Abstract:
A method for forming a frontside substrate contact on a Silicon-On-Insulator wafer in the presence of planarized contact dielectric is described. The method offers the improvement of reducing the etch selectivity requirements while allowing simultaneous etching and metallization of gate, source, drain and substrate contacts.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     U.S. GOVERNMENT RIGHTS 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to device processing of Silicon-On-Insulator (SOI) wafers, and more particularly, relates to a method of forming a frontside contact to the silicon substrate of a SOI wafer. 
     2. Description of the Prior Art 
     SOI Technology has been shown to provide many advantages over bulk silicon technology. These advantages include higher speed, lower power and resistance to certain forms of radiation. 
     In many applications it is desirable to ground or bias the bottom substrate of a SOI circuit. In order to make an electrical connection at the backside of the wafer it requires additional wafer processing. In most of the more advanced packaging technologies such as flip chip and die stacking, making a connection to substrate silicon via the back of the die is cost prohibitive. Therefore, connecting the substrate to a standard topside metallization pad is necessary. However, the challenge of forming this contact during normal processing flow without adding major requirements to the standard processes without this topside contact is significant. This is especially true in the presence of highly planarized contact dielectric processes (i.e. Chemical Mechanical Polishing (CMP) and photoresist planarized contacts) required for today&#39;s deep submicron lithography where the thickness of the contact dielectric over the substrate contact may be in the range of 0.2 to 1.0 micron more than the thickness over the thickest region in the normal circuit area, which would typically be the source/drain contact areas. Scaling designs into the deep submicron range normally requires a high degree of planarity in the contact dielectric in order to be able to lithographically define contact and metal interconnect layers. Even without a frontside contact to the substrate, the selectivity of the contact etch is already required to be quite high. In the prior art, in order to make simultaneous contact to gate polysilicon, source and drain regions, and the underlying substrate, the selectivity of contact etch to gate polysilicon must be increased significantly in order to be able to etch the planarized contact dielectric all the way down to the silicon substrate. 
     Brady, et al, in U.S. Pat. No. 5,314,841, “Method of Forming a Frontside Contact to the Silicon Substrate of a SOI Wafer” describes a frontside contact formation process. This prior art has the weakness of increasing the etch selectivity requirements of the process significantly when compared to a process that does not include the frontside substrate contact. 
     It is therefore an object of the present invention to provide a frontside substrate contact without significantly increasing the selectivity requirements of the etch of the planarized contact dielectric. 
     It is another object of the present invention to teach a simple method to create a sloped oxide edge using reflowed photoresist with an oxide etch non-selective to photoresist. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention solves these and other needs by providing a method which both forms the substrate contact as part of the normal processing sequence without exposing the substrate silicon to the gate etch and uses a local interconnect to raise said substrate contact to a level above the source/drain areas prior to depositing the contact dielectric. It is assumed in the process flow stated below that the top silicon is removed at some time prior to Step  2 . 
     1. Proceed with standard device processing steps up through gate etch. 
     2. Define a substrate contact region with photoresist. 
     3. Bake the photoresist to create a positive edge profile. 
     4. Etch a hole or trench down to the substrate through the exposed hole in the photoresist. 
     5. Remove the photoresist and continue with normal processing. 
     6. Cover the substrate contact region while implanting a dopant of the opposite type as the substrate. 
     7. Open the substrate contact region while implanting a dopant of the same type as the substrate. 
     8. Open the substrate contact region for normal silicidation. 
     9. Deposit and define a local interconnect from the silicide region within the substrate contact region up to the top of the oxide layers. 
     10. Deposit and planarize a contact dielectric. 
     11. Within normal metalization processing, etch through the planarized contact dielectric down to the top end of the local interconnect, as well as the source, drain, and gate contact locations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view of the placement of photoresist defining the substrate contact site on the SOI wafer after reflow of said photoresist. 
     FIG. 2 is a cross sectional view of the wafer after oxide etching. 
     FIG. 3 is a cross sectional view of the doping taking place in the substrate contact site. 
     FIG. 4 is a cross sectional view of the final topside substrate contact after silicidation, local interconnect processing, contact dielectric planarization, and metallization. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The first requirement is the removal of the top silicon of the SOI wafer in the site where the frontside contact is to be formed. Most SOI technologies remove the silicon from some portion of the wafer either by etching or by oxidizing the silicon to form a field oxide. In the process described in the preferred embodiment of the present invention, it is assumed that the silicon was removed at some earlier process step via Reactive Ion Etching (RIE) that is selective to the buried oxide, and that a field oxide similar to that used for Shallow Trench Isolation (STI) is deposited above the buried oxide layer. 
     It can be seen in FIG. 1 that there now exists a wafer consisting of a field oxide layer  4  over a buried oxide layer  6  over a silicon substrate  8 . Defining material  2  is placed on the top surface of the field oxide  4  to define the region  1  where the topside contact is to be placed. In the preferred embodiment, the defining material  2  is in the form of a photoresist mask, and will be referred to as such from this point forward. The wafer is baked to cause a partial reflow of the photoresist  2 , giving it the positive edge profile  7 . A positive edge profile means that, when the edge is viewed in a cross-sectional profile view, the profile is either strictly increasing or strictly decreasing. Relating this in mathematical terms, if the profile was thought of as being the plot of a function, it could be referred to as being a monotonic function. 
     A hole or trench  3  is then etched down through the oxide layers  4  and  6  to the substrate  8  through the opening in the photoresist  2  as shown in FIG.  2 . This hole or trench can be used for STI between adjacent active devices on the wafer. While this could be done with a wet etch, a Reactive Ion Etch is used in order to maximize size and edge profile control. This is especially needed in the presence of a Chemical Vapor Deposition (CVD) STI top field oxide layer  4  which can wet etch at a much higher rate than the underlying buried oxide layer  6 . The positive edge profile  9  needed in order to be able to create a connection later from the substrate contact point up to the top of the field oxide  4  is created through the combination of the aforementioned high temperature bake of the photoresist  2 , typically in the 155-190 degree C. temperature range, and a RIE in which the oxide to photoresist selectivity is adjusted to transfer the positive slope (smoothed from the bake) of the photoresist edge  7  downward into the oxide layers  4  and  6 . Oxide to photoresist etch rates of 1-2:1 are preferred but higher selectivity may be used depending upon the photoresist reflow profile and the desired step coverage of the local interconnect. 
     Most RIE chemistries that are non-selective to photoresist are also non-selective to silicon. Therefore, the non-selective RIE should not be carried all the way down to the silicon substrate  8 . It is desirable to maintain the non-selective etch as long as possible before switching to a highly selective etch. An isotropic last step (either wet or dry) is preferred to ensure the etch profile remains positive all the way to the top. It should be noted that the original photoresist  2  must be thick enough to not be consumed by the RIE, and to protect the underlying circuitry from wet etch if used. The exact location of the previous step in the process flow is quite flexible, but it should be placed before P-channel source/drain implant cut and implant (assuming a P-type substrate), and either after gate etch or before gate oxidation so as to avoid any attack of the substrate silicon  8  by the gate polysilicon over etch. 
     Alternatively an RIE etch which creates a sufficiently positive profile in field oxide  4  and buried oxide  6  could be used in lieu of the previously described photoresist reflow. In a manner consistent with a standard process flow, the substrate contact site  3  is left uncovered by photoresist  10  as in FIG. 3 when implanting a dopant  14  of the same type as the substrate  8 , but is covered by photoresist (not shown) when implanting a dopant of the opposite type as the substrate  8 . The result is a doped region  12  of the same type as the substrate  8  in the substrate contact site  3 . 
     The silicidation of the substrate contact, gate, source and drain can all be made simultaneously in a commonly known manner. In FIG. 4, the silicide  24  can be seen in the doped region  12 . A local interconnect layer  20 , common to many advanced CMOS submicron processes, is then placed to route the connection to the substrate  8  along the positive edge profile  9  of buried oxide layer  6  and field oxide layer  4  up to the top of the field oxide layer  4 , placing it at a level typically somewhere between source, drain and gate polysilicon (not shown). This eliminates the need for any changes (i.e. increased etch selectivity) in over etch of the planarized contact dielectric  22  relative to a process without the substrate contact. Planarized contact dielectric  22  may be of a variety of oxides, for example, phosphorous-doped silicon dioxide (P-glass) borophosphosilicate glass (BPSG), or high density plasma (HDP) which has been planarized by a CMP process. Local interconnects have typically been used for very short routing connections to source drain and gate. Local interconnects usually have a self-aligned contact capability which improve density. Local interconnect  20  may be formed using various silicides, for example, titanium silicide (TiSi 2 ). In one method, a titanium amorphous silicon layer is deposited and the amorphous silicon is defined. The amorphous silicon is then reacted with the titanium to form the titanium silicide local interconnect. The forming of the local interconnect can be part of the salicide process itself or it can be a separate process performed after salicidation. A variety of methods including simply depositing titanium silicide or titanium nitride could be used to form the local interconnect of the present invention. In a commonly known manner, a metallized contact is formed including a contact plug  18  typically formed of tungsten and contact trace  16  which is typically aluminum. 
     Thus since the invention disclosed herein may be embodied in other specific forms without departing from the spirit or general characteristics thereof, some of which forms have been indicated, the embodiments described herein are to be considered in all respects illustrative and not restrictive. The scope of the invention is to be indicated by the appended claims, rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.