Dual wafer attachment process

[0003] The invention pertains to microstructure wafers. Particularly, it pertains to attachment of wafers, and more particularly to temperature sensitive wafers.

[0004] The invention involves the bonding of devices or materials fabricated on separate wafers. A microstructure's front surface may be bonded to another microstructure; however, a microstructure on one substrate may be incompatible with the process used to produce the other microstructure. The invention is designed to avoid problems caused by such incompatibility.

[0005] The need for such a process is driven by the performance needs in several areas of military and industrial applications, including thermal and mechanical sensors, magnetoresistive memory arrays, and superconducting channels.

[0006] Wafer bonding technology has existed for some time. Therefore, other patented processes exist producing more and less similar structures. The idea of bonding wafers processed with incompatible processes has been tried. Existing bolometer technology requires that the readout electronics, CMOS and metalizations survive the processing conditions used for the detector materials. The development of high temperature coefficient of resistance (TCR) materials, which require very high processing temperature, provided an incentive to develop a technique for coupling these materials into bolometer technology. The present technique is superior to related art single-wafer technology because the detector film is processed at temperatures much higher than 450 degrees Celsius (C.), which is the practical limit of CMOS devices.

[0007] The present invention consists of producing the desired microstructures on separate substrates and coating them with a suitable bonding material. These structures may be CMOS electronics or a pure microstructure. One embodiment includes a high-temperature thermal sensor on one wafer and low-temperature CMOS electronics with some electrical and thermal features on another wafer. In the case where the bonding material is polyimide, the polyimide on both surfaces to be bonded are soft baked. The wafers are placed in a wafer bonder and, using precision alignment, brought into contact. The application of pressure and heat forms a bond between the two coatings of polyimide.

[0008] A wafer may need to be removed from a combined structure. A particularly advantageous technique is to build one of the bonded structures on a sacrificial layer that can be etched away to facilitate removal of a wafer without grinding. Further processing can be done on either or both structures.

[0009] After wafer removal, a contact from the backside of one of the structures now on polyimide to the other on the wafer has been demonstrated. This contact, electrical or physical, is one of many kinds, which could be made. Sacrificial material, for example, polyimide, may be removed from between the structures that are bonded via a contact. It may also be desirable to bond a microstructure with something that is not a microstructure, such as single- or multi-layer material, crystalline or amorphous. The present process provides a good method of bonding of these items, while incorporating the materials having a temperature coefficient of resistance that range from a typical value of 2%/C to a high value of 3.5%/C on the present wafers. The TCR may be measured at a value of 12%/C on bulk substrates with much lower 1/f (k=10⁻¹⁴) noise than VO_x(vanadium oxide) (k=10⁻¹³) films. The material may be thinned after wafer removal to improve performance with lower mass.

[0010] FIGS. 1-6 are cross-sectional views of a high-temperature thermal sensor being formed on a silicon wafer.

[0011] FIGS. 7-10 are cross-sectional views of an adaptive structure being formed over CMOS electronics on a silicon wafer.

[0012] FIG. 11 shows the sensor and electronics after having been bonded together.

[0013] FIGS. 12-13 demonstrate a certain method of removing a silicon wafer, using a sacrificial layer, after the structures have been bonded.

[0014] FIG. 14 demonstrates further processing on the structure formed on the removed wafer.

[0015] FIG. 15 shows the removal of part of the original bonding mediums.

[0016] FIG. 16 shows the removal of dielectric material in a region into which a contact is to be deposited.

[0017] FIG. 17 shows the deposition of a structural and electrical contact on the final bonding medium.

[0018] FIG. 18 shows the removal of other sacrificial material.

[0019] FIG. 19 shows the deposition of a reflector and a polyimide sacrificial layer on the wafer.

[0020] FIG. 20 shows an attachment of two wafers via their polyimide layers.

[0021] FIG. 21 shows the etch of an access hole through a silicon wafer to an etch stopping layer.

[0022] FIG. 22 shows the removal of the silicon wafer with the removal of the etch stopping layer of FIG. 21.

[0023] FIG. 23 reveals a YSZ crystal orientation layer and the polyimide layer thinned by milling.

[0024] FIG. 24 shows the removal of the polyimide layer and a punch-through of another polyimide layer to contacts or reflectors of electronic devices.

[0025] FIG. 25 reveals a self-aligned cut of dielectric layers.

[0026] FIG. 26 shows a structural and electrical contact between a reflector or contact of the electronic devices and the leg metal.

[0027] FIG. 27 reveals the removal of the sacrificial polyimide resulting in a thermal isolation between the leg metal and the device electronics.

[0028] FIG. 28 is a planar view of a pixel use in the resultant device of the dual wafer attachment process.

[0029] The dual wafer microstructure attachment process (DWaMA Process) is described in the context of fabricating a bolometer. The process, in this context and in general, is partitioned into three phases of fabrication.

[0030] The first phase is to create a microstructure for thermal sensing. A high temperature coefficient of resistance (TCR) film is necessary for high performance microstructures, requiring high temperature processing. The microstructure will be formed on a first, temporary wafer on which a release layer will first be deposited. These steps are illustrated in FIGS. 1-6 for one device of many on a given wafer.

[0031] In FIG. 1, a silicon wafer 2 of a suitable quality is obtained and prepared to be used as a substrate for a thermal sensing microstructure. A layer 4 of hastalloy is first deposited across the surface of wafer 2. This material is compatible with the processes required to form the sensor, though it is not part of it, and can be etched by a method with good selectivity properties against the materials to be in contact with it, excluding the wafer. It also provides a suitable surface on which to form the sensor. Any material used for this sacrificial layer should have these three properties. A yttria stabilized zirconia (YSZ) crystal orientation film 6, appropriate for the application, is then deposited across the surface of hastalloy layer 4 and patterned, leaving a mesa 6, per this particular sensor's design. Since this wafer's front surface will be bonded with another's front surface, the sensor is actually being constructed from the top down. This property of the proposed method can be considered when designing according to this methodology. Present wafer 2 is designed according to the mirroring effect, which takes place when bonding wafers face-to-face.

[0032] A layer 8 of Bi_xTiO_yis deposited on the exposed areas of hastalloy layer 4 as well as on the surfaces of the YSZ mesa 6, using a solgel process, as shown in FIG. 2. On Bi_xTiO_ylayer 8, a CMR (colossal magneto-resistance) film 10 is deposited on all surfaces with a solgel process and the best thermal processing available. A Si₃N₄layer 12 is deposited on CMR film 10, thereby passivating the film.

[0033] A photoresist is applied to the surface of Si₃N₄layer 12, and patterned to form a mesa of layers Bi_xTiO_y8, CMR 10, and Si₃N₄12 on YSZ 6 mesa. A subsequent etch of these three layers on the YSZ and hastalloy layers 6 and 4. Polyimide 14 is deposited to a depth greater than the height of the entire mesa and is planarized to the level of the mesa, with the resulting structure in FIG. 3.

[0034] Si₃N₄layer 12 is patterned in the same way, using a mask defining contacts to CMR film 10, as shown in FIG. 4. Metal 16 is deposited and patterned such that polyimide layer 14 is exposed in region 15 in the areas where layers 6, 8, 10, and 12 were previously removed. The width of the cut in metal 16 should be enough to allow deposition of a contact structure later in the process. A dielectric 18 providing passivation of the metal 16 is deposited.

[0035] Dielectric 18 is patterned and removed in the region of space 15, providing direct access to polyimide layer 14 from the surface of the microstructure. FIG. 5 shows this structure.

[0036] Finally, a polyimide layer 19 is deposited across the surface to a depth of about 1000 angstroms, extending to the previously deposited polyimide layer 14 and to a height greater than the depth of space 15. This wafer and its microstructures now undergo thermal processing to about 100 degrees C. in order to partially cure polyimide layer 14. See FIG. 6. At this point, the fabrication of the sensor itself is complete.

[0037] In the second phase, the readout or drive electronics are formed on a wafer 22 using standard CMOS processes. An electronic connection is formed at the top level of the CMOS devices in order to make electrical contact with a microstructure that is formed in phase one. The present wafer carries a thick layer of sacrificial bonding material which when removed, with the thin polyimide layer on the other wafer, provides a cavity for thermal isolation of the CMOS devices from the microstructure and for optical reflection. These process steps are shown in FIGS. 7-10.

[0038] CMOS devices 20 are formed on a first silicon wafer 22, being depicted in FIG. 7. A reflector 69 is deposited and patterned on top of the CMOS devices 20 to define the optical cavity between the reflector and the microstructure above it. A thick layer of polyimide 24 is deposited and patterned, producing a basket 21 above a CMOS lead 23 for the purpose of providing a form for metal to be deposited. Polyimide layer 24 also acts as a sacrificial layer, occupying a space between electronics 20 and the microstructure.

[0039] Aluminum 26 is deposited across the entire surface of polyimide layer 24, as seen in FIG. 8, making electrical contact with lead 23 at the bottom of basket 21, and thus to electronics 20, and filling basket 21 to a level above the level of polyimide layer 24.

[0040] Aluminum post 27 is planarized in FIG. 9 by means of a chemical mechanical polish (CMP). This CMP is of a duration so as to re-expose polyimide 24 but not thin it a significant amount.

[0041] A polyimide layer 28 is deposited across the surfaces of planarized polyimide 24 and aluminum post 27, as shown in FIG. 10. This polyimide layer 28 is partially cured by baking at 100° C. for two minutes. The thickness of this layer is nominally about 1000 angstroms, but may need to be thicker in the presence of a non-planar surface. This temporarily completes processing on wafer 22.

[0042] The first half of the third and final phase is to attach the high temperature microstructure film to the CMOS devices by bonding polyimide layers 19 and 28, respectively, which are the top layers on each wafer. This is shown in FIG. 10. Next, wafer 2 is removed to allow exposure of the sensor.

[0043] Being coated with soft-baked polyimide, the wafers are aligned to within one micron of their front surfaces opposite each other in a wafer-to-wafer bonding apparatus while being held a few microns (e.g., <100 microns) apart. They are fused together with the application of pressure and heat. Here is the essence of the present invention. This process discloses a simple method to bond these two structures. Also, it is a wafer level process that enables a quarter of a million devices to be fabricated in parallel. Pressures can range from 20 to 100 psi, 60 psi being nominal. Temperatures don't exceed 400 degrees C. Temperatures should not exceed 450 degrees C. when microelectronics are involved. The bonding process is done either in a vacuum or in the presence of non-oxidizing or inert gases. The resulting cross section is seen in FIG. 11.

[0044] Access holes 40 are cut into temporary second wafer 2 from the back side using a deep RIE (reactive ion etch) silicon etch, stopping on hastalloy release layer 4, as seen in FIG. 12. These holes provide local exposure to hastalloy layer 4, thereby allowing the hastalloy to be etched in a shorter period of time. The release layer is chemically etched to allow the removal of temporary wafer 2 from the bonded assembly, being shown in FIG. 13.

[0045] YSZ crystal orientation layer 6 and surrounding polyimide layer 14 are thinned by a means of blanket milling to a thickness as desired for the sensor. FIG. 14 illustrates this.

[0046] The second half of the third and final phase is to open a region between the microstructure and post 27 created on the readout wafer, make an electrical and physical connection, and remove sacrificial polyimide layers 19, 24 and 28, so as to provide thermal isolation between the microstructure and the readout substrate.

[0047] FIG. 15 displays the bonded assembly after some of the polyimide 14, 19, 24 has been removed. The anisotropic etch removes a portion of polyimide 14 layer at the surface of the structure and is allowed to continue through space 15 in metal 16. The etchant stops at aluminum post 27 while removing portions of polyimide layers 19 and 28 that had been at the surface of the wafers at the time of bonding and below space 15.

[0048] A cut, self-aligned to YSZ film 6 and leg metal 16, is made of dielectric layer 18 in region 42, as demonstrated in FIG. 16.

[0049] The second-to-last stage of processing involves depositing and patterning an electrical and structural contact 44 in the discussed region 42, affixed to post 27 and leg metal 16. FIG. 17 shows the contact.

[0050] Finally, FIG. 18 shows the final product after a dry etch has removed the remaining parts of polyimide layers 24, 28, 19, creating the final desired thermal isolation.

[0051] The above procedure can be modified to eliminate the use of post 27. The following illustrates that modification. In FIG. 19, as in FIG. 1, reflector 69 is deposited and patterned on top of CMOS electronics 20 and their interconnect metals 50 to form the optically resonant cavity between the reflector deposited and patterned on top of wafer 22 and the microstructure of wafer 2. A layer of polyimide 51 is deposited on top of the resulting structure, but here it is not patterned or etched.

[0052] As before, wafers 2, 22 of FIGS. 19 and 20, respectively, are bonded. Holes 52 are etched to hastalloy layer 53 through wafer 2, as shown in FIG. 21. Wafer 2 is removed with an etch of hastalloy layer 53 in FIG. 22. YSZ crystal orientation layer 56 is thinned by blanket milling along with polyimide layer 54 in FIG. 23.

[0053] In FIG. 24, as in FIG. 15, polyimide layer 54 and portions of layers 51 and 60 are etched, exposing leg metal 58 and punching through to CMOS lead 50, which is electrically connected to CMOS devices 20.

[0054] A cut of dielectric layer 62, self-aligned to YSZ film 56 and leg metal features 58, is made and is shown in FIG. 25.

[0055] Finally, contact 64 is deposited and patterned, making electrical and structural contact between CMOS lead metal 50 and leg metal 58, of FIG. 26. Remaining polyimide layers 51 and 60 are etched away, in FIG. 27. The resulting assembly uses only one deposited material to make the backside contact from leg metal 58 to CMOS lead metal 50.

[0056] The above procedure can be further modified to substitute the use of hastalloy with molybdenum for faster etching. Molybdenum requires less etching time for release but may not promote crystalinity in the YSZ and thereby the CMR films as well as hastalloy.

[0057] FIG. 28 reveals a plan view of a pixel of the resultant device of the described process. It shows reflector 69, YSZ 6 and CMR resistor 10. Also, contacts 44 and 64, and leg metals 16. This figure shows interconnects 71, which are not shown in the cross-section view of the device, to pads.

[0058] This process allows the separation of the non-CMOS-compatible process steps, which in this case is the high temperature processing of the sensor material, from those needed to form CMOS electronics. It also demonstrates the assembly of these structures through thin film contact layers and sacrificial release layers. Although the process has been described using CMOS and non-CMOS processes, a much broader range of applications can be made.

[0059] Though the invention has been described with respect to a specific preferred embodiment, many variations and modifications will become apparent to those skilled in the art upon reading the present application. It is therefore intended that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.