A new technique, at temperatures of 150°C or 450°C, that provides planarization after a very deep etching step in silicon is presented. Resist spinning and layer patterning as well as realization of bridges or cantilevers across deep holes becomes possible. The sacrificial wafer bonding technique contains a wafer bond step followed by an etch back. Results of (1) polymer bonding followed by dry etching and (2) anodic bonding combined with KOH etching are discussed. The polymer bond method was applied in a strain based membrane pressure sensor to pattern the strain gauges and to provide electrical connections across a deep corrugation in a thin silicon nitride membrane by metal bridge

Berenschot, J.W.

Elwenspoek, M.

Fluitman, J.H.J.

Spiering, V.L.

English

A new technique, at temperatures of 150°C or 450°C, that provides planarization after a very deep etching step in silicon is presented. Resist spinning and layer patterning as well as realization of bridges or cantilevers across deep holes becomes possible. The sacrificial wafer bonding technique contains a wafer bond step followed by an etch back. Results of (1) polymer bonding followed by dry etching and (2) anodic bonding combined with KOH etching are discussed. The polymer bond method was applied in a strain based membrane pressure sensor to pattern the strain gauges and to provide electrical connections across a deep corrugation in a thin silicon nitride membrane by metal bridges

Elwenspoek, M.C.

University of Twente Research Information

LOW TEMPERATURE SACRIFICIAL WAFER BONDING FOR PLANARIZATION AFTER VERY DEEP ETCHING. A th, in h i d  in V.L. Spiering, J.W. Berenschot, M. Elwenspoek and J.HJ. Fluitman new technique, at temperatures of 150°C or 450°C t provides planarization after a very deep etching step silicon is presented. Resist spinning and layer palerning as well as realization of bridges or cantilevers across deep holes becomes possible. The sacrificial wafer bonding technique contains a wafer bond step followed by an etch back. Results of (1) polymer bopding followed by dry etching and (2) anodic bonding combined with KOH etching are discussed. The polymer method was applied in a strain based membrane prelssure sensor to pattern the strain gauges and to provide electrical connections across a deep corrugation thin silicon nitride membrane by metal bridges. MESA Research Institute, University of Twente, P. 0. Box 21 7, 7500 AE Enschede, the Netherlands ~ ABSTRACT I I NTR OD UCTlO N chined devices is often by the impossibility of layer patterning by spinning on a wafer containing large surface steps. Usually it is then tried to carry out the tion of layers and patterning before the deep etch wever is not possible if the deep etch step is create a mould to shape the device, by he actual device material after the etch step. methods to apply and pattern photoresist are ng, or producing dry resist coatings by ation, photo polymerization, or plasma polymeri- eventually followed by dry development ill .  , the shadow mask technique can be used to patterns on wafers containing deep etch holes. methods allow patterning of the wafer surface the creation of bridges technique to acquire filling the holes with an organic rial, e.g., a polymer, eventually followed by a ng step. The flatness of the structures across the e defined by the shape of the planarized ich can be a serious restriction in the 69 By means of the (low temperature) sacrificial wafer bonding technique, layer deposition, resist spinning, layer patterning combined with realization of bridges and cantilevers is possible. By varying the bond layer and etch back method, it allows a wide application in micro electso mechanical systems. The bond step is carried out whether with polymers (at 15OOC) or by anodic bonding (at 450°C). The advantage compared to applications of silicon fusion bonding [2, 31 is the low bonding temperature, especially when the polymer bonding is applied. First the technique using polymer and anodic wafer bonding will be illustrated by realizing metal bridge connections across a groove in silicon with a depth of 100 pm. Next the application of the sacrificial wafer bonding technique, using the polymer wafer bond step, is demonstrated in the realization of a strain based membrane pressure sensor with low residual stress and largely reduced package influences [41. SACRIFICIAL WAFER BONDING Basically the technique consist of four steps, see also fig. 1. First a wafer, containing a thin stop layer, is bonded (1) on top of a device with the deep etch holes. After wafer bonding, the top wafer is etched down (2) to the (thin) stop layer. Spinning of photoresist is now possible on this layer, which was not the case with the original bottom wafer that contains the deep etched features. The stop layer can easily be patterned (3) in such a way that all etch holes are covered, see also fig. 2. New layers can directly be deposited on the wafer surface (b), but also original layers on the bottom wafer can be patterned (a). Finally the coverage of the etch holes can be removed (4) and the sacrificial bonding step has been completed. In this last case the bonding wafer as well as the bonding layer is removed completely. The result is the original wafer containing deep etch holes, but now added with patterned original layers, patterned new deposited layers and eventually bridges or cantilevers of new deposited materials. 0-7803-1833-1/94 $3.00 @ 1994 IEEE Authorized licensed use limited to: UNIVERSITEIT TWENTE. Downloaded on November 11, 2008 at 06:23 from IEEE Xplore.  Restrictions apply.1 2 3 a b 4 Fig. 1 Main steps of the sacrificial wafer bonding process: 1. on top bonding of a wafer containing a stop layer 2. etching of the wafer down to the stop layer 3. patterning of the stop and bond layer a. original layers (e.g., layer 1) can be patterned IJ. new layers (layer 2) can be deposited and patterned 4. removing of the stop and bond layer a and b are device process steps. a 4 Fig. 2 Top view of a device after step a (lejit) and step 4 of the sacrificial wafer bonding process. It is possible to combine several functions in one layer. Depending on the bonding and etch back process, the bond layer could the bond layer cou bond layer material b groove. Note that the strength, lifetime and bond area of the bonding is of less importance, since the bond is temporary. More important however is the etch stop function of the stop layer. EXPERIMENTAL RESULTS The technique is d d with two bonding processes and etch bac s. First the combination of polymer bonding f Another successful silicon wafers with a glass, followed by an (SF,:N,) plasma in the reactive ion etch mode. Polymer bonding Before application i was studied. The omission of th layer by infrared imaging. - The procedure consists of 1. spinning the resist for 3" wafers; laye 2. prebake of 140 environment, 3. contacting of the two wafers in 4. postbake of 90 minutes at environment. 70 Authorized licensed use limited to: UNIVERSITEIT TWENTE. Downloaded on November 11, 2008 at 06:23 from IEEE Xplore.  Restrictions apply.prebake step the main problem was the solvent in the photoresist during the heat contacting. This resulted in strong ous bond layers, containing many small areas. Gassing out the solvent by prebaking hardening of the top layer, due to n of the resist. The bond layer looses its sticking n that case. In order to overcome this problem, a of 140 minutes at 15OOC in a nitrogen ent was carried out directly after spinning the on the (flat) top wafer. Then the solvents can rate without oxidation of the polymer layer. The gontactinz of the two wafers in a bonding tool took pla e at room temperature, followed by a postbake step of 90 minutes at 150°C in the same nitrogen oven as the pre ake step. Note that the actual bond step takes place d 'ng the higher temperature postbake step. Without the pos bake step the sticking quality, and therefore the bond stre gth, is very poor. The heat treatment resulted in a ho iogeneous bond layer with sufficient strength. Fig. 3 shows an example of two bare silicon wafers bon ed with negative resist. An infrared image of the bon ing layer and a photograph of the split wafers are sho n. The wafers broke apart in the interior of the pho oresist layer, which indicates a large bond strength. i Bond (top) and Reactive ion etching Reactive ion etching was used as etch back method since it has the advantage of a large etch rate combined with an almost perfect etch stop on metal. A flat wafer with evaporated aluminum and spun negative resist was polymer bonded on top of a second wafer that contains deep etched grooves. The top wafer was etched down on the aluminum (see also step 1 and 2 of fig. 1). A 0.5 pm thick aluminum layer was sufficient to serve as the etch stop layer. The etching was done in a 50:lO sccm SF6:N2 plasma, at 20 Pa and 25"C, in a parallel plate electrode configuration of the type Plasmafab 340. First the (lower) electrode was powered for about 3/4 hours with 0.83 W/cm2, until the aluminum stop layer was reached at the outer radius. Then the process was continued at a reduced power of 0.42 W/cm2, until the whole top wafer was etched away. The limit of the etch rate (power) is the temperature. If the temperature increases too much, the polymer and metal layer can degenerate. This is the reason of the power-reduction as soon as the aluminum layer has been reached at the outer radius of the wafer. Wafers with a thickness of 280 pm were removed in circa 2% hours. The radial non-uniformity of the etch process is caused by the "loading effect" [71, a radius dependent electrical field, and an inhomogeneous gas flow pattern. The loading effect is characterized by a decrease in etch rate as the quantity of material to be etched is increased, for a given set of etching conditions. In this particular case, the etch process is locally limited by the supply of fresh radicals. It causes a much larger rate at outer radius of the wafer compared to the centre etch rate. The coverage layer of 0.5 pm thick aluminum across grooves with a width of 150 pm showed a little downwards bending. The centre deflection was about 2.2 prn and is due to stresses in the layer, or plastic deformations as a result of the temperature increase during the dry etch back. For the applications in this research it is expected that these deformations have no relevance. Fabrication of bridges The sacrificial wafer bonding technique, using the negative resist as bond layer, is demonstrated by realizing aluminum bridges across deep grooves in a silicon wafer. After bonding and etch back (step 1 and 2) the aluminum etch stop layer is directly patterned in such a way that aluminum bridges remain, see also fig. 4. The non-covered negative resist is stripped afterwards in a oxygen plasma (step 3). Note that in this case aluminum bridges with a resist foundation are realized, because there was no new deposition of aluminum. Fig. 5 shows the metal bridges across silicon grooves, fabricated with the sacrificial polymer bonding method. The grooves have a depth of 100 pm. 71 Authorized licensed use limited to: UNIVERSITEIT TWENTE. Downloaded on November 11, 2008 at 06:23 from IEEE Xplore.  Restrictions apply.c 2 3 I 0 Silicon neg. resist 0 Aluminum I Fk. 4 Process steps for the realization of aluminum bridges across deep silicon grooves by the sacrificial wafer bonding process usingpolymer bonding and dry etch back: 1. on top bonding of a wafer containing a stop layer 2. etching of the wafer down to the stop l q e r  3. patterning of the stop and bond layer. Fk .  5 Aluminum bridges across deep silicon grooves, realized by the sacrificial wafer bonding process using polyner bonding and dry etch back Anodic bonding followed by Another successful combination silicon wafers with a thin la etch stop function in one layer. Recently it has been shown that it is possible to use very thin sputtered layers (thickness down to 20 nm possible), acquiring a bonding as well as a selective etch stop in KOH [8]. Anodic bonding Borosilicate glass films were 8330 glass target in a Nordiko density was 1 Pa and the targe W/cm2. The layers applied in this research were sputtered in 100% Ar with a thicknes pm and no anneal step was done. However an i t reduction of compressive stress from 130 MPa to 5 MPa is achieved after an anneal step of 3.5 hours at 550°C in nitrogen ambient [8]. The bonding set-up was equipp camera, which made in-situ infrared For the 1.4 pm thick glass layer a needed to start the bonding process. sufficient to bond silicon wafers successfullv with glass about 1.3 pm for a groove width of 125 pm and is due to compressive stresses in the glass layer. material and as mask material for etching the underlying glass layer. 7 2  Authorized licensed use limited to: UNIVERSITEIT TWENTE. Downloaded on November 11, 2008 at 06:23 from IEEE Xplore.  Restrictions apply.ion etching with a SF6-plasma. At that was largely decreasing, possibly density. The final 100 nm were solution, without I-I . 6 Metal bridges across deep silicon grooves, realized I DISCUSSION o examples of the sacrificial wafer bonding method that the technique is a powerful tool in the attractive are the wide range of several functions in one layer. to realize bridges across realization of micro electro mechanical and etch back methods as well as A ter the etch back of the top wafer, the system fa rication process can be continued without any c nstraint except the limitations brought in by the a k plied materials itself. This flexibility opens many possibilities for flat layer deposition, resist spinning, and layer patterning combined with the realization of bridges and cantilevers, all after a very deep etching step. Another different application is to create a shadow mask, after the etch back, for patterning the bottom of a deep etch hole. The demonstrated methods, using a polymer or sputtered glass as bond layer respectively, have different advantages. The polymer bonding is attractive for its simple and fast technology and the very low process temperature (150°C). On the other hand the succeeding process steps are limited by the maximal tolerable temperature of the negative resist, i.e. about 300°C. For anodic bonding the bond temperature is higher (45OoC), but there is no stringent temperature limitation for succeeding steps. The serious constraint in this case is the presence of Na+-ions; these can disturb the electric qualities of other components. In the experiments the aluminum bridges were fabricated with a bond layer base. Next, an application will be presented with one of the original bottom layers as direct foundation for aluminum bridges. In the final process step the bond layer is removed completely so the whole sacrificial wafer bonding step has been carried out. APPLICATION The polymer bonding technique, in combination with the dry etch back, is applied in a membrane pressure-sensor containing a deep corrugation. The corrugation is added to reduce package stresses and to obtain a residual stress relief in the inner circular membrane without loosing pressure sensitivity [4]. The sacrificial wafer bonding facilitates patterning of polysilicon strain gauges on a silicon nitride membrane as well as metal bridge connections across the deep thin silicon nitride corrugation. A schematic outline of the fabricated device is shown in fig. 7. A square silicon nitride membrane (thickness of 1 pm), containing a 100 pm deep circular corrugation, has been realized. The corrugations were shaped by first etching deep grooves in silicon by reactive ion etching, followed by a low pressure chemical vapour deposition of silicon nitride [9]. On top of this layer polysilicon was grown and doped, and further thermally grown oxide was deposited as temporary protection. So there are three original layers like layer 1 in fig. 1. At this point the polymer sacrificial wafer bonding technique was applied and after the planarization the polysilicon strain gauges were patterned. A new aluminum layer was evaporated directly on the nitride and patterned to create bridge-connections between strain gauges inside the corrugation and bond paths outside the corrugation. Again finally the complete bond layer is removed. 73 Authorized licensed use limited to: UNIVERSITEIT TWENTE. Downloaded on November 11, 2008 at 06:23 from IEEE Xplore.  Restrictions apply.CONCLUSIONS A new flexible technique at low temperature for planarization after a very dee demonstrated. Resist g and layer patterning as well as realization of or cantilevers across deep holes becomes possible. Because of the many different Aluminum Poly-Si possibilities and combinations the sacrificial wafer bonding technique can have a application in micro electro mechanical systems. Results of polymer bonding e contributions of Fig. 7 Basic outline of cross-section and top view of the membrane-pressure sensor, containing a deep corrugation. The patterning of the polysilicon strain gauges and the aluminum contact bridges were realized using the sacriFcia1 wafer bonding technique. Fig. 8 shows SEM photographs of the top view of the first realized samples. The characterization and testing will be discussed elsewhere. Fig. 8 SEM photographs of the top view of realized membrane-pressure sensors with a deep corrugation. Henri Jansen concerning the optimization of the polymer bonding process. These investigations in the programme of the Foundation for Fundamental Research on Matter (FOM) have been Technology Foundat 111 121 IEEE Solid-Sta I31 S.  Kawahit Micromachined solenoids for h itive magnetic sensors, Proc. 6th Int. Con$ Sensors and Actuators (Transducers W), San Francisco, U.S.A., June 24- 141 V.L. Spiering S.  Bouw ger and M. Elwenspoek, Membranes fabricated p single corrugation for package stress reduction and residual stress relief, J. 271991, pp. 1077-1080. 161 181 . -  Micromech. Microeng., R.L. Smith and S.D. Collins, Micr chemical sensors, IEEE Trans. Ele packaging for ces, 35 (1988) 787-792. C. den Besten, R.E.G. Polymer bonding of mi Micro Electro Mechanica Electrochein Soc. 124 Actuators A, 37-38 (1993), 800-804. 74 Authorized licensed use limited to: UNIVERSITEIT TWENTE. Downloaded on November 11, 2008 at 06:23 from IEEE Xplore.  Restrictions apply.

Low temperature sacrificial wafer bonding for planarization after very deep etching

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https://doi.org/10.1109/MEMSYS.1994.555600

Low temperature sacrificial wafer bonding for planarization after very deep etching

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