Abstract:
A method for sealing a transducer of a type having a diaphragm with an active region and an inactive region, a stress sensing network associated with the active region of the diaphragm, contacts associated with the inactive region of the diaphragm, and lead-outs for coupling the stress sensing network to the contacts. The method comprises oxidizing the transducer to provide a first oxide layer which covers the diaphragm, the stress sensing network, the lead-outs and the contacts. Next, a layer of semiconductive material is deposited over the first oxide layer and is then planarized to provide a planar surface having a substantially flat and bondable surface. Finally, a cover member is bonded to the planar surface of the layer which covers the inactive region of the diaphragm to hermetically seal the stress sensing network and thereby provide a sealed transducer.

Description:
This is a continuation of application Ser. No. 08/709,139, filed Sep. 6, 1996, entitled COVERED, SEALED PRESSURE TRANSDUCERS AND METHOD FOR MAKING SAME, which is a continuation of Ser. No. 08/502,125, filed on Jul. 13, 1995, now abandoned. 
    
    
     FIELD OF INVENTION 
     The present invention relates generally to semiconductor pressure transducers, and more particularly to semiconductor pressure transducers having sealed stress sensing networks. 
     BACKGROUND OF THE INVENTION 
     Semiconductor pressure transducers are frequently used in applications which require operation in harsh environments that are corrosive and/or involve high temperatures. Accordingly, the stress sensing network of transducers used in such applications, must be protected from these harsh environmental conditions in some way in order for the transducer to remain operational over extended periods of time. In the past, surface over-coatings such as silicon nitride, silicon dioxide and the like, have been provided over the stress sensing network to protect it from harsh operating environments. However, such coatings provide only partial protection as the metallized contact areas of the sensing network remain exposed, which presents problems in applications which involve corrosive environments. 
     One method which enables the transducer to remain operational in corrosive and high temperature environments involves exposing the backside of the transducer to ambient pressure while hermetically sealing the stress sensing network located on the frontside thereof, in the active portion of the diaphragm surface. Such a method is described in co-pending U.S. patent application Ser. No. 08/458,405 filed on Jun. 2, 1995 entitled: HERMETICALLY SEALED TRANSDUCER AND METHODS FOR PRODUCING THE SAME, by A. D. Kurtz and assigned to Kulite Semiconductors Products, the assignee herein. The co-pending application describes a hermetic seal which utilizes a PYROCERAM glass frit or similar means, to bond a cover member to the frontside of the transducer to hermetically seal the sensing network. 
     The glass must be used in the above-described application to bond the cover member to the inactive portion of the diaphragm because the fabrication processes used in forming the sensing network of the transducer result in raised or lowered surface features depending on whether the transducer has a silicon-on-oxide (silicon-on-silicon) structure or a diffused non-implanted structure. The raised or lowered surface features such as where the sensing network extends into the contact areas, are not planar with the underlying substrate. This makes it virtually impossible to form a hermetic seal by electrostatically or fusion bonding a cover member to the inactive portion of the diaphragm. For instance, in a silicon-on-oxide transducer structure, the entire silicon network and particularly, the regions on the inactive portion of the diaphragm leading to the metalized contact areas (lead-outs) are elevated from the oxide by several microns. Moreover, a finite space is provided between these raised lead-outs on the order of mils to prevent them from electrically shorting together. The step height of the lead-outs and the finite spaces therebetween create gaps between the cover member and the inactive portion of the diaphragm. These gaps make it virtually impossible to form a hermetic seal between the cover and the inactive portion of the diaphragm by electrostatic or fusion bonding and thus, a glass frit must be used to fill the gaps to create a hermetic cover. 
     Although the glass frit enables the cover member to be bonded to the inactive portion of the diaphragm to hermetically seal the stress sensing network, there are problems associated with this method. For example, the glass frit has a tendency to flow onto the diaphragm during the high temperature sealing process which causes the performance of the sensors to degrade. There are also thermal mismatching issues and other process compatibility issues which require consideration when using a glass frit. All of this makes the glass frit a difficult material to use for fabricating hermetically sealed covers. 
     It is, therefore, an object of the present invention to provide an improved method for hermetically sealing a semiconductor pressure transducer which substantially eliminates the use of a glass frit to bond a cover member to a transducer. 
     SUMMARY OF THE INVENTION 
     A method for hermetically sealing a transducer of a type having a diaphragm with an active region and an inactive region, stress sensing means associated with the active region of the diaphragm, contact means associated with the inactive region of the diaphragm, and interconnection means for coupling the stress sensing means to the contact means. The method comprises oxidizing the transducer to provide a first oxide layer which covers the diaphragm, the stress sensing means, the interconnection means and the contact means. A layer of semiconductive material is then deposited over the first oxide layer. The layer of semiconductive material is then planarized to provide a planar surface having a substantially flat bondable surface. Finally, a cover member is bonded to the planar surface of the semiconductive layer in the area covering the inactive region of the diaphragm to hermetically seal the stress sensing network and thereby provide a hermetically sealed transducer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a detailed understanding of the present invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings wherein: 
     FIG. 1A is a top view depicting a silicon-on-oxide pressure transducer; 
     FIG. 1B is a cross-sectional view depicting of the silicon-on-oxide pressure transducer of FIG. 1A; 
     FIG. 1C is an isometric view of the silicon-on-oxide pressure transducer of FIGS. 1A and 1B; 
     FIG. 1D is a top view depicting a diffused pressure transducer; 
     FIG. 1E is a cross-sectional view depicting the diffused pressure transducer of FIG. 1D; 
     FIGS. 2A-2E are cross-sectional views depicting the fabrication of a silicon-on-oxide pressure transducer; 
     FIG. 3A is a top view of a transducer structure initially processed according to the method of the present invention; 
     FIG. 3B is a cross-sectional side view through line  3 B— 3 B of FIG. 3A; 
     FIGS. 3B-3F are cross-sectional views depicting the fabrication steps which enable a cover to be hermetically bonded to the transducer of FIGS. 3A and 3B using fusion or electrostatic bonding; 
     FIG. 3G is a top view of the transducer shown in FIG. 3F; 
     FIG. 3H is a cross-sectional side view through line  3 H— 3 H of FIG. 3G; 
     FIG. 4A is a cross-sectional side view through line  4 A— 4 A of FIG. 3G; 
     FIG. 4B is a cross-sectional view depicting the hermetic bonding of the cover to the transducer of FIG. 4A; 
     FIG. 4C is a perspective view of transducer of FIG. 4B; and 
     FIGS. 5A and 5B are isometric views which depict the hermetically sealed transducer made in accordance with the present invention mounted on a header. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The method of the present invention can be utilized to hermetically seal the raised feature sensing network of a silicon-on-oxide pressure transducer  10  as shown in the illustrative embodiment of FIGS. 1A-1C. The present invention can also be utilized to hermetically seal the depressed feature sensing network of a diffused pressure transducer  20  as shown in the illustrative embodiment of FIGS. 1D and 1E. 
     The pressure transducer  10  shown in FIGS. 1A-1C, includes a p+ Si stress sensing network  18  which is dielectrically isolated from the carrier substrate  12  by a dielectric isolation layer  16 . The pressure transducer  20  shown in FIGS. 1D and 1E, includes a p+ stress sensing network which is diffused into the carrier substrate  12 . 
     For the purpose of illustration only, the present invention will be described in conjunction with a silicon-on-oxide semiconductor pressure transducer similar to that shown in FIGS. 1A-1C. It should be understood, however, that the method of the present invention can also be used to hermetically seal a diffused semiconductor pressure transducer  20  similar to the one shown in FIGS. 1D and 1E. 
     The fabrication of a single silicon-on-oxide pressure transducer is depicted in FIGS. 2A-2E for illustration only. It should be understood, however, that in the description which follows, more than one transducer can be obtained from the composite wafer structure to be described. 
     A silicon-on-oxide pressure transducer such as the one shown in FIG. 1A, can be fabricated using conventional processes known in the art. A preferred process is described in U.S. Pat. No. 5,286,671 entitled FUSION BONDING TECHNIQUE FOR USE IN FABRICATING SEMICONDUCTOR DEVICES, to A. Kurtz et al., issued February 1994, the subject matter of which is incorporated herein by reference. In accordance with the &#39;671 patent, a silicon-on-oxide pressure transducer is fabricated as follows. 
     In FIG. 2A, an n-type sacrificial/pattern wafer  30  has been selected and high conductivity p+ areas, which form a stress sensing piezoresistive pattern  32 , lead-outs (not visible) and electrical contact regions  34 , have been created therein by diffusion using oxide and/or nitride masking and photolithography. After diffusion, the surface of the wafer  30  is treated with a conductivity-selective etch which does not attack the p+ areas, leaving them raised from the surface as shown in FIG.  2 A. The sacrificial wafer  30  is then fusion-bonded to a carrier wafer  36  shown in FIG. 2B which has been previously treated to obtain a 5000 Å to 15,000 Å silicon dioxide layer  38  thereon. The oxide layer  38  can be formed in a direct oxidation technique whereby the wafer  36  is heated in a high temperature furnace to approximately 1000° C.-1300° C. and passing oxygen over the surface thereof. After fusion bonding as shown in FIG. 2C, the n-type material of the sacrificial wafer  30  is removed using a conductivity-selective etch, leaving only the p+ piezoresistive sensor patterns  32 , the lead-outs (not visible) and the contact areas  34  bonded to the carrier wafer  36  as shown in FIG.  2 D. 
     In FIG. 2E, the carrier wafer  36  itself has been shaped to provide a diaphragm structure  40  using well known etching techniques as taught in U.S. Pat. No. 4,236,137 entitled SEMICONDUCTOR TRANSDUCER EMPLOYING FLEXURE FRAMES to Kurtz et al., November, 1980, and assigned to the assignee herein, which patent is incorporated herein by reference. As can be seen, the stress sensing network  32  and its associated contacts  34  (which have been metallized with PtSi 2 /Ti/Pt layer) are stepped up and form a surface with an uneven topography in combination with the oxide layer  38  making it virtually impossible to form a hermetic seal by electrostatically or fusion bonding a cover member (not shown) to the inactive portion of the diaphragm. In accordance with the method of the present invention, as depicted in FIGS. 3A and 3B, the entire sensing surface of the carrier wafer  36  (as depicted in prior art FIG. 2 e ) has been reoxidized and nitrided resulting in a silicon dioxide (SiO 2 ) and/or silicon nitride (Si 3 N 4 ) layer  42 , also referred to herein as the underlying Si layer  42 , as it underlies the sputtered silicon layer  44 , as further explained below preferably on the order of 1-5 thousand Angstroms on the raised portion of the sensing surface. This insures that the planar surface layer to be later formed, will be dielectrically isolated from the sensor networks, the lead-outs and the metalized contact pads. 
     In FIG. 3C, a layer of silicon  44  is sputtered or otherwise formed over the underlying Si layer  42 . The thickness of the sputtered silicon layer must be greater than the depth of the underlying Si layer  42 , as shown in FIG.  3 B. For example, if the depth of the underlying Si layer is approximately 2 microns, then the thickness of the sputtered Si layer is preferably between approximately 4 and 10 microns. 
     This can be seen in FIG.  3 B. For example, looking at FIG. 3B, the depth of the underlying Si layer  42 , reflected by the distance A, is depicted as being much larger than the depth of the sputtered Si layer  44 , reflected by the distance B in FIG.  3 B. 
     As can be seen in FIG. 3C, the resulting sputtered Si layer  44  is non-planar, as it follows the general contours of the underlying surface features. In FIG. 3D the Si layer  44  is planarized using well known polishing or equivalent techniques which removes the high areas of the sputtered Si and provides a substantially flat planar surface  46 . The resulting thickness of the Si layer  44  after planarizing is preferably between 1 and 2 microns. The resulting planar surface  46 , as shown in FIG. 3D, covers inactive and active regions of the diaphragm structure  40 . In FIG. 3D, the active region overlays the portion of the diaphragm structure that deflects upon the application of a force thereto and is designated between the dotted lines at  48 . The inactive region is designated between the dotted lines at  50 . 
     As is well known in the art, the piezoresistive stress sensing patterns  32  described above, consist of a plurality of piezoresistive elements, each of which essentially forms a variable resistor in one of the four legs of a Wheatstone bridge circuit where the respective resistances of each piezoresistive element varies in proportion to a force or pressure applied to the transducer. As can be seen from FIG. 3D, the piezoresistive sensing pattern  32  is located over the active areas  48  of the diaphragm structure. The electrical contacts  34  are located in the non-active areas  50  of the diaphragm structure and form the circuit nodes of the Wheatstone bridge circuit. The lead-outs (not visible) interconnect the contacts  34  with the piezoresistive elements of the sensing pattern  32 . 
     The planar surface  46  provided in the inactive region  50  of the diaphragm  40  enables a cover wafer to be hermetically sealed thereto using electrostatic or fusion bonding techniques. Prior to hermetically bonding a cover wafer to the sensing side of the carrier wafer  36 , the polished Si layer  44  is coated with an oxide layer  52  as shown in FIG.  3 E. Next as shown in FIGS. 3F and  3 G, holes or windows  90  are etched through the oxide layer  52  the sputtered Si layer  44  and layer  42  to expose the contacts  34 . The contacts  34  are then metalized (metallization is designated by  54 ) so that external leads can be readily attached later on during transducer processing as shown in FIG.  3 H. 
     After the contacts  34  have been metalized, a determination is made as to whether the sputtered Si layer  44  is to remain or be totally removed from the active area  48  of the diaphragm  40 . This determination depends upon desired final diaphragm thickness. If the desired diaphragm thickness is on the order of 1 mil or more, the extra stiffness imparted by leaving the sputtered Si layer  44  over the active portion  48  of the diaphragm  40  will be negligible. Thus, it needn&#39;t be removed as it will act as a protective coating over the sensing network  32  to facilitate further transducer processing. If, however, the desired diaphragm thickness is between approximately 0.2 mils and 0.8 mils, the sputtered Si layer  44  lying over the active portion  48  may be removed in order to obtain the requisite sensor sensitivity. In FIGS. 3H and 3G, the sputtered Si layer  44  lying over the active portion  48  of the diaphragm  40  is shown removed after using well known etching techniques. The lead-outs designated by numeral  33  are visible in FIG. 3G, which is a top view of the carrier wafer  36  of FIG.  3 F. 
     The transducer structure as depicted in FIG. 4A, which is a cross-sectional side view through line  4 A— 4 A of FIG. 3G, will now hermetically sealed. A cover  56  preferably made from Si, PYREX, or PYREX-coated Si is to be bonded to the sensing surface of the carrier wafer  36 . If the cover  56  is to be fusion bonded, a Si cover is used. If, however, the cover  56  is to be electrostatically bonded, then a PYREX or PYREX-coated Si cover is used. 
     As can be seen in FIG. 4B, the cover  56  includes a centrally located depression  58  which has been etched a given depth into the bottom surface thereof. The depth of the depression is selected to allow for full-scale deflection of the diaphragm without making contact with the diaphragm and to act as an over pressure stop at any greater deflection. Further, a series of corner cut-outs (not visible) are formed in the cover  56  which correspond to the metallized contacts  34 . The corner cut-outs  60  can be seen in FIG. 4C which shows the transducer after hermetically bonding the cover  56  to the carrier wafer  36 . 
     In FIG. 4B, the cover  56  is placed onto the sensing surface of the carrier wafer so that it is in position on the peripheral portion or inactive region of the carrier wafer  36 . The cover  56  is then electrostatically or fusion bonded to the carrier wafer  36 . Electrostatic bonding is well known in the art and involves heating the wafers while simultaneously providing a small current flow through the composite assembly. Fusion bonding involves heating the wafers at a temperature of between approximately 900° C. and 1000° C. or less, for approximately 5 to 10 minutes to bond the two wafers together. A fusion bonding technique is described in the earlier mentioned U.S. Pat. No. 5,286,671. 
     If the sputtered layer is not removed over the active portion of the diaphragm, the method of the present invention is additionally useful for pressure transducers used in applications where mounting as depicted in FIG. 5A, is required and the sensor network of the transducer  62  may be exposed to a fluid or other medium that can potentially degrade the sensor performance. For this case, the bonded cover  64  must have an aperture  66  that will connect to the sensing portion  68  of the diaphragm  70 , as well as the corner cutouts  72  previously described. In addition, the carrier wafer  74  is bonded to either a piece of glass or silicon  76  on the carrier wafer&#39;s non-sensing surface. This third wafer  76  will provide the diaphragm clamping as well as make a hermetic seal on the non-sensing surface. If a gage pressure transducer is required, the third wafer structure  76  will contain an aperture (not shown) connecting to the active portion of the non-sensing side of the diaphragm  70 . In the preferred embodiment, the sensor chip is mounted on a header  78  such as was disclosed in U.S. Pat. No. 4,764,747 entitled GLASS HEADER STRUCTURE FOR A SEMICONDUCTOR PRESSURE TRANSDUCER, to A. Kurtz et al., issued Aug. 16, 1988, the subject matter of which is incorporated herein by reference. Leads (not shown) are affixed to the pins  80  of the header  78  and to the contact regions of the sensor chip  62 . It should be noted that the pins  80  of the header  78  are below the level of the contact pads  82  to the sensor  62 . Moreover, the top surface of the cover is above the level of the contact pads  82 , therefore, both the area around the contact pads and all of the space within the header down to the surface of the pins, can be potted with an insulator  84  as shown in FIG. 5B, thus, providing dielectric isolation for the contacts, the leads and the pins and, therefore, affording environmental isolation. 
     Further, the method of the present invention also provides an unexpected advantage in embodiments where the sputtered Si layer remains over the sensor network as earlier described. In such an embodiment, the thermal expansion coefficient of the sputtered Si is greater than the thermal expansion coefficient of the oxide to which the sensor network is bonded. For thin diaphragms, this will result in less curvature as a function of temperature and hence, improve both the temperature coefficient of zero shift as well as the temperature coefficient of sensitivity. 
     It should be understood that the embodiments described herein are merely exemplary and that a person skilled in the art may make many variations and modifications to these embodiments utilizing functionally equivalent elements to those described herein. Any and all such variations or modifications as well as others which may become apparent to those skilled in the art, are intended to be included within the scope of the invention as defined by the appended claims.