Abstract:
A method of preventing copper transport on a semiconductor wafer, comprising the following steps. A semiconductor wafer having a front side and a backside is provided. Metal, selected from the group comprising aluminum, aluminum-copper, aluminum-silicon, and aluminum-copper-silicon is sputtered on the backside of the wafer to form a layer of metal. The back side sputtered aluminum layer may be partially oxidized at low temperature to further decrease the copper penetration possibility and to also provide greater flexibility in subsequent copper interconnect related processing. Once the back side layer is in place, the wafer can be processed as usual. The sputtered back side aluminum layer can be removed during final backside grinding.

Description:
BACKGROUND OF THE INVENTION 
     A full-faced nitride film may be maintained on the backside of a wafer to prevent diffusion of copper species into the wafer during further processing. 
     U.S. Pat. No. 5,614,433 to Mandelman describes a method for forming an silicon-on-insulator (SOI) complimentary metal oxide semiconductor (CMOS) integrated circuit (IC), containing NFET&#39;s (N-type field effect transistors) and PFET&#39;s (P-type field effect transistors). An ion implantation (I/I) of aluminum (Al) is formed below the channel areas of NFET&#39;s that suppresses backside leakage in the PFET&#39;s and NFET&#39;s. 
     U.S. Pat. No. 5,296,385 to Moslehi et al. describes a method for semiconductor wafer backside preparation to prevent infrared light transmission through the wafer by either heavily damaging the wafer backside or heavily doping the back side wafer, and forming a top layer of silicon nitride over the wafer backside. 
     U.S. Pat. No. 5,360,765 to Kondo et al. describes a method for forming electrodes of a semiconductor device by forming a contact metal film over a silicon substrate after a cleaning process using a reverse sputtering of argon ions, and then forming a nickel film as a soldering metal on the contact metal film. The amount of argon atoms at the interface between the silicon substrate and the contact metal film is controlled to less than 4.0×10 14  atoms/cm 2 . 
     U.S. Pat. No. 5,964,953 to Mettifogo describes a process for removing aluminum contamination from the surface of an etched semiconductor wafer. The semiconductor wafer is first lapped in a lapping slurry containing aluminum, then etched, and then immersed in an aqueous bath comprising an alkaline component and a surfactant. 
     U.S. Pat. No. 5,958,796 to Prall et al. describes a method of cleaning waste matter from the backside of a semiconductor wafer substrate. A cover layer is deposited over the front side of the wafer and waste matter is removed from the backside of the wafer either by etching the waste matter from the backside with a suitable etchant, or by planarizing the backside of the wafer with a chemical-mechanical planarization (CMP) process. 
     U.S. Pat. No. 3,997,368 to Petroff et al. describes a method of eliminating stacking faults in silicon devices by a gettering process. Before any high temperature processing steps, a stressed layer is formed on the backside of the wafer and is annealed to cause the nucleation sites to diffuse to a localized region near the back surface. The stressed layer may comprise silicon nitride or aluminum oxide. Enhanced gettering is achieved if interfacial misfit dislocations are introduced into the back surface by, for example, diffusion of phosphorus therein, before forming the stressed layer. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to prevent transportation of copper on the back side of a wafer by a gettering process comprising sputtering a thin gettering layer on the wafer back side. 
     Other objects will appear hereinafter. 
     It has now been discovered that the above and other objects of the present invention may be accomplished in the following manner. Specifically, a semiconductor wafer having a front side and a backside is provided. Metal, selected from the group comprising aluminum, (aluminum alloys, i.e.: aluminum-copper, aluminum-silicon, and aluminum-copper-silicon) is sputtered on the backside of the wafer to form a layer of metal. The back side sputtered aluminum layer may be partially oxidized at low temperature to further decrease the copper penetration possibility and to also provide greater flexibility in subsequent copper interconnect related processing. Once the back side layer is in place, the wafer can be processed as usual. The sputtered back side aluminum layer can be removed during final backside grinding. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the method of preventing copper transportation by sputtering a gettering layer on the back side of a wafer according to the present invention will be more clearly understood from the following description taken in conjunction with the accompanying drawings in which like reference numerals designate similar or corresponding elements, regions and portions and in which: 
     FIGS. 1 and 2 schematically illustrate in cross-sectional representation a first embodiment of the present invention. 
     FIGS. 3 and 4 schematically illustrate in cross-sectional representation a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Accordingly FIG. 1 shows a schematic cross-sectional diagram of semiconductor wafer  10  that may be comprised of silicon or silicon-on-insulator (SOI). Wafer  10  has front side  11  and backside  13 . Front side  11  of wafer  10  is the side of wafer  10  upon which semiconductor devices will be formed, e.g. source, drain, FETs, etc. 
     Unless otherwise specified, all structures, layers, etc. may be formed or accomplished by conventional methods known in the prior art. 
     First Embodiment 
     FIG. 2 (with the common FIG. 1) illustrates the first embodiment of the present invention. 
     As shown in FIG. 2, aluminum or an aluminum alloy such as aluminum-copper, aluminum-silicon, or aluminum-copper-silicon more preferably aluminum is deposited on the backside  13  of wafer  10  to form aluminum/aluminum alloy layer  12  before electroplating between copper (shown as layer  15 ) on the front side  11  of wafer  10  as discussed below. A barrier layer (not shown) such as binary and tertiary metal nitrides, e.g. TiN or TaN, or metal silicon nitrides, e.g. Ta x Si y N z , may also be used in conjunction with aluminum/aluminum alloy layer  12 . 
     For purposes of illustration, aluminum/aluminum alloy layer  12  will be referred to as aluminum layer  12 . Wherever “aluminum” is used hereafter, it will be considered to be either aluminum or any of the aluminum alloys specified above unless specifically noted otherwise. 
     A key point of the invention is that aluminum is a ‘natural getterer’ for copper and will bind elemental copper very securely. That is mobile ionic or elemental copper become attached to, or trapped in, aluminum layer  12  and are thus prevented from traveling or transporting into wafer  10  surfaces  11 ,  13 . This phenomenon is called gettering. Thus, cross contamination from copper will be eliminated from, for example, the fabrication tools. 
     The aluminum is preferably deposited by sputtering to form thin aluminum layer  12 , but in any case before deposition of copper, at the parameters including: pressure—from about 0.1 to 9000 mTorr; DC power—from about 500 to 10,000 W; nitrogen gas flow—from about 0 to 30 sccm; and argon gas flow—from about 0 to 100 sccm. 
     Aluminum layer  12  is from about 20 to 10,000 Å thick, more preferably from about 750 to 7000 Å thick, and most preferably from about 1000 to 5000 Å thick. 
     Copper may then be deposited over the front side  11  of wafer  10  to form, for example, plugs, lines, bonding pads, etc., by, for example electroplating, PVD or CVD. For purposes of illustration, any copper so deposited will be represented by copper layer  15 . 
     If wafer  10  is immersed into a copper electroplating bath, the pH of the electroplating bath must be compatible with aluminum to protect aluminum layer  12 . Specifically, the electroplating bath pH must be from about 4 to 9. Alternatively, hot plating of aluminum can be performed. 
     Wafer  10  may then be further processed to form semiconductor devices thereon. During the passivation layer deposition (not shown), any chemi-absorbed copper  14  on surface  12   a  of thin aluminum layer  12  will combine with aluminum and thus would not be mobile any more. 
     Absorbed copper  14  is only shown for illustration purposes. Aluminum layer  12  may getter other impurities besides absorbed copper  14 . 
     Thin aluminum layer  12  may then be removed from backside  13  of wafer  10  during a back-grinding step, e.g. after BEOL (back-end of line). 
     Second Embodiment 
     FIGS. 3 and 4 (with the common FIG. 1) illustrate the alternate embodiment of the present invention wherein the sputtered aluminum layer is partially oxidized to extend the permissible pH range of the copper electroplating bath. (As in the first embodiment “aluminum” is considered to be either aluminum or any of the aluminum alloys specified in the first embodiment unless specifically noted otherwise.) 
     As shown in FIG. 3, aluminum is deposited on the backside  13  of wafer  10  to form aluminum layer  16  before electroplating copper on the front side  11  of wafer  10  as discussed below. 
     The aluminum is preferably deposited by sputtering to form thin aluminum layer  16  at the following parameters: pressure—from about 0.1 to 9000 mTorr; DC power—from about 500 to 10,000 W; nitrogen gas flow—from about 0 to 30 sccm; and argon gas flow—from about 0 to 100 sccm. 
     Aluminum layer  16  is from about 20 to 10,000 Å thick, more preferably from about 750 to 7000 Å thick, and most preferably from about 1000 to 5000 Å thick. 
     Aluminum layer  16  is then partially oxidized by treating with oxygen to form oxidized aluminum layer  17  over aluminum layer  16  at the following parameters: 
     oxygen flow: from about 100 to 100,000 sccm; RF or microwave power: from about 200 to 10,000 W 
     time: from about 5 to 3600 seconds: and 
     temperature: from about 50 to 400° C. 
     Other gasses that are added to oxygen include a forming gas (N 2 /H 2 ) and fluorocarbon. The flow ratio of oxygen to fluorocarbon, and the flow ratio of oxygen to nitrogen within the N 2 /H 2  forming gas is from about 1:1 to 1000:1. 
     Oxidized aluminum layer  17  has a thickness of preferably from about 100 to 500 Å, and more preferably from about 200 to 400 Å. 
     Alternatively, aluminum oxide can be deposited by sputtering an aluminum oxide target at the parameters including: pressure—from about 0.1 to 9000 mTorr; RF power—from about 500 to 10,000 W; argon gas flow from about 0 to 100 sccm; an nitrogen gas flow from about 0 to 30 sccm. 
     Alternatively, aluminum oxide can also be formed by reactive sputtering of aluminum target with oxygen conditions. 
     As shown in FIG. 4, copper may then be deposited over the front side  11  of wafer  10  to form, for example, plugs, lines, bonding pads, etc., by, for example electroplating. For purposes of illustration, any copper so deposited will be represented by copper layer  19 . 
     If wafer  10  is immersed into a copper electroplating bath, the pH of the electroplating bath must be compatible with partially oxidized aluminum to protect aluminum layer  16 /oxidized aluminum layer  17 . Since layer  16  is partially oxidized (i.e. oxidized layer  17 ), the permissible pH range of the electroplating bath is greater than in the first embodiment discussed above. 
     Specifically, the electroplating bath pH may be from about 4.1 to 9.1. 
     Wafer  10  may then be further processed to form semiconductor devices thereon. During the passivation layer deposition (not shown), any chemi-absorbed copper  18  on surface  17   a  of thin oxidized aluminum layer  17 /aluminum layer  16  will combine with aluminum and thus would not be mobile any more. 
     Absorbed copper  18  is only shown for illustration purposes. Oxidized aluminum layer  17 /aluminum layer  16  may getterer other impurities besides absorbed copper  18 . 
     Thin oxidized aluminum layer  17 /aluminum layer  16  may be removed from backside  13  of wafer  10  during a back-grinding step, e.g. after BEOL (back-end of line). 
     Regardless whether the first embodiment of second embodiment of the present invention is selected, a single aluminum sputter layer  12 ,  16  acts as a gettering layer for copper and can last through BEOL (back-end of line) processing. Further, aluminum gettering layer  12 ,  16  may be finally removed during back grinding step. 
     While particular embodiments of the present invention have been illustrated and described, it is not intended to limit the invention, except as defined by the following claims.