. This latter voltage can be comparatively high and will typically be of the order of 60 to 100V. As part of the operation the second resonant circuit generates the high frequency required to excite the mirror structure.

The circuit is arranged such that gain is obtained at the ripple frequency by ensuring that the ripple frequency and resonant frequency are substantially similar. Thus small changes in capacitance result in measurable voltages.

Figure 2 shows a schematic circuit of the electrostatic drive for the comb structure.

The circuit comprises a series resonant circuit formed by the inductor L2 and capacitors C3 and C4. The load of this circuit is the transformer X4x, the bridge rectifier D1-D8 and the LC filter L3 and C9.

When the series resonant circuit is excited by a voltage of 12V at a frequency of 2MHz, a voltage of 96V is produced by the resonant circuit at the secondary circuit of the transformer. This is rectified and smoothed by the bridge to produce a DC voltage of 61 V, which can be used to drive the mirror. As the voltage is rectified by a full bridge, a ripple voltage will be present on the DC voltage. The ripple voltage will have a frequency twice that of the drive frequency, i. e. 4MHz. This is sufficiently high that it will not affect the mechanical resonance of the MEMS. The ac component of the ripple voltage is then extracted and amplified.

Figure 3 shows the basic schematics of the mirror resonant circuit. The resultant amplified ripple voltage is then rectified by a precision full bridge rectifier, smoothed by an LC filter (L2, C91), the characteristics of which filter are determined by the mechanical properties of the MEMS.

The resultant DC voltage (rip~dc~2) is then added to a Position Set voltage by an error amplifier, thereby resulting in an error signal or voltage dependent on the mirror

position. The resultant error voltage is then multiplied by the RF voltage to provide a low voltage drive to the comb power supply unit. The amplitude of the RF voltage drive at 2MHz is now a function of the error signal, hence closing the loop of the circuit.

It should be noted that to simulate the closed loop control, the movement of the mirror must be allowed for by converting the stiffness and mass of the mirror into their electrical equivalents. This results in an LC filter with a resonant frequency set by the mass and stiffness. This filter is then used to smooth the applied voltage to the mirror so that the resultant voltage is proportional to the mirror comb position with the added benefit of mechanical damping.

Fig. 4 shows a schematic cross-section of the MEMS, which comprises a silicon substrate layer 401 having a bonded silicon on insulator layer (BSOI) 402. The mirror layer 403 is elastically suspended above the BSOI layer 402 by insulating layers of silicon dioxide 404. The mirror layer comprises a concentrically located moveable mirror 405 at the centre of the layer. A recticular structure surrounds the mirror 405 and comprises a plurality of polysilicon comb fingers 406, which fingers are inserted into trenches formed in the bonded silicon layer, thereby acting as vertically oriented comb electrode fingers. These electrode fingers 406 are moveable in response to an applied electric field, which movement will vary the capacitance between the electrode fingers. Located intermediate to each electrode finger is a fixed electrode 407. Electrical contacts are provided on the mirror layer and the bonded layer.

In use the device is actuable by an applied voltage, which is applied between the suspended polysilicon electrode and the BSOI layer. By subdividing the respective layers into co-terminous sectors a linear actuation force may be applied to each sector allowing the vertical orientation of the mirror to be adjusted. Alternative methods of adjusting the vertical orientation are of course possible, such as applying torques about orthogonal tilt axes.

In a typical optoelectronic application such as an optical filter, it is necessary to move the comb structure by up to 3 urn to within an accuracy of lnm. In view of the physical dimensions of such a device, the capacitance between comb fingers is of the

order of IpF and hence in view of the stiffness of polysilicon, drive voltages of the order of 60V are required.

Fig. 5 shows an alternative drive concept in which the original dual resonant drive concept is replaced by a simple switch-mode system. Typically in this embodiment a 2V peak-to-peak 100kHz square wave is generated, amplified and stepped up by a transformer. Utilising a full bridge rectifier, a dc voltage up to 160 volts can be generated. The resultant HF ripple is smoothed by an LC filter. The required RF signal to excite the MEMs resonant circuit is coupled into the DC power supply by the capacitor C2 and is isolated from the high voltage by the diode D14. The benefits of this system are such that the RF injected voltage can be adjusted in amplitude and frequency as necessary and the injected ripple remains constant independent of dc drive voltage. The detector and servo system will be as described above with the addition of a pass filter to extract the RF ripple from the general power supply unit ripple.

Although the invention has been particularly described in relation to a tuneable filter, it could be used in other tuneable MEMS or MOEMS such as a variable optical attenuator.

Claims

1. A micro-electro-mechanical system (MEMS) comprising a positional drive adapted to precision position a component of the MEMS, wherein the positional drive is provided with a control system dependent on the electrical resonance of a resonance circuit incorporating a capacitance formed by at least two components of the MEMS.

2. A micro-electro-mechanical system according to Claim 1, wherein the system comprises a further circuit having a rectifier, which rectifier is adapted to provide a drive voltage for the MEMS, which drive voltage has a ripple voltage, which ripple voltage excites the resonance circuit, changes in the ripple voltage being detectable by the control system.

3. A micro-electro-mechanical system according to Claim 1 or Claim 2, wherein the resonant frequency of the circuit is close to the ripple frequency, thereby providing gain at the ripple frequency.

4. A micro-electro-mechanical system according to Claim 3, wherein the ripple frequency is sufficiently greater than the mechanical resonant frequency of the MEMS that the ripple voltage does not substantially positionally affect the MEMS.

5. A micro-electro-mechanical system according to any one of Claims 1 to 4, wherein the control system further comprises a signal processing circuit adapted to extract the ripple voltage, rectify and smooth the ripple voltage, which ripple voltage is then added to the position set voltage, thereby providing an error signal dependent on position of the MEMS component.

6. A micro-electro-mechanical system according to any one of Claims 1 to 5, wherein the error signal is multiplied by an RF voltage, thereby providing a drive voltage for the positionable MEMS component.

7. A micro-electro-mechanical system according to any one of Claims I to 6, wherein the resonant circuit is provided with an LC filter adapted to smooth the drive

voltage for the MEMS, the resonant frequency of which LC filter is determined by the mass and stiffness of the MEMS components.

8. A micro-electro-mechanical system according to any one of Claims 1 to 7, wherein the capacitance comprises a comb structure.

9. A micro-electro-mechanical system substantially as described herein with reference to the accompanying drawings.