Patent Document

FIELD OF THE INVENTION 
     The invention relates to the general field of MEMS with particular reference to filling deep trenches with oxide and with the formation of silicon beams. 
     BACKGROUND OF THE INVENTION 
     The fabrication of MicroElectro Mechanical Systems (MEMS) on silicon substrates often necessitates electrical isolation between various suspended. mechanical structures that need to be electrically actuated. In a previous patent (U.S. Pat. No. 5,930,595), we disclosed a deep trench isolation process for sensors and actuators application. The isolation between microstructure and surrounding silicon was realized by oxide filled trenches. That process is suitable for deep trench isolation of up to 10-12 microns. However for higher sensitivity, there is a need for much deeper trenches (40-45 microns). This makes it necessary to have a deeper trench isolation process. 
     The principal limitation imposed by the process described in U.S. Pat. No. 5,930,595 is illustrated in FIG. 1 a . Seen there is silicon body  12  in which deep trenches 104 have been formed. Through use of a release etch, the trench bottoms were widened to form expanded regions 103 which penetrated one another, thereby connecting all trenches and forming silicon islands  110  (FIG. 1 b ). 
     The filling of the trenches is illustrated in FIG. 1 b . First the trench walls were lined with thermal oxide layer  106 . Then filling was completed with the deposition of an oxide layer (TEOS) by means of LPCVD (low pressure chemical vapor deposition). This resulted in the formation of voids such as  109 , whose presence is undesirable because of potential reliability problems. The void formation problem increases in severity with increasing trench depth. 
     A routine search of the prior art was performed. The following references of interest were found: 
     In U.S. Pat. No. 6,051,866, Shaw et al. show a sensor-process with initial steps similar to the present invention. Muenzel et al in U.S. Pat. No. 5,723,353 teach another sensor isolation system. Sakai et al. (U.S. Pat. No. 5,549,785) also describe an isolation process. Sparks et al. in U.S. Pat. No. 5,427,975, teach a method for filling trenches with polysilicon but said polysilicon is not oxidizable, being inserted in order to make electrical connections. In U.S. Pat. No. 5,426,070, Shaw et al. discuss the formation of beams in a silicon body by using trenches followed by a release etch step. These trenches do not get filled with oxide. MacDonald et al. (U.S. Pat. No. 5,393,375) show a similar beam formation process. 
     SUMMARY OF THE INVENTION 
     It has been an object of the present invention to provide a process for filling a deep trench in a silicon body with silicon oxide. 
     A further object of the invention has been that said filling process be free of void formation within the filler oxide. 
     A still further object has been to apply said process to the formation of embedded silicon beams that are surrounded by silicon oxide. 
     Another object of the invention has been to apply said process to the formation of silicon beams that are partly cantilever and partly embedded in silicon oxide. 
     These objects have been achieved by means of a process of deep trench etch and silicon undercut release etch by using oxide spacers to protect the silicon beam sidewalls during release etch. An oxide layer is then formed followed by deposition of a pre-determined thickness of polysilicon which is then thermally oxidized. The polysilicon layer inside the trenches gets fully oxidized resulting in void-free trench isolation. This process creates a silicon island on three sides leaving the third side for interfacing with the sensor/actuator beams. The sensor/actuator is formed by a similar process of deep trench etch and release etch process on the same substrate. These suspended beams of the sensors and actuators are bridged with the silicon islands from the fourth side. The above process finally results in suspended silicon beams connected to electrically isolated silicon islands called anchors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 a  and  1   b  illustrate how voids form when a deep trench is filled using a process of the prior art. 
     FIGS. 2 through 4 show how the trenches are defined and formed. 
     FIG. 5 a  shows trenches that have been lined with thermal oxide. 
     FIGS. 5 b  and  5   c  refer to a second embodiment and illustrate how trenches are filled by first depositing, and then thermally oxidizing, polysilicon. 
     FIG. 5 d  shows the structure after oxide liner formation and polysilicon deposition. 
     FIG. 5 e  shows the structure after thermal oxidation of the polysilicon. 
     FIG. 5 f  shows the structure after removal of surface oxide. 
     FIG. 6 shows the structure after two adjacent trenches have been connected by means of a release etch thereby forming a beam. 
     FIGS. 7 and 8 refer to embodiments two through four and illustrate how trenches are filled by first depositing, and then thermally oxidizing, polysilicon. 
     FIG. 9 shows the structure of FIG. 8 after removal of surface oxide. 
     FIGS. 10 through 12 illustrate different silicon beam configurations achieved through application of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As discussed earlier, there is need for deep, oxide-filled, trenches that are free of trapped voids. The present invention achieves this by forming the trench-filling oxide by thermal oxidation. This approach has several advantages (in addition to eliminating void formation). It makes possible the filling of deeper trenches with oxide. By using thermal oxide (as compared to the LPCVD oxide that is standard in the prior art), a less porous oxide of much better quality is obtained. Additionally, the dielectric strength of thermal oxide is better than LPCVD oxide. 
     For applications involving accelerometers and gyroscopes, the oxide shell that holds the isolation island can be reinforced by a thin unoxidized polysilicon layer. This greatly improves the mechanical stability of the isolation structure for such applications. This new approach can provide excellent trench isolation for 40-50 micron deep trenches with aspect ratios as high as 20. 
     The process of the present invention will now be disclosed through a description of four embodiments. 
     First Embodiment 
     Referring now to FIG. 2, the process of this embodiment begins with the provision of silicon body 12 on whose upper surface silicon oxide layer  21  is deposited. Photoresist layer  22  is then laid down and patterned to form an etch mask that includes openings such as  23  that define the trench. All unprotected silicon oxide is then removed, resulting in the formation of hard mask 31 as seen in FIG.  3 . 
     Using both masks, trenches 41 are then etched to a depth of at least 40-50 microns. The hard mask ensures accurate dimensioning of the trenches even if the photoresist is attacked by the etchant. Etching of the trenches was achieved as follows: 
     In an STS SCP system, using a cyclical etch process—etching with SF 6  (100 SCCM), O 2  (10 SCCM), and C 4 F 8  (40 SCCM); 600 watt coil power, 14 watt platen; 7.5 seconds cycle time; passivating with C 4 F 8  (180 SCCM); 600 watt coil power; 0.5 seconds cycle time; pressure 22-30 mtorr at 54% valve position, for a total process time between about 40 and 50 minutes (depending on required trench depth), for an etch rate of about 1.2 microns per minute. 
     After removing all photoresist, all silicon oxide (notably hard mask  31 ) is also removed. This sequence of steps are shown in FIGS. 2,  3 ,  4   a  and  4   b . Then, a new layer of silicon oxide (layer  51  in FIG. 5 d ) is formed on all exposed surfaces by means of thermal oxidation or LPC TEOS deposition. Layer  51  has a thickness between about 4,000 and 6,000 Angstroms. This is followed by a key feature of the invention which is the deposition of conformal polysilicon layer  52  over silicon oxide layer  51 . The polysilicon is deposited to a thickness between about 4,000 and 6,000 Angstroms by means of the following LPCVD process: 
     using ASM furnace LB45 at a temperature about 580° C.; pressure about 300 mtorr; with SiH 4  at about 165 SCCM, for a deposition rate of about 0.33 microns/minute, deposition time depending on requirements. 
     As shown in FIG. 5 e , in another key step, polysilicon layer  52  is then thermally oxidized though its full thickness by means of wet thermal oxidation. This is achieved by means of a stabilizing temperature of about 700° C. which is ramped up to about 1,050° C., for wet oxidation using oxygen at 500 SCCM and hydrogen at 6 SLPM (standard liters per minute) for as long as needed, followed by a nitrogen anneal (5 SLPM) at about 1,050° C. for about 30 minutes, after which the temperature is ramped back down to about 700° C. 
     Through careful choice of the thicknesses of layers  51  and  52  (the trench has a width that is between about 2 and 3 times the combined thickness of layers  51  and  52 ), as the oxide grows, opposing sides of the trench will just meet, causing the trench to fill with oxide  55  and avoiding the formation of any voids. The surface oxide layer can then be removed, giving the final structure the appearance seen in FIG. 5 f.    
     As illustrated in FIG. 10, oxide filled trench  55  can stand alone within silicon body 12 or can take the form of multiply connected trenches that surround a series of silicon beams  12 , as shown in FIG.  11 . 
     Second Embodiment 
     As in the first embodiment, the process of this embodiment begins with the provision of silicon body  12  on whose upper surface silicon oxide layer  21  is deposited. Photoresist layer  22  is then laid down and patterned to form an etch mask that includes openings such as  23  that define the trench. All unprotected silicon oxide is then removed, resulting in the formation of hard mask  31  as seen in FIG.  3 . 
     Using both masks, trenches  41  are then etched to a depth of at least 40-50 microns, The hard mask ensures accurate dimensioning of the trenches even if the photoresist is attacked by the etchant. A typical aspect ratio for the trench would be between about 15 and 25, depending on trench depth and width. Etching of the trenches was achieved by means of a deep RIE process, as follows: 
     using an STS multiplex ICP system, etching with SF 6  (100 SCCM), O 2  (10 SCCM), C 4 F 8  (40 SCCM), 600 watt coil power, and 140 watt platen power. This was followed by passivation in C 4 F 8  (180 SCCM) with 600 watt coil power for a 5 second cycle time. Pressure was 22-30 mtorr at 54% fixed valve position, for an etch rate of about 1.2 microns/minute, with total etch time depending on required trench depth. 
     After removing all photoresist, the silicon oxide (notably hard mask  31 ) is still retained. Then, a new layer of silicon oxide (layer  51  in FIGS. 5 a  and  5   b ) is formed on all exposed surfaces by means of thermal oxidation or deposition, following which it is selectively removed from the top and floor of the trench. Layer  51  is deposited to a thickness between about 4,000 and 6,000 Angstroms. 
     This is followed by the formation of beam  61  (FIG. 5 c ) which is achieved by subjecting all exposed silicon to a release etch which causes all silicon  62  between the floors of trenches  41  to be removed. 
     At user&#39;s discretion, either of the following processes may be used for the release etch (STS multiplex ICP system}: 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 XeF 2  release etch 
                 SF 6  plasma release etch 
               
               
                   
                   
               
             
             
               
                   
                 XeF 2  1.6 torr 
                 SF 6  160-200 SCCM 
               
               
                   
                 pressure 90 mtorr 
                 pressure 30-40 mtorr 
               
               
                   
                 temperature 10° C. 
                 coil power 1,000 watts 
               
               
                   
                 time 15-30 minutes 
                 time 2-3 minutes 
               
               
                   
                 etch rate 1 micron/minute 
                 platen power 10 watts 
               
               
                   
                   
               
             
          
         
       
     
     Then, all silicon oxide (notably layers  31  and  51 ) is removed, following which fresh silicon oxide layer  64  is formed on all exposed surfaces by means of thermal oxidation or LPCVD TEOS deposition. The latter consisted of TEOS (about 100 SCCM), at a temperature of about 720° C. and a pressure of about 150 mtorr, for a deposition rate of about 25 Angstroms/minute. 
     Then, in a key feature of the invention, conformal polysilicon layer  52  is deposited over silicon oxide layer  64 . The polysilicon is deposited to a thickness between about 4,000 and 6,000 Angstroms by means of LPCVD (SiH 4  at 165 SCCM, at about 580° C. and a pressure of about 300 mtorr) for a deposition rate of about 0.33 microns/minute. 
     As shown in FIG. 8, in another key step, polysilicon layer  52  is then thermally oxidized though its full thickness by means of wet thermal oxidation. This is achieved by means of a stabilizing temperature of about 700° C. which is ramped up to about 1,050° C., for wet oxidation using oxygen at 500 SCCM and hydrogen at 6 SLPM (standard liters per minute) for as long as needed, followed by a nitrogen anneal (5 SLPM) at about 1,050° C. for about 30 minutes, after which the temperature is ramped back down to about 700° C. 
     Through careful choice of the thicknesses of layers  51  and  52  (the trench has a width that is between about 2.5 and 3 times the combined thickness of layers  51  and  52 ), as the oxide grows, opposing sides of the trench will just meet, causing the trench to fill with oxide  85  and avoiding the formation of any voids. This is illustrated in FIG.  8 . The surface portion of  85  can then be removed, giving the final structure the appearance seen in FIG.  9 . 
     As illustrated in FIG. 11, a plurality of silicon beams 61 may be formed. These are anchored to main silicon body  12  at one end and are surrounded everywhere else (except their top surface) by silicon oxide  85 . 
     Third Embodiment 
     This embodiment is similar to the second embodiment but with additional process steps after the structure has reached the stage illustrated in FIG.  9 . This additional processing comprises the formation of a cantilever beam (such as 112 in FIG. 12) by masking the upper surface  113  (seen here as a lower surface with mask not shown) and then etching unprotected parts of surface  113  so that part of the beam becomes cantilever  112 , which extends away from surface  113 , as shown. 
     Fourth Embodiment 
     This embodiment is also similar to the second embodiment, but with one key difference. After the polysilicon layer  52  has been laid down, as illustrated in FIG. 7, the next step of fully oxidizing it by means of wet thermal oxidation, is modified to only partial oxidation so that, instead of filling the trench entirely with silicon oxide, it becomes filled with a laminate of silicon oxide-polysilicon-silicon oxide (not shown) that has greater mechanical strength than silicon oxide alone. Said laminate surrounds the silicon beam on four sides and, as in the oxide only case, is free of voids. 
     As in the earlier embodiments, the trench has a width that is between about 2.5 and 3 times the combined thickness of silicon oxide layer  64  and the polysilicon layer  52 , while silicon oxide layer  64 , the remaining (unoxidized) polysilicon layer, and the oxidized polysilicon layer that form the laminate, have relative thicknesses of about 5,000 to 1,000 to 8,000 Angstroms, respectively. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.

Technology Category: 7