WO2005122217A1

WO2005122217A1 - Thermosetting polymer bonding for micro electro-mechanical systems

Info

Publication number: WO2005122217A1
Application number: PCT/US2005/020704
Authority: WO
Inventors: Arun Majumdar; Srinath Satayanarayana; Rohit N. Karnit
Original assignee: The Regents Of The University Of California
Priority date: 2004-06-09
Filing date: 2005-06-09
Publication date: 2005-12-22

Abstract

A method of bonding a pair of members (20, 30) together to form a microdevice, by: applying a layer of uncured thermosetting polymer (12) onto a first member (10); placing a second member (20) onto the first member (10); removing the second member (20) from the first member (10), thereby transferring portions of the uncured thermosetting polymer (12) from the first member (10) onto the second member (20); placing the second member (20) onto a third member (30); and then thermally curing the thermosetting polymer (12), thereby bonding the second member (20) to the third member (30).

Description

THERMOSETTING POLYMER BONDING FOR MICRO ELECTRO-MECHANICAL SYSTEMS

TECHNICAL FIELD The present invention relates to bonding of MEMS (Micro Electro-Mechanical Systems) and microfluidic devices and structures.

BACKGROUND OF THE INVENTION Multilayer MEMS and microfluidic designs use diverse materials that require separate fabrication and assembly. The use of fluids in such devices places special restrictions on the bonding processes that can be used to assemble these devices.

To date, there are two main wafer and chip bonding techniques that are used to assemble MEMS (and microfluidic) devices, being "direct bonding" and "bonding with an intermediate layer".

Direct bonding includes "anodic bonding", "fusion bonding" and "activated surface bonding". Each of these methods have disadvantages. For example, anodic bonding requires extensive wafer cleaning, high voltages (>l V) and high temperatures (>400 degrees C). Fusion bonding relies on attractive forces between extremely clean flat surfaces. Thus, it also requires extensive pre-cleaning. In addition, it is typically carried out in vacuum conditions with external pressure being applied to the wafers. Lastly, surface activation bonding requires pre-treatment of the bonding surfaces with oxygen plasma, hydration processes or other chemicals to increase reactivity. Unfortunately, the effect of such surface treatment processes lasts for only a small window of time and therefore the bonding and alignment need to be completed within a small time window.

Bonding with an intermediate layer includes "adhesive bonding", "eutectic bonding", and "solder compression bonding". Each of these methods have disadvantages too. For example, standard adhesion bonding techniques rely on spin coating a thin adhesive layer onto the substrate. Unfortunately, spin coating is problematic when the wafers to be bonded have a non-uniform topography. Eutectic bonding requires a thin gold layer as an adhesive when bonding silicon wafers. Lastly, solder compression bonding involves the deposition and patterning of additional layers like solder or other soft metals, and may require pressure to bond the substrates together.

Yet another form of bonding is thermal glass-to-glass bonding in which two glass substrates are heated to a temperature at which melting starts to occur and then pressed together. A problem with this approach is that as the glass starts to deform, problems may be caused for the fluidic circuit formed therein.

As can be appreciated, the above discussed bonding techniques are unsuitable for assembling microdevices that have non-uniform surface topography and pressure sensitive components. In addition, the above discussed bonding techniques are typically unsuitable for use with labile bio-molecules.

SUMMARY OF THE INVENTION The present invention provides a method of bonding a pair of members together to form a microdevice, by: applying a layer of uncured thermosetting polymer onto a first member; placing a second member onto the first member; removing the second member from the first member, thereby transferring portions of the uncured thermosetting polymer from the first member onto the second member; placing the second member onto a third member; and then thermally curing the thermosetting polymer, thereby bonding the second member to the third member.

In one embodiment, each of the thermosetting polymer and the second and third members are all made of PDMS. In another embodiment, either or both of the second or third members are made from silicon, glass, PDMS, quartz, silicon nitride or plastic.

In one embodiment, the first member is a transfer wafer, the second member is a cover wafer and the third member is a device wafer. Thermally curing the thermosetting polymer may optionally be accomplished by raising the temperature of the thermosetting polymer to between 70 to 90 degrees C. In various embodiments, the second member has a microfluidic channel therein, and the third member includes an optional micromechanical structure mounted thereon. This micromechanical structure may optionally be a sensor which may include a deflectable microcantilever or micromembrane.

The present invention also provides a method of bonding a pair of members together to form a microdevice, by: applying a layer of a first material onto a first member; placing a second member onto the first member; removing the second member from the first member, thereby transferring portions of the first material from the first member onto the second member; applying a layer of a second material onto a third member; and placing the second member onto the third member, wherein the first and second materials each comprise one of a resin and a curing agent, such that the second member bonds to the third member after the first and second materials contact one another. Optionally, heating may be used to facilitate bonding between the resin and the curing agent.

A first advantage of the present invention is that it is not time constrained. In other words, the present bonding method need not be carried out in a short time frame. This is in contrast to a system proposed by Schlautmann, as follows.

In the stamped adhesive bonding system disclosed by Schlautmann (in his paper entitled "Fabrication of a microfluidic chip by UV bonding at room temperature for integration of temperature-sensitive layers, Journal of Micromechanics and Microengineering 13 (2003) S81-S84"), a volatile adhesive compound is used in the adhesive stamping process. (This volatile adhesive compound is cured by UV exposure). Due to the volatility of this adhesive compound, Schlautmann states that his stamping bonding process was usually carried out in less than one minute (p. S83) "to avoid evaporation of the glue".

In contrast, the present invention uses a thermosetting polymer such as uncured PDMS as the stamped adhesive. Rapid evaporation of this adhesive is not a problem. Therefore, it is not necessary to carry out the present adhesive stamping transfer process within a very short time period. This advantageously provides more opportunity for proper alignment between the members prior to their bonding together. A second advantage of the present method is that it can be used with a wide variety of materials. Moreover, such materials may be transparent, translucent or opaque. This is due to the fact that heat is used to bond the members together. In contrast, the Schlautmann system uses exposure to UV radiation to bond its members together. As a result, at least one of Schlautmann members must permit UV radiation to pass therethrough (i.e.: to reach the UV curable adhesive between the two members). Thus, the Schlautmann system could not be used to bond together two opaque members (such as two silicon wafers).

A third advantage of the present invention is that it can be used on surfaces with uneven topography. This is especially relevant in building microfluidic systems, where good sealing can be difficult to obtain when fluid channels are disposed in uneven member surfaces.

Other advantages of the present bonding system may include the fact that it: (a) does not require extensive pre-cleaning procedures; (b) need not be performed in a vacuum; and that (c) external pressure on the two members to be bonded together is not required.

In various optional embodiments of the present invention, both of the members to be bonded together are PDMS and the thermosetting polymer used to bond them together is also PDMS. This is particularly advantageous in that the microdevice so formed is a homogeneous block of material. This has the advantage of providing high structural strength. hi various other embodiments of the invention, including the bonding of PDMS to glass, PDMS to silicon and PDMS to nitride, the present bonding may also be reversible.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional side elevation view of a transfer wafer with an adhesive layer applied thereon.

Fig. 2 is a sectional side elevation view of a cover wafer being placed onto the structure of Fig. 1. Fig. 3 is a sectional side elevation view of the cover wafer of Fig. 2 being removed from the structure of Fig. 1 (showing the transfer of adhesive from the transfer wafer to the cover wafer).

Fig. 4 is a sectional side elevation view of the cover wafer of Fig. 3 being placed onto a device wafer.

Fig. 5 is a sectional side elevation view of the assembled structure of Fig. 4 with heat applied thereto.

Fig. 6 A is a top plan view of a first microdevice formed in accordance with the present bonding method.

Fig. 6B is a sectional side elevation view of the microdevice of Fig. 6 A.

Fig. 7A is a top plan view of a second microdevice formed in accordance with the present bonding method.

Fig. 7B is a sectional side elevation view of the microdevice of Fig. 7A.

Fig. 8 is an illustration of burst pressure test results achieved with the microdevice of Figs . 6 A and 6B.

Fig. 9 is an illustration of burst pressure test results achieved with the microdevice of Figs . 7A and 7B.

Fig. 10 is a sectional side elevation view of a component wafer with a layer of resin (or curing agent) thereon being placed onto another wafer having curing agent (or resin) thereon.

Fig. 11 is a sectional side elevation view of the assembled structure of Fig. 10 with the resin and curing agent causing bonding between the component wafer and the other wafer disposed thereunder. b

DETAILED DESCRIPTION OF THE DRAWINGS Figs. 1 to 5 illustrate a first aspect of the present bonding system using an uncured thermosetting polymer. Figs. 6A to 7B illustrate devices bonded by the resent method, the bonding strength of which was experimentally determined. Figs. 8 and 9 show the experimental data on bonding strength for these two devices. Lastly, Figs. 10 and 11 illustrate an alternate aspect of the preferred bonding system using a resin and a curing agent.

Referring first to fig. 1, a transfer wafer 10 is provided. A layer of uncured thermosetting polymer 12 is applied onto transfer wafer 10. Thermosetting polymer 12 may be applied to transfer wafer 10 by spinning, spraying or dipping.

Thermosetting polymer 12 may optionally comprises PDMS (poly(dimethylsiloxane)) but is not so limited. Rater, any suitable thermosetting polymer may be used.

Next, as show in Fig. 2, a cover wafer 20 is placed on top of the layer of thermosetting polymer 12. Cover wafer 20 may also be made of PDMS, but is not so limited. For example, cover wafer 20 may equally be made from glass, silicon, quartz, silicon nitride or plastic, or from any other suitable material. In the illustrated embodiment of Fig. 2, cover wafer 20 has a plurality of microchannels 22 and 24 formed therein. Such microchannels 22 and 24 may comprise a microfluidic circuit, as will be shown below.

Next, as shown in Fig. 3, cover wafer 20 is pulled away from transfer wafer 10. As can be seen, portions of thermosetting polymer 12 move from transfer wafer 10 to cover wafer 20. (Taken together, the processes shown in Figs. 2 and 3 may be referred to as "stamping" the adhesive (i.e.: thermosetting polymer 12) onto cover wafer 20).

Next, as shown in Fig. 4, cover wafer 20 is aligned with and then placed onto. a device wafer 30. Device wafer 30 may also be made of PDMS, but is not so limited. For example, device wafer 30 may equally be made from glass, silicon, quartz, silicon nitride or plastic, or from any other suitable material. In the illustrated embodiment of Fig. 4, device wafer 30 may optionally include a sensor 32 mounted thereon, h various optional embodiments, sensor 32 may comprise a deflectable microcanilever or micromembrane. Next, as shown in Fig. 5, thermosetting polymer 12 is cured by heating the assembled system. In preferred embodiments, such heating may involve raising the temperature of the microdevice to between 70 to 90 degrees C. Other suitable temperatures (and their associated heating time periods) are encompassed within the scope of the present invention.

As a result, thermosetting polymer 12 is cured, thereby bonding cover wafer 20 to device wafer 30. As can be seen in this construction, sensors 32 are thus ideally positioned within an enclosed fluid chamber formed by microchannel 22. A separate fluid channel 24 is also shown.

In those optional embodiments where wafers 20 ad 30 and thermosetting polymer 12 are all the same material (e.g.: PDSM), the resulting microdevice stricture (as shown in Fig. 5) will be formed as an integral block of the same homogenous material (with sensors 32 disposed wthin fluid microchannel 22).

In various optional embodiments, none, either or both of cover wafer 20 and device wafer 30 may comprise microfluidic channels. Moreover, there is no requirement that device wafer 30 need to have an actual sensor 32 or other device mounted thereon. For example, wafer 30 may simply comprise a flat wafer used for forming a sealed chamber when placed against microfluidic channel 24 in cover wafer 20.

In various optional embodiments of the present invention, the uncured thermosetting polymer 12 is pre-cured to increase its viscosity. This is advantageous in reducing the potential for the PDMS to flow into the microfluidic channels. In various experiments performed by the inventors, such pre-curing has proved to be very successful in improving bonding performance of the device, hi the experiments performed by the present inventors, such pre-curing was carried out from 15 minutes to one hour, however, the present invention is not so limited.

Figs. 6A and 6B illustrate a first device built by the present inventor to test the effectiveness of the present bonding technique. Figs. 7A and 7B illustrate a second device built by the present inventor to test the effectiveness of the present bonding technique. In Figs. 6A to 7B, a PDMS cover wafer 20 was bonded onto a glass slide 30 using uncured PDMS as the thermosetting polymer. Pressure was introduced through apertures 25, and the burst pressure of the two devices were measured.

In different tests of the device illustrated in Figs. 6A and 6B, microchannel 21 is varied in size (S) from 1mm² to 2.5 mm². MicroChannel 21 has a height (H) of 40μm. The burst pressures for these devices is illustrated in Fig. 8, where each data point represents one device.

Similarly, the device illustrated in Figs. 7A and 7B, has a microchannel 21 that is lmm square and has a height (H) of 40μm. A narrow elongated channel 23 extends from microchannel 21. Elongated channel 23 has a length (L) of 20μm. In different tests of the device, the width (W) of elongated channel 23 is varied from 20μm to lOOμm. The burst pressures for these devices is illustrated in Fig. 9, where each data point represents one device.

As can be seen in Figs. 8 and 9, burst pressure tests performed by the present inventors suggested that wafer bonds using PDMS could withstand 200 to 700 kPa (i.e.: 2 to 7 atmospheres) depending upon the geometry and configuration of the device.

Further details of the fabrication and testing of the microdevices illustrated in Figs. 6A to 7B are as follows. It is to be understood that these details are merely exemplary, and do not limit the scope of the present invention. First, a microchannel mold was made by patterning a silicon wafer with a photoresist masking and etching the silicon approximately 40 μm deep in a DRIE system to leave a positive relief of channels. The silicon mold was then placed in a desiccator with a few drops of tridecafluouro-l,l,2,2-tetrahydrooctyl-l- trichlorosilane (United Chemical Technologies, Bristo, PA) to create a monolayer on the surface of the silicon. The PDMS was mixed in a 10: 1 ration of monomer and curing agent per manufacturer's instructions (Sylgard 184, Dow Corning, Midland, MI) and poured over the mold. The PDMS was degassed and cured at 120 degrees C for 20-25 minutes and then removed from the mold to form cover wafer 20. The inlet holes 25 were formed by stamping the molded PDMS cover 20 with a 20-gauge intramedic luer stub adapter (Beckton Dickinson, Sparks, MD). The PDMS cover 20 was then bonded to glass slide 30 using uncured PDMS as the thermosetting polymer using the stamping adhesive transfer technique as outlined above. In particular, 1 to 2 ml of freshly mixed PDMS was poured onto a polished silicon wafer. The wafer was then spun at 8000 rpm for 8 to 9 minutes. This resulted in a thin layer of uncured PDMS having a thickness of 1 to 1.5 μm. The previously fabricated PDMS cover wafer 20 was then cleaned with isopropyl alcohol. The PDMS cover wafer 20 with the thin layer of uncured PDMS was then placed onto glass slide 30 and cured at 90 degrees C for 10 to 5 mins.

In additional tests performed by the present inventors, PDMS bonding was successfully used to bond channels running over steps lμm in height without any leakage along the step. Moreover, the present bonding technique was found to work both on silicon and silicon nitride surfaces as well.

Lastly, Figs. 10 and 11 illustrate an alternate aspect of the present method. Figs. 10 and 11 illustrate the sequential steps of placing a cover wafer 20 onto a wafer 30 similar to the stamping method shown in Figs. 1 to 5 above. However, instead of stamping a thermosetting polymer 12 onto cover wafer 20, and then placing cover wafer 20 onto wafer 30, a first material 14 is instead stamped onto cover wafer 20, and a second material 16 is placed onto wafer 30.

In accordance with the present invention, one of material 14 and 16 comprises a resin, and the other of material 14 and 16 comprises a curing agent. Thus, bonding of wafers 20 and 30 occurs when materials 14 and 16 come into contact with one another. In optional aspects of the present invention, heat may be applied to the resin and curing agents 14 and 16; however, such heating may not be necessary to achieve bonding.

In various embodiments, second material 16 may cover the entire surface of wafer 30, or alternately, second material 16 may optionally be stamped onto selected portions of wafer 30 (for example, when wafer 30 also contains channels or recesses formed therein).

Claims

What is claimed is:

1. A method of bonding a pair of members together to form a microdevice, comprising: applying a layer of uncured thermosetting polymer onto a first member; placing a second member onto the first member; removing the second member from the first member, thereby transferring portions of the uncured thennosetting polymer from the first member onto the second member; placing the second member onto a third member; and then thermally curing the thermosetting polymer, thereby bonding the second member to the third member.

2. The method of claim 1, wherein the first member is a transfer wafer, the second member is a cover wafer and the third member is a device wafer.

3. The method of claim 1 , wherein the thermosetting polymer is made of PDMS.

4. The method of claim 3, wherein the second and third members are made of PDMS.

5. The method of claim 1, wherein the second member is made of at least one of the group consisting of silicon, glass, PDMS, quartz, silicon nitride and plastic.

6. The method of claim 1, wherein the third member is made of at least one of the group consisting of silicon, glass, PDMS, quartz, silicon nitride and plastic.

7. The method of claim 1 , wherein thermally curing the thermosetting polymer comprises raising the temperature of the thermosetting polymer to between 70 to 90 degrees C.

8. The method of claim 1, wherein the second member comprises a microfluidic channel.

9. The method of claim 1, wherein the third member comprises a microfluidic channel.

10. The method of claim 1, wherein the third member comprises a micromechanical structure mounted thereon.

11. The method of claim 10, wherein the micromechanical structure on the third member is a sensor.

12. The method of claim 11, wherein the sensor is a deflectable microcantilever or micromembrane.

13. The method of claim 1, wherein the uncured thermosetting polymer is applied onto the first member by spinning, spraying or dipping.

14. A method of bonding a pair of members together to form a microdevice, comprising: applying a layer of a first material onto a first member; placing a second member onto the first member; removing the second member from the first member, thereby transferring portions of the first material from the first member onto the second member; applying a layer of a second material onto a third member; and placing the second member onto the third member, wherein the first and second materials each comprise one of a resin and a curing agent, such that the second member bonds to the third member after the first and second materials contact one another.

15. The method of claim 14, further comprising: heating the first and second materials to facilitate bonding.

16. The method of claim 14, wherein the first member is a transfer wafer, the second member is a cover wafer and the third member is a device wafer.

17. The method of claim 14, wherein the second member is made of at least one of the group consisting of silicon, glass, PDMS, quartz, silicon nitride and plastic.

18. The method of claim 14, wherein the third member is made of at least one of the group consisting of silicon, glass, PDMS, quartz, silicon nitride and plastic.

19. The method of claim 14, wherein the second member comprises a microfluidic channel.

20. The method of claim 14, wherein the third member comprises a micromechanical structure mounted thereon.

21. The method of claim 20, wherein the micromechanical structure on the third member is a sensor.

22. The method of claim 21, wherein the sensor is a deflectable microcantilever or micromembrane.

23. A microdevice, comprising: a top member; a bottom member; and a bonding layer between the top and bottom members, the bonding layer formed from a cured thermosetting polymer, wherein the cured thermosetting polymer is formed from an uncured thermosetting polymer that has been stamped onto one of the top or bottom member and then thermally cured.

24. The microdevice of claim 23, wherein the bonding layer is made of PDMS.

25. The microdevice of claim 23, wherein the top and bottom members are made of PDMS.

26. The microdevice of claim 23, wherein the top member is made of at least one of the group consisting of silicon, glass, PDMS, quartz, silicon nitride and plastic.

27. The microdevice of claim 23, wherein the bottom member is made of at least one of the group consisting of silicon, glass, PDMS, quartz, silicon nitride and plastic.

28. The microdevice of claim 23, wherein the top member comprises a microfluidic channel.

29. The microdevice of claim 23, wherein the bottom member comprises a micromechanical structure mounted thereon.

30. The microdevice of claim 29, wherein the micromechanical structure is a sensor.

31. The microdevice of claim 30, wherein the sensor is a deflectable microcantilever or micromembrane.