WO2009133506A2

WO2009133506A2 - Mems device and manufacturing method

Info

Publication number: WO2009133506A2
Application number: PCT/IB2009/051685
Authority: WO
Inventors: Petrus H. C. Magnee
Original assignee: Nxp B.V.
Priority date: 2008-04-29
Filing date: 2009-04-24
Publication date: 2009-11-05
Also published as: WO2009133506A3

Abstract

A method of forming a MEMS device, in which a MEMS device portion (20) is formed as a patterned portion of the silicon layer (14) of an SOI substrate (10,12,14). The MEMS device portion (20) is covered with a sacrificial layer (22) and a cap layer (26), so that further processing above or around the MEMS device can be implemented using standard CMOS processing, to form integrated circuit components (30). The MEMS device is completed by forming an opening (33) in the base layer (10) of the SOI substrate beneath the MEMS device portion and using the opening to remove the insulator layer of the SOI substrate and the sacrificial layer, thereby providing a space above and below the MEMS device portion. The opening in the base layer is closed with a further substrate (34).

Description

DESCRIPTION

MEMS DEVICE AND MANUFACTURING METHOD

The invention relates to Micro Electrical-Mechanical Systems (MEMS) devices.

In particular, the invention relates to MEMS devices having designs which enable integration into integrated circuits formed using IC processes. The basic requirement of MEMS devices are typically that a cavity needs to be formed that is hermetically sealed, for example under a vacuum. The cavity needs to be small, for example a wafer-level chip-scale package (WL-CSP), and needs to be strong. The strength is required so that cavity will not collapse due to moulding pressures experienced during the formation of plastic packages.

There have been different approaches to the integration of MEMS devices with standard IC processes, such as CMOS processes.

Typically, either very thick layers are used to provide the required strength of the cavity, or a separate cap is placed at the end of the process. The use of thick layers gives rise to large topography, which makes integration with standard IC processes very difficult. A separate cap requires additional space around the MEMS device.

The integration of MEMS devices onto Silicon-on-lnsulator (SOI) substrates has been considered, as this enables the integration of high voltage components with the MEMS devices. This invention relates to a design based on such SOI technology.

According to the invention, there is provided a method of forming a MEMS device, comprising: providing a silicon-on-insulator substrate comprising a base layer, an insulator layer and a silicon layer; defining a MEMS device portion as a patterned portion of the silicon layer of the substrate; covering the MEMS device portion with a sacrificial layer and a cap layer; forming an opening in the base layer beneath the MEMS device portion; using the opening in the base layer to remove the insulator layer of the substrate and the sacrificial layer, thereby providing a space above and below the MEMS device portion; and covering the opening in the base layer with a second base layer, wherein the method further comprises forming integrated circuit components using parts of the silicon layer not used for the MEMS device.

This method uses a silicon on insulator process. This is very well suited to make thin-SOI MEMS devices, but also enables such devices to be combined with high-voltage electronics. The method provides the MEMS device with a cap so that conventional

IC processing can be implemented around the MEMS device so that full integration can be achieved. The removal of the sacrificial layer is performed from the other side of the substrate, so that the sacrificial layer can remain in place while the IC processing is conducted. This prevents damage to the MEMS device, and the processing of the MEMS device can be completed near the end of the manufacturing steps.

Defining the MEMS device portion preferably comprises etching trenches in the silicon layer down to the insulator layer, to define a MEMS portion and anchor portions of the silicon layer. Covering the MEMS device portion preferably comprises filling the trenches with the sacrificial layer. This provides the support for the MEMS device portion during subsequent processing steps, until the sacrificial layer is removed. The sacrificial layer can comprise silicon oxide, and this does not impose any constraints on the processing to be used, as this is a standard material used in IC processing, such as CMOS processing. Similarly, the cap layer can comprise a standard IC processing material, such as silicon nitride or silicon carbide. Covering the opening in the base layer can comprises vacuum sealing the space. Furthermore, the back of the substrate base layer can be thinned before forming the opening in the base layer. This is because the base layer height defines the depth of the cavity beneath the MEMS device portion (e.g. beam).

Removing the insulator layer of the substrate and the sacrificial layer is performed using an etchant which does not remove the cap layer, so that cap layer remains in the final structure.

The integrated circuit components can be formed after covering the MEMS device portion with the sacrificial layer and the cap layer, and before forming an opening in the base layer beneath the MEMS device portion. Thus, the sacrificial layer remains in place for the formation of the integrated circuit components. By encapsulating the MEMS device with rigid material, no special care is required for wafer handling during processing. Part of the processing of the integrated circuit components can be before defining the

MEMS device portion.

The invention also provides an integrated circuit comprising a MEMS device and other integrated circuit components, comprising: a silicon-on-insulator substrate comprising a base layer, an insulator layer and a silicon layer; a MEMS device portion comprising a patterned portion of the silicon layer of the substrate; a cap layer extending over the MEMS device portion; an opening in the base layer beneath the MEMS device portion; a second base layer which covers the opening in the base layer; a space above MEMS device portion which is closed by the cap layer, and a space below the MEMS device portion which is closed by the second base layer; and integrated circuit components defined using parts of the silicon layer not used for the MEMS device, and provided in layers over and/or around the cap layer. Examples of the invention will now be described in detail with reference to the accompanying drawings, in which:

Figures 1 to 16 show the manufacturing steps to produce a MEMS device of the invention; and Figure 17 shows one example of MEMS device of the invention in plan view.

The same reference numbers have been used in different figures to denote the same components and layers, and the description is not repeated. The invention provides a method of forming a MEMS device in which a

MEMS device portion is formed as a patterned portion of the silicon layer of an SOI substrate. The MEMS device portion is covered with a sacrificial layer and a cap layer, so that further processing above the MEMS device can be implemented using standard IC processing. The MEMS device is completed by forming an opening in the base layer of the SOI substrate beneath the MEMS device portion and using the opening to remove the insulator layer of the SOI substrate and the sacrificial layer, thereby providing a space above and below the MEMS device portion. The opening in the base layer is closed with a further substrate. Figures 1 to 16 show the manufacturing steps to produce a MEMS device of the invention.

Figure 1 shows the starting material, which is a standard thin-film SOI substrate, consisting of a mono-crystalline silicon substrate 10, a buried silicon oxide layer 12 (BOX, typically 1 μm thick) and a thin mono-crystalline silicon layer 14 (SOI, typically 1.5μm). The substrate thus comprises a base layer 10, an insulator 12 and a silicon top layer 14.

This substrate is the default for high-voltage processes such as described in the article "A-BCD: An economic 100 V RESURF silicon-on- insulator BCD technology for consumer and automotive applications", van der Pol, J.A. et al, Power Semiconductor Devices and ICs, 2000. Proceedings. The 12th International Symposium on 22-25 May 2000 Page(s):327 - 330 The SOI layer 14 is also ideal for good. quality MEMS structures, such as resonators. The material parameters are much better controlled than for instance when using poly-crystalline silicon (see for example the article

"Progress in Integrated Gyroscopes", John E Geen, IEEE A&E SYSTEMS MAGAZINE, NOVEMBER 2004, Page(s): 12 - 17.

As shown in Figures 2 to 4, the first step is to define the MEMS structure.

A photoresist layer 16 is deposited, and trenches 15 are formed as shown in Figure 2. These are used to etch trenches 17 (Figure 3) in the SOI layer 14 around the structure that later on in the process will be released. The trenches 17 are etched down to the BOX layer 14, thereby completely isolating the MEMS structure, except for some anchor points, which are not shown in the cross sections of Figures 2 to 4. The photoresist is removed to form the structure shown in Figure 4, which shows the MEMS device portion 20. Figure 5 shows the trenches filled with silicon oxide and the entire structure is then also covered with a silicon oxide layer 22. This enables further processing, without leaving the MEMS device portion 20 released and thus very fragile. The oxide layer 22 may be smoothed for example by a chemical-mechanical polishing (CMP) step, but this is not required. Figures 6 and 7 show the oxide layer 22 being removed from the area outside the MEMS device, using a photoresist layer 24 as shown in Figure 6, to leave a cover only over the MEMS device 20 as shown in Figure 7.

Figures 8 to 10 show the oxide cover being encapsulated in a different cap layer, which could be silicon nitride, silicon carbide, or any other layer that is resistant against an etch-step to remove the oxide layer 22 later on (which is can for example be a hydrofluoric acid (HF) based etch). In Figure 8, the cap layer 26 covers the device. A photoresist layer 28 shown in Figure 9 is used to remove the cap layer areas outside the area of the MEMS device, as shown in Figure 10. Figure 11 shows standard CMOS devices 30 which can be formed using conventional CMOS processes. Since the MEMS device 20 is still completely fixed, and not mechanically released, no special attention is required in wafer handling. The use of front-end compatible materials (such as silicon oxide, silicon nitride, and/or silicon carbide) also means there are no limitations required to the standard high temperature steps used, for example for implantation anneals, gate-oxide growth etc. After the standard CMOS processing has finished, the silicon substrate

(the base layer of the SOI substrate) is grinded down to a thickness preferably less than 100μm, more preferably less than 50μm. The thickness of the base substrate will define the cavity height beneath the MEMS device. The thinned substrate is shown as 10' in Figure 12, together with a photoresist layer 32. The photoresist layer 32 is used to remove the silicon base layer 10' from beneath the MEMS device to define an opening 33, up to the BOX layer 12, as shown in Figure 13.

The MEMS device is then released by removing the oxide 22 of the insulator layer 12 and of the sacrificial layer beneath the cap 26. This can be implemented by using e.g. a HF-based wet or vapour etch-technique. Because the trenches were etched down to the BOX layer 12, filled with oxide, and covered with an oxide block, the release can be done in one etch-step.

The cap layer 26 limits the oxide removal to just around the MEMS device, leaving a cavity around it, in particular above and below the MEMS device portion 20. The opening in the base layer 10' is used for this purpose.

Finally, as shown in Figure 15, the substrate 10',12,14 is bonded to a bare silicon wafer 34 (i.e. a second base layer), thereby closing the cavity. This bonding step can be done at different pressure levels or in a different gas environment, allowing the cavity to be filled with choice of gasses at a desired pressure.

Alternatively, a getter-material 35 can be placed on top of this second base layer 34, to maintain high vacuum levels for an extended lifetime, as shown in Figure 16.

The described sequence of steps is one preferred implementation. However, it may be altered for various reasons. The trench etch and oxide filling (Figures 2 to 5) can be moved to a later stage, for instance after the poly- silicon gates of the standard CMOS processes have been defined. The covering oxide layer 22 may be combined with a first planahzation step prior to the interconnect levels, etc.

Changing the order may require some fine-tuning in the exact processing steps, but this will be routine to those skilled in the art. The height of the cavity above the MEMS device is equal to the thickness of the oxide layer 22, and this is selected depending on the achievable strength and the maximum mechanical displacement of the MEMS device being encapsulated.

Depending on whether the required height fits below the first interconnect level of the CMOS process or not, the associated processing steps can be shifted to a later stage in the CMOS process.

The dimensions of MEMS device can be defined early on in the process, allowing certain regions to be properly doped to make good contacts, to function as connection leads. The cavity may still require extra material for strength, but this can be added completely at the end of the process, for example by using a thicker passivation stack, without incurring additional topography.

In the example above, the contacts to MEMS device are not shown. Figure 17 shows a plan view of one example of MEMS resonator configuration. Line X-X gives the location of the schematic cross sections of Figures 1 through 16.

In the example of Figure 17, the resonator body 20 is anchored at its four corners. Electrical connections are made to the resonator readout electrodes using standard interconnect metal lines 36. These metal lines contact regions of highly doped silicon 37 and 38 of the MEMS device. Regions 37 extend to the outside of the trench 17, and region 38 defines the resonator electrode (as well as the corner anchor portions). Regions 37 and 38 thus form the electrical connections from the outside of the cavity 26 to the inside, thereby forming the electrical connection to MEMS body 20, and the electrical connection to the outside of the trench 17. The electrodes can then be used for measuring the capacitance value across the trench 17, which is used to characterise the resonance within the device.. The MEMS device is shown in the example above as an isolated portion, suspended by anchors. The basic structure of an isolated portion can be used to define a MEMS switch (with a beam that makes or breaks contact), a MEMS capacitor (with a dielectric spacing that depends on the beam position) or a MEMS resonator as shown above (with a beam that oscillates), or other MEMS structures like gyroscopes or accelerometers. Of course, different types of resonator are known other than that shown schematically in Figure 17.

As mentioned above, standard IC processes (i.e. materials, deposition techniques, doping techniques and etching techniques) can be used both for the MEMS device and the other components to be integrated on the same substrate. These IC processes (such as CMOS processes) have not been described in detail as they will be completely routine to those skilled in the art.

Only a few example of materials have been given above. However, the main requirement is for a cap to be provided over the MEMS device which is not etched when the sacrificial layer beneath is removed, through an opening provided in the base substrate. Many different combinations of materials and etchants to achieve this will be apparent to those skilled in the art.

Various other modifications will also be apparent to those skilled in the art.

Claims

1. A method of forming a MEMS device, comprising: providing a silicon-on-insulator substrate comprising a base layer (10), an insulator layer (12) and a silicon layer (14); defining a MEMS device portion (20) as a patterned portion of the silicon layer (14) of the substrate; covering the MEMS device portion with a sacrificial layer (22) and a cap layer (26); forming an opening (33) in the base layer (10') beneath the MEMS device portion; using the opening (33) in the base layer to remove the insulator layer of the substrate and the sacrificial layer, thereby providing a space above and below the MEMS device portion (20); and covering the opening (33) in the base layer (10') with a second base layer (34), wherein the method further comprises forming integrated circuit components (30) using parts of the silicon layer (14) not used for the MEMS device.

2. A method as claimed in claim 1 , wherein defining a MEMS device portion comprises etching trenches (17) in the silicon layer (14) down to the insulator layer (12), to define a MEMS portion and anchor portions of the silicon layer (14).

3. A method as claimed in any preceding claim, wherein covering the MEMS device portion comprises filling the trenches (17) with the sacrificial layer.

4. A method as claimed in claim 2 or 3, wherein the sacrificial layer (22) comprises silicon oxide.

5. A method as claimed in any preceding claim, wherein covering the opening in the base layer (10) comprises vacuum sealing the space.

6. A method as claimed in any preceding claim, further comprising thinning the back of the substrate base layer (10) before forming the opening in the base layer (10').

7. A method as claimed in any preceding claim, wherein removing the insulator layer (12) of the substrate and the sacrificial layer (22) is performed using an etchant which does not remove the cap layer (26).

8. A method as claimed in any preceding claim, wherein the integrated circuit components (30) are formed after covering the MEMS device portion (20) with a sacrificial layer (22) and a cap layer (26) and before forming an opening (33) in the base layer (10') beneath the MEMS device portion (20).

9. An integrated circuit comprising a MEMS device and other integrated circuit components, comprising: a silicon-on-insulator substrate comprising a base layer (10'), an insulator layer (12) and a silicon layer (14); a MEMS device portion (20) comprising a patterned portion of the silicon layer (14) of the substrate; a cap layer (26) extending over the MEMS device portion (20); an opening (33) in the base layer (10') beneath the MEMS device portion (20); a second base layer (34) which covers the opening (33) in the base layer (10'); a space above MEMS device portion (20) which is closed by the cap layer (26), and a space below the MEMS device portion (20) which is closed by the second base layer (34); and integrated circuit components (30) defined using parts of the silicon layer (14) not used for the MEMS device, and provided in layers over and/or around the cap layer (26).

10. A circuit as claimed in claim 9, wherein the MEMS device portion (20) comprises a MEMS portion and anchor portions of the silicon layer.

11. A circuit as claimed in claim 9 or 10, wherein the space is vacuum sealed.

12. A circuit as claimed in claim 9, 10 or 11 , where a getter layer (35) is included on the second base layer (34).