WO2013026899A1

WO2013026899A1 - Replica temperature sensing for oven-controlled mems device

Info

Publication number: WO2013026899A1
Application number: PCT/EP2012/066400
Authority: WO
Inventors: Jonathan Borremans
Original assignee: Imec
Priority date: 2011-08-23
Filing date: 2012-08-23
Publication date: 2013-02-28

Abstract

System and method for controlling the temperature of a MEMS device, using a first ovenized system, at least one second ovenized system and a temperature control loop. The first ovenized system comprises an ovenized MEMS device, a first heat source and first means for adjusting the heating power. Each second ovenized system comprises an ovenized replica MEMS device, a second heat source, second means for adjusting the heating power and temperature sensing means. The temperature control loop adjusts the heating power of the first and second heat sources by generating a control signal in response to a signal received from the temperature sensing means.

Description

Replica temperature sensing for oven-controlled MEMS device

TECHNICAL FIELD

The present disclosure relates generally to a system and method for stabilizing the temperature of micro-electromechanical devices.

BACKGROUND ART

It is well known in the art that micro-electromechanical (MEM or MEMS) devices can be used to build oscillators, due to the high Q factor, and can serve as frequency reference devices, as illustrated in Figure 1. Commonly, most applications employ quartz crystal oscillators as frequency reference devices because of their better temperature stability. However, such quartz based oscillators suffer from low level of integration due to the large area overhead occupied, the heterogeneous components and the packaging requirements. In comparison, MEMS resonators have a more compact design in IC-compatible technology, which allows ease of integration and thus significantly lowers the overall cost.

One of the main issues with MEMS oscillators/resonators is that they exhibit a high sensitivity of frequency drift over temperature (e.g. +-5000ppm over 100°C), and are thus less stable, as compared to a quartz crystal (e.g. +-1 ppm over 100°C), as illustrated in Figure 2. Hence, they need to be stabilized over the temperature. One way of achieving this is through an oven-controlled setup, whereby the MEMS device is heated up. The temperature of the device is monitored with a temperature sensing device, and kept fixed within a range. Hence the temperature of the MEMS device is stable, and its parameters are fixed. The concept of the oven-controlled set-up is illustrated in Figure 3. In order to measure the temperature of the MEMS device accurately a temperature sensing device is required to be integrated having a good thermal contact to the MEMS device itself.

One possible implementation can be realised by placing a (temperature dependent) resistor on top of the MEMS device. The resistor's value is then representative for the actual MEMS device temperature. Such an example implementation is illustrated in Figure 4, in which the MEMS device is attached to the ambient through isolating springs, and heated up through Joule heating by sending a current (i) through the springs (legs). A resistor located on top of the MEMS resonator is used to sense the temperature of the resonating device. Unfortunately, the placement of the resistor on top of the device can greatly affect or even deteriorate the MEMS device performance. In general, placing a temperature sensing device in close thermal proximity of a mechanical device may influence its parameters by lowering its Q factor. In addition, this implementation may require the springs (legs) of the MEMS devices to be wider than mechanically ideal for facilitating the minimum resistor width and spacing.

SUMMARY OF THE DISCLOSURE

It is an aim of the present disclosure to provide a system and method for stabilizing the temperature of micro-electromechanical devices with which impact on the device performance of the micro-electromechanical devices can be reduced.

This aim is achievable with the subject-matter of the independent claims. This disclosure thus provides a system for controlling the temperature of a MEMS device, comprising a first ovenized system, at least one second ovenized system and a temperature control loop,

the first ovenized system comprising an ovenized MEMS device, a first heat source arranged for heating up the MEMS device to a predetermined temperature and first means for adjusting the heating power of the first heat source according to a control signal;

the at least one second ovenized system each comprising an ovenized replica MEMS device (herein also called "replica semiconductor device" or "replica sensing device") being substantially a replica of said MEMS device, a second heat source arranged for heating up the replica MEMS device to said predetermined temperature, second means for adjusting the heating power of the second heat source according to a control signal and temperature sensing means arranged in close proximity to the replica MEMS device for sensing an electric property indicative of the temperature of the replica MEMS device;

the temperature control loop being connected to said temperature sensing means and to said first and second means for adjusting the heating power of respectively the first and second heat source and comprising a control signal generator arranged for generating said control signal in response to a signal received from said temperature sensing means, such that the temperature of the MEMS device and each replica MEMS device can be kept substantially at said predetermined temperature.

With the system of this disclosure, direct temperature sensing of the functional MEMS device (that of the first ovenized system) can be omitted. The temperature can be sensed at one or more replica MEMS devices which are heated up to the same predetermined temperature. Hence, the need to provide temperature sensing means or structures at or on top of the functional MEMS device can be avoided and, as a result, any impact on the device performance that the provision of such means or structures may have.

Since the temperature sensing can be performed by means of replica MEMS devices which substantially or exactly replicate the functional MEMS device, the temperature behaviour of the functional MEMS device can be replicated and accurate temperature sensing can be achieved. This disclosure thus in fact also relates to the use of such replica MEMS devices as temperature sensors for a MEMS device.

In embodiments according to the disclosure, the number of second ovenized systems may be greater than one, each second ovenized system being arranged in close proximity to the first ovenized system. A plurality of second ovenized systems may increase the accuracy of the sensing. In such embodiments, the first ovenized system may be arranged in the centre and the second ovenized systems may be arranged around the first ovenized system. In such embodiments where the number of second ovenized systems is greater than one, the control signal may be generated by averaging the temperature sensing readings of all second ovenized systems measured. The heating power of the first ovenized system may be adjusted accordingly to match the average heating power of the second ovenized systems. Other mathematical algorithms (other than averaging) may also be used to define the heating control signal.

In embodiments according to the disclosure, the first and second ovenized systems may be arranged in a single package or on the same substrate or die.

In embodiments according to the disclosure, each replica MEMS device may be an exact replica of the MEMS device with the exception of the temperature sensing means which may be processed on top of the replica MEMS device.

In embodiments according to the disclosure, the temperature sensing means of each second ovenized system are formed by resistors or capacitors processed on top of the replica MEMS device, or at least in good thermal contact therewith.

In embodiments according to the disclosure, the MEMS and replica MEMS devices may be suspended in vacuum by means of support legs or springs. These legs or springs may be optimized for thermal insulation towards the substrate above which they are suspended. In embodiments according to the disclosure, the support legs or springs may be used as said heat sources by means of Joule heating. In such embodiments, the means for adjusting the heating power may for example be formed by controllable voltage or current supplies, responsive to said control signal, and arranged for supplying a variable voltage over or current through these legs or springs.

In embodiments according to the disclosure, the MEMS device of the first ovenized system is a resonator, such as for example a Bulk Acoustic Wave (BAW) bar resonator. The disclosure is however also applicable to other types of MEMS devices, such as for example filters.

This disclosure further provides a system for controlling the temperature of a MEMS device, comprising the steps of:

heating up an ovenized MEMS device of a first ovenized system to a predetermined temperature by means of a first heat source arranged for heating up the MEMS device;

heating up at least one ovenized replica MEMS device of at least one second ovenized system to said predetermined temperature by means of each time a second heat source arranged for heating up the replica MEMS device, each ovenized replica MEMS device being substantially a replica of said MEMS device; sensing an electric property indicative of the temperature of each replica

MEMS device by means of each time a temperature sensing means arranged in close proximity to the replica MEMS device;

generating, in a temperature control loop, a control signal in response to a signal received from said temperature sensing means; and

providing said control signal to first and second means for adjusting the heating power of respectively the first and second heat source, thereby adjusting the heating power of the first and second heat sources, such that the temperature of the MEMS device and each replica MEMS device can be kept substantially at said predetermined temperature.

Advantages and embodiments described above for the system according to this disclosure are also applicable for the method according to this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will be further elucidated by means of the following description and the appended figures.

Figure 1 shows, in general, a MEMS resonator applied in an oscillator and filter configuration. Figure 2 shows, in general, a plot of the temperature drift of a quartz and MEMS resonator.

Figure 3 illustrates, in general, a Micro-oven stabilized resonator system. Figure 4 illustrates a MEMS device with resistor on top to sense temperature, suitable for use as a replica MEMS device according to this disclosure.

Figure 5 illustrates an embodiment according to this disclosure, wherein replica temperature sensors are placed around a resonator.

Figures 6 and 7 depict embodiments of methods of measuring and controlling the temperature of an ovenized MEMS device, according to embodiments of this disclosure.

Figure 8 illustrates a MEMS device and a replica MEMS device according to one of the embodiments of this disclosure.

Figure 9 illustrates another embodiment of a replica temperature sensor. Figure 10 illustrates another embodiment of a replica temperature sensor.

MODES FOR CARRYING OUT THE DISCLOSURE

The present disclosure will be described with respect to particular embodiments and with reference to certain drawings but the disclosure is not limited thereto. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not necessarily correspond to actual reductions to practice of the disclosure.

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. The terms are interchangeable under appropriate circumstances and the embodiments of the disclosure can operate in other sequences than described or illustrated herein.

Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. The terms so used are interchangeable under appropriate circumstances and the embodiments of the disclosure described herein can operate in other orientations than described or illustrated herein.

The term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It needs to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression "a device comprising means A and B" should not be limited to devices consisting of only components A and B. It means that with respect to the present disclosure, the only relevant components of the device are A and B.

The present disclosure relates to a system and a method for measuring and controlling the temperature of a micro-electromechanical device with which impact on its performance can be reduced or avoided. This is realized by including preferably a plurality of temperature sensors - replica MEMS devices - around a central MEMS device.

According to one embodiment of the present disclosure, a first ovenized system and at least one second ovenized system are provided. The first ovenized system may comprise:

- an oven,

- a MEMS device inside the oven,

- a heat source for heating up the MEMs device to a predetermined temperature, and

means for adjusting the heating power according to a control signal.

The at least one second ovenized system may comprise:

- an oven,

- a replica semiconductor device inside the oven,

- a heat source for heating up the replica semiconductor device to a predetermined temperature,

means for temperature sensing, and

- a temperature control loop arranged for sensing the temperature and generating a control signal for steering the heating power such that the temperature of the first ovenized system and the at least second ovenized system is kept at a fixed or desired predetermined value, regardless of the ambient temperature.

All elements may be present in close thermal proximity, in a single package or susceptible to a closely related ambient temperature.

The MEMS device incorporated in the first ovenized system can be a resonator of any type, for example a Bulk Acoustic Wave (BAW) bar resonator, processed in SiGe or SOI technology, suspended in vacuum by means of support legs or springs. The disclosure is not limited to MEMS resonators. The MEMS device can also be any other kind of micro-electro mechanical system, including filters.

The at least one second ovenized system may operate as a temperature sensing system and may be optimized for thermal performance. However, other operational modes, such as mechanical, are not excluded.

In some embodiments, the number of second ovenized systems employed is greater than one and they may be arranged in close proximity and around the first ovenized system, to ensure good thermal conductivity.

The replica semiconductor device incorporated in the at least one second ovenized system can be a MEMS device similar and even exact to the one of the first ovenized system. However, any replica devices with similar and appropriate thermal characteristics can be considered. The temperature sensing means of the second ovenized system may be realized by the use of resistors or capacitors processed on top of the semiconductor device.

In some embodiments, the first ovenized system and the at least one second ovenized system can be placed on the same package to achieve close thermal proximity.

According to another aspect of the present disclosure, a method is provided. The method may comprise:

- measuring and stabilizing the temperature of the at least one second ovenized system with a temperature control loop,

providing a control signal generated by the temperature control loop to the first ovenized system's heating control system, and

- adjusting the heating power of the first ovenized system device to that of the at least one second ovenized system.

In embodiments where the number of second ovenized systems is greater than one, the control signal may be generated by averaging the temperature sensing readings of all second ovenized systems measured. The heating power of the first ovenized system is adjusted accordingly to match the average heating power of the second ovenized systems. Other mathematical algorithms (other than averaging) may be used to define the heating control signal.

An example embodiment of the disclosed system is depicted in Figures 4 and 5. In this embodiment, a central MEMS device 10, e.g. a resonator, is used for its mechanical properties. According to the state-of-art, such a device needs to be ovenized to keep it at a predetermined temperature, preferably well above ambient. As shown in Figure 5, the MEMS device (resonator) is located in 'the center' of a plurality of replica sensing devices 20. The term 'center' is used to indicate that the device is placed in a local average or with known or deterministic thermal relation to the replica devices. In this case, the replica sensing devices are used to measure the temperature. The 'center' device 10 is used as a resonator and thus good mechanical properties are imperative. In this example, the resonator can be used in an oscillator setup, though the present disclosure extends beyond the scope of this example. The MEMS device (resonator) 10 and the replicas 20 operate in vacuum and may be well thermally isolated from the ambient temperature, except through mechanical support legs (or springs) 21 , which are connected to the ambient world. The devices can be heated up through Joule heating by sending current through these support legs 21 . Other heating means can also be considered. In one embodiment, the MEMS device 10 is a Bulk Acoustic Wave (BAW) bar resonator processed in SiGe or SOI technology, suspended in vacuum by means of support legs or springs. The replicas 20 may have actuation electrodes 23 like the resonator 10, but only for the purpose of replicating the structure.

To overcome the problems of the temperature sensing affecting the performance of the MEMS device, as identified in the prior art, the temperature sensing means may be placed on a set of N (N>=1 ) replica sensing devices 20 surrounding the functional MEMS device 10. The term replica sensing in this case may be interpreted to mean that the temperature sensing devices are exact replicas of the MEMS device (resonator) placed in the centre with the exception of the means 22 for sensing the temperature. As such these replica sensing devices may also be each time a MEMS device, which is not used for its mechanical properties, though it is not excluded. The replica devices can also be any other type of semiconductor devices with similar or identical thermal characteristics as the MEMS device (resonator) placed in the centre. These replica sensing devices are heated up to keep them individually at a fixed, predetermined temperature.

In one embodiment, the means for sensing the temperature, as incorporated in the replica sensing devices, is a resistor 22 processed on top of a semiconductor device, for example a MEMS device. In another embodiment, the means for sensing the temperature is a capacitor processed on top of a semiconductor device, for example a MEMS device.

In an embodiment, the means for sensing the temperature is preferably fabricated in an Al, Cu, Si, Pt or W. Other types of material that meet the requirements for temperature sensing and are compatible with standard fabrication steps can also be used. It is desirable that the temperature sensing devices make good thermal contact with the MEMS device. It is sometimes desirable for the temperature sensing devices to make electrical contact with the MEMS device.

A heating steering control signal for adjusting the heating power of the MEMS device is generated by a temperature control system. The heating steering control signal relates to the individual steering signals of the replica sensing devices. The replica devices in this case are assumed not to be used as resonating MEMS device but for temperature sensing, thus their mechanical performance is of lesser concern.

Assuming that all devices (MEMS devices and replica sensing devices) are subject to the same ambient temperature, and that they are steered to keep each individual device at a fixed predetermined temperature, the central MEMS device (resonator) temperature can also be kept at a fixed temperature. In this way the temperature of the central MEMS device is indirectly sensed, by measuring the temperature on the replica devices and adjusting the heating power accordingly. As a result there is no need to place the temperature sensing means on top of the central MEMS device, thus leaving its mechanical performance unaffected. For enhancing the accuracy of the temperature measurements, all devices can be placed in close thermal proximity, e.g. in the same package.

However, temperature gradients or fluctuations can exist, making each device subject to a slightly different ambient temperature, depending on the packaging and size of the devices (e.g. 0.1 °C difference). To address this issue, the measured temperature of all replica devices may be averaged over a period of time. The averaged temperature more accurately indicates the temperature of the central MEMS device.

Since the central MEMS device is the only one to be used mechanically, all other temperature sensing devices can be optimized for thermal performance. Thus they can be somewhat different in respect to their design. It is crucial, however, to heat up all devices with a heating power related to (e.g. proportional to) the same control signal.

Methods of measuring and controlling the temperature are depicted in

Figures 6 and 7. The MEMS device 1 , 1 ' needs to be stabilized at a predetermined temperature. Its temperature can be set by a heat control signal 8, 8', which is generated as follows; the temperature measured (or electric property indicative thereof) 4, 4' by each replica sensing device 2, 2' is continuously monitored and used in a temperature control loop to keep the temperature of the device fixed to a predetermined value by means of a heating control signal 5, 5'. A plurality of sensors can be used on each replica device to sense the individual temperatures. A plurality of replica devices (N>=1 ) can also be used. A control system, which may be distributed 3, 7 as in figure 6 or centralized T as in figure 7, is arranged for processing the heat control signals 5, 5' provided by the replica sensing devices and synthesize accordingly a heat control signal 8, 8' that sets the temperature of the MEMS device 1 , X without requiring sensing its temperature. In one embodiment, the control system applies an averaging function. The temperature control loop can in general contain any element or mechanism needed to effectively operate the device or tune the output signal as desired.

In an embodiment, two resistors 31 , 32 of different material may be processed on top of the replica MEMS device 30, as illustrated in Figure 8. The device on the left shows a MEMS resonator 10 without temperature sensors. The device on the right shows the preferred embodiment, whereby two sensing elements, in this case resistors 31 and 32 with TCRi (thermal coefficient of resistance for 31 ) and TCR₂ (thermal coefficient of resistance for 32), are processed on top of the resonator replica 30. This embodiment can use the ratiometric principle to minimize the frequency drift of the system. In this case the ratiometric principle can be provided in that both sensing elements have different temperature dependent characteristics such that when measured the measurement curves intersect in a predefined intersection point corresponding to the predefined set temperature T_set. In this way, it is possible to easily determine if the actual temperature of the oscillator is above or below or equal to predetermined temperature T_set, and to control the loop correspondingly.

In some embodiments, the first sensing element may be sensed with a first sensing signal, and the second sensing element may be sensed with a second sensing signal. The first and second sensing signals may be substantially identical. For example, the first and second sensing signals may have substantially the same amplitude, but not necessarily the same sign or phase or duration.

According to another aspect, a temperature-controlled timing oscillator is also provided.

In another embodiment, illustrated in Figure 9, the replica device 40 is optimized for thermal aspects, and less for mechanical factors. A top view in Figure 9 depicts a center square bar 41 , suspended in vacuum through long springs on the side 42. The springs provide thermal isolation and allow for low- power Joule heating of the bar by sending current from one side to the other, as illustrated in Figure 9. Figure 9 illustrates the current flow for heating in the embodiment of Figure 8, through the springs 42 and the isolated beam 41 . In this embodiment, there are likewise two resistors 43, 44 with different temperature dependencies processed on top of the replica device 40 to obtain the possibility of ratiometric control.

Claims

1 . System for controlling the temperature of a MEMS device, comprising a first ovenized system, at least one second ovenized system and a temperature control loop,

the at least one second ovenized system each comprising an ovenized replica MEMS device being substantially a replica of said MEMS device, a second heat source arranged for heating up the replica MEMS device to said predetermined temperature, second means for adjusting the heating power of the second heat source according to a control signal and temperature sensing means arranged in close proximity to the replica MEMS device for sensing an electric property indicative of the temperature of the replica MEMS device;

2. System for controlling the temperature of a MEMS device according to claim 1 , wherein the number of second ovenized systems is greater than one, each second ovenized system being arranged in close proximity to the first ovenized system.

3. System for controlling the temperature of a MEMS device according to claim 2, wherein the first ovenized system is arranged in the centre and the second ovenized systems are arranged around the first ovenized system.

4. System for controlling the temperature of a MEMS device according to any one of the preceding claims, wherein the first and second ovenized systems are arranged in a single package.

5. System for controlling the temperature of a MEMS device according to any one of the preceding claims, wherein each replica MEMS device is an exact replica of the MEMS device with the exception of the temperature sensing means which are processed on top of the replica MEMS device.

6. System for controlling the temperature of a MEMS device according to any one of the preceding claims, wherein the temperature sensing means of each second ovenized system are formed by resistors or capacitors processed on top of the replica MEMS device.

7. System for controlling the temperature of a MEMS device according to any one of the preceding claims, wherein the MEMS and replica MEMS devices are suspended in vacuum by means of support legs or springs.

8. System for controlling the temperature of a MEMS device according to claim 7, wherein the support legs or springs are used as said heat sources by means of Joule heating.

9. System for controlling the temperature of a MEMS device according to any one of the preceding claims, wherein the MEMS device of the first ovenized system is a resonator.

10. System for controlling the temperature of a MEMS device according to claim 9, wherein the resonator is a Bulk Acoustic Wave (BAW) bar resonator.

1 1 . Method for controlling the temperature of a MEMS device comprising the steps of:

12. Method for controlling the temperature of a MEMS device according to claim 1 1 , wherein the number of second ovenized systems is greater than one, each second ovenized system being arranged in close proximity to the first ovenized system.

13. Method for controlling the temperature of a MEMS device according to claim 12, wherein the first ovenized system is arranged in the centre and the second ovenized systems are arranged around the first ovenized system.

14. Method for controlling the temperature of a MEMS device according to any one of the claims 1 1 -13, wherein the first and second ovenized systems are arranged in a single package.

15. Method for controlling the temperature of a MEMS device according to any one of the claims 1 1 -14, wherein each replica MEMS device is an exact replica of the MEMS device with the exception of the temperature sensing means which are processed on top of the replica MEMS device.

16. Method for controlling the temperature of a MEMS device according to any one of the claims 1 1 -15, wherein the temperature sensing means of each second ovenized system are formed by resistors or capacitors processed on top of the replica MEMS device.

17. Method for controlling the temperature of a MEMS device according to any one of the claims 1 1 -16, wherein the MEMS and replica MEMS devices are suspended in vacuum by means of support legs or springs.

18. Method for controlling the temperature of a MEMS device according to claim 17, wherein the support legs or springs are used as said heat sources by means of Joule heating.

19. Method for controlling the temperature of a MEMS device according to any one of the claims 1 1 -18, wherein the MEMS device of the first ovenized system is a resonator.

20. Method for controlling the temperature of a MEMS device according to claim 19, wherein the resonator is a Bulk Acoustic Wave (BAW) bar resonator.