Abstract:
A method for forming salicides with lower sheet resistance and increased sheet resistance uniformity over a semiconductor process wafer including providing a semiconductor process wafer having exposed silicon containing areas at a process surface; depositing a metal layer including at least one of cobalt and titanium over the process surface; carrying out at least one thermal annealing process to react the metal layer and silicon to form a metal silicide over the silicon containing areas; and, wet etching unsilicided areas of the metal layer with a wet etching solution including phosphoric acid (H 3 PO 4 ), nitric acid (HNO 3 ), and a carboxylic acid to leave salicides covering silicon containing areas at the process surface.

Description:
FIELD OF THE INVENTION 
   This invention generally relates to semiconductor processing methods for forming salicides (self-aligned silicides) over silicon or polysilicon areas of a deep-submicron CMOS semiconductor device and more particularly to a selective wet etching method to remove residual metal and metal nitrides during salicide formation. 
   BACKGROUND OF THE INVENTION 
   In the integrated circuit industry today, hundreds of thousands of semiconductor devices are built on a single chip. Contact resistances between functioning areas of a CMOD device, such as a source or drain region, is critical to the functioning of a CMOS device, for example a transistor. For example, metal interconnect features are formed to connect source and drain regions to other parts of a functioning semiconductor device. Source and drain regions of a transistor are doped portions of a semiconductor substrate, for example single crystal silicon. The source and drain regions are typically formed by implanting ions in the silicon substrate to achieve n-doped regions or p-doped regions. To prevent the contamination of the silicon substrate by contacting metal interconnects, an intermediate layer of a metal silicide is formed over the silicon substrate, for example, titanium silicide or cobalt silicide. Metal silicides are thermally stable at higher temperatures and prevent metals from diffusing into the silicon substrate which will alter electrical properties. 
   One requirement of the metal silicide is the necessity for low sheet resistance or contact resistance to the silicon substrate. In this regard, cobalt silicide (e.g., CoSi2) and titanium silicide (TiSi2) have the lowest resistivity and therefore provide a lower contact resistance to the silicon semiconductor substrate. The severity of the effect of increased resistance on the drain side of the transistor depends on whether the transistor is operating in the saturated region or the linear region, where the reduction of drain voltage has less effect if operation is in the saturated region. Increased contact resistance on the source side of the transistor is generally more severe, reducing the effective gate voltage and severely degrading device performance. It has been found that self aligned silicides (salicides) covering the entire source/drain area is the one of the most effective solutions to decreasing contact resistance and improving device performance allowing device scaling below 0.25 microns. 
   One problem in forming salicides having line widths less than about 0.5 microns, is the tendency of titanium silicides to agglomerate when formed overlying gate, source, and drain regions and subjected to high annealing temperatures, typically using a rapid thermal anneal (RTA), also referred to as a rapid thermal process (RTP). For example, in the formation of titanium silicide, typically a two-step process is required to form the low electrical resistance phase of titanium silicide, frequently requiring annealing temperatures of up to 800° C. In smaller line width areas, the titanium silicide has difficulty achieving the nucleation and growth of the crystalline phase required for low electrical resistance, requiring higher annealing temperatures, which frequently causes agglomeration of the silicide. Consequently, cobalt silicide is a preferred material for forming salicides for sub-quarter micron devices since the required phase transformation to form the low electrical resistance crystalline phase takes place at lower temperatures, for example, from about 600° C. to about 700° C. without the problem of silicide agglomeration. 
   In a typical salicide process, a metal, for example titanium or cobalt is deposited to cover the gate, source and drain regions. The metal is then subjected to a two step high temperature anneal where a metal silicide is formed by the diffusion of silicon from underlying areas including silicon or polysilicon into the overlying metal area thereby forming metal silicides. Carrying out the annealing process in nitrogen causes formation of metal nitrides within the metal, for example titanium nitride, slowing the silicon diffusion to prevent what is referred to as bridging, where silicon diffuses into the sidewall regions of the deposited metal along the gate causing a short electrical circuit between the gate region and the source/drain region. The likelihood of bridging increases as the annealing temperature is increased, providing another factor favoring the use of cobalt silicide at least for the formation of salicides over the gate and source/drain regions. 
   Following formation of the silicided areas over the gate and source/drain regions, a wet etching process is carried out to etch away remaining metal of metal nitrides from unsilicided areas, to form self aligned silicides (salicides) over the respective gate and source/drain regions. One problem with the prior art wet etching process for forming salicides is the poor selectivity demonstrated by prior art etching solutions including, for example, standard cleaning 1 (SC-1) and SC-2, which are typically used as sequential cleaning solutions including mixtures of NH4OH—H2O2—H2O, and HCL—H2O2—H2O, respectively. Poor selectivity of the wet etching solution of the metal and metal nitride portions with respect to the silicided portions will result in, on the one hand, underetching, where residual metals or metal nitrides remain on the sidewall spacers of the gate structure, and on the other hand, overetching of the silicided portions over the gate and source/drain regions. As a result, non-selective etching causes non-uniformities over the wafer resulting in out of specification electrical resistances including sheet resistances over a large percentage of the silicided wafer areas. In addition, poor etching selectivity can detrimentally affect gate oxide integrity. The problem of poor selectivity is especially a concern with cobalt silicide formed over narrow line width areas where silicide defects caused by overetching have a significant effect on electrical behavior, for example, forming nanometer sized voids, leading to increased junction leakage. 
   There is therefore a need in the semiconductor processing art to develop a method for a reliable and uniform selective wet etching process to form low sheet resistance salicides over sub-quarter micron semiconductor devices with reliable and uniform electrical behavior. 
   It is therefore an object of the invention to provide a method for a reliable and uniform selective wet etching to form to form low resistance salicides over sub-quarter micron semiconductor devices thereby improving electrical behavior including sheet resistances while overcoming other shortcomings of the prior art. 
   SUMMARY OF THE INVENTION 
   To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention provides a method for forming salicides with lower sheet resistance and increased sheet resistance uniformity over a semiconductor process wafer. 
   In a first embodiment, the method includes providing a semiconductor process wafer having exposed silicon containing areas at a process surface; depositing a metal layer including at least one of cobalt and titanium over the process surface; carrying out at least one thermal annealing process to react the metal layer and silicon to form a metal silicide over the silicon containing areas; and, wet etching unsilicided areas of the metal layer with a wet etching solution including phosphoric acid (H 3 PO 4 ), nitric acid (HNO 3 ), and a carboxylic acid to leave salicides covering silicon containing areas at the process surface. 
   These and other embodiments, aspects and features of the invention will be better understood from a detailed description of the preferred embodiments of the invention which are further described below in conjunction with the accompanying Figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A-1C  are cross sectional side views of a portion of a CMOS transistor showing manufacturing stages for forming salcides according to an embodiment of the present invention. 
       FIG. 2A  is a cumulative distribution graph of sheet resistance measurements over a process wafer surface taken following prior art wet etching processes for forming salicides. 
       FIG. 2B  is a cumulative distribution graph of sheet resistance measurements over a process wafer surface taken following the wet etching process for forming salicides according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The method of the present invention is explained with respect to processing steps included in forming salicides in a sub-quarter micron technology CMOS transistor. It will be appreciated that the method may be used with larger device technologies, but that it is most advantageously used with sub-quarter micron technologies, or where the salicide linewidth is less than about 0.5 microns. It will be appreciated that although direct benefits will be realized according to the method of the present invention by avoiding non-selective etching of silicided portions of the gate and source/drain regions in forming salicides, that other indirect benefits will additionally be realized including the improved electrical performance and reliability of the semiconductor device such as increased gate oxide integrity, more uniform sheet resistance, and reduced junction leakage. It will further be appreciated that although the method of the present invention is advantageously used, and an exemplary implementation detailed with respect to, the formation of and etching of cobalt silicide to form salicides, that the wet etching process of the present invention may be advantageously used for the selective etching of other metal silicides including, for example, titanium silicide. 
   In an exemplary embodiment of the present invention, reference is made to  FIGS. 1A-1C  where cross sectional side views of portions of a semiconductor device are shown at stages in the manufacturing of salicides over gate and source/drain regions of the semiconductor device. For example, referring to  FIG. 1A , is shown a portion of a CMOS transistor structure having a polysilicon gate  12  formed over a gate oxide  14 , and having sidewall oxide spacers  16 A and  16 B formed according to known processes in the art. For example, doped regions in the silicon substrate  10 , include a doped source region, e.g.,  18  and a doped drain region, e.g.,  20 , including lightly doped regions (LDD)  18 A,  20 A, which are formed by conventional ion implantation techniques following patterning of polysilicon gate  12 . subsequently, sidewall spacers  16 A and  16 B, formed of a silicon oxide or other dielectric, are patterned and formed along the sidewalls of the polysilicon gate. Another ion implantation is carried out using the sidewall spacers  16 A and  16 B as an implantation mask to form more heavily doped regions, e.g.,  18 B,  20 B, adjacent to the LDD regions, e.g.,  18 A,  20 A. Electrical interconnects are later created over the gate and source/drain regions to provide electrical communication between the transistor and other device areas where proper electrical functioning is critically dependent on the formation of low contact resistance to the gate and source/drain regions including ohmic-like behavior. 
   Referring to  FIG. 1B , a metal layer  22 , preferably cobalt, is blanket deposited by a conventional PVD method to a thickness of about 20 nanometers to about 100 nanometers. It will be appreciated that titanium metal may be used to form a titanium silicide, however, cobalt is preferred for forming self-aligned silicides (salicides) having linewidths of less than about 0.5 microns due to the more reliable phase transformation to form low sheet resistance salicides at lower temperatures compared to titanium silicide. For example, following deposition of the cobalt metal layer  22 , the semiconductor wafer is subjected to a rapid thermal anneal (RTA) where the wafer is heated in a multi-step process first to about 450° C. and then to about 700° C. to about 750° C. preferably in a nitrogen atmosphere. During the RTA process cobalt silicide is formed over the areas having underlying silicon or polysilicon areas, e.g., the gate  12 , source  18 , and drain  20  regions by diffusion of silicon to react with the overlying metal layer  22  to form a cobalt silicide (CoSi x , e.g., CoSi 2 ). In the case titanium salicide is formed, the metal layer  22  is a titanium layer followed by a multi-step RTA process where a first RTA process is carried out at a temperature of about 620° C. to about 680° C. followed by a second RTA process at temperatures higher than about 750° C. to form a low resistance phase of titanium silicide (e.g., TiSi 2 ) over silicon containing portions of the substrate, i.e., the gate, source and drain regions, and titanium and titanium nitride over non-silicon containing portions of the substrate. 
   Referring to  FIG. 1C , according to the present invention a wet etching process is carried out to selectively etch away the unsilicided portions of the cobalt or titanium metal layer  22 . According to the present invention, an acidic mixture including phosphoric acid (H 3 PO 4 ), nitric acid (HNO 3 ), and a carboxylic acid, preferably acetic acid (CH 3 COOH), is used to selectively etch away the unsilicided portions of the metal layer  22  leaving self aligned silicides, e.g.,  24 A,  24 B, and  24 C over the gate, source, and drain regions, respectively. It will be appreciated that other suitable carboxylic acids include formic acid, propionic acid, valeric acid, oxalic acid, malonic acid, succinic acid, glutaric acid, maleic acid, fumaric acid, phthalic acid, glycolic acid, lactic acid, citric acid, tartaric acid, gluconic acid, adipic acid, and combinations thereof. It is believed the carboxylic acid serves a complexing function that aids the selectivity of the etching process. 
   In one preferred embodiment, the acidic etching mixture includes about 65 to about 75 weight percent phosphoric acid, about 5 to about 15 weight percent nitric acid, about 1 to about 5 weight percent carboxylic acid, preferably acetic acid, with the remaining portion water, preferably deionized water. In a more preferred embodiment, the acidic etching mixture includes about 70 weight percent phosphoric acid, about 10 weight percent nitric acid, about 2 to about 3 weight percent carboxylic acid, preferably acetic acid, with the remaining portion water, preferably deionized water. 
   In another embodiment, the wet etching process includes at least a dipping process with optionally applied ultrasonic scrubbing, for example megasonic scrubbing for at least a portion of the dipping process. In a preferred embodiment, a sequential wet etching process is carried out where a first etching process including dipping and optional ultrasonic scrubbing is carried out in a first etching solution including a hydrogen peroxide (H 2 O 2 ) solution of about 25 to about 35 weight percent H 2 O 2  with the remaining portion water, preferably deionized water. The first etching process is carried out at a temperature of from about 20° C. to about 80° C., more preferably about 40° C. to about 60° C., most preferably about 50° C., for a period of about 1 to about 10 minutes, more preferably, about 3 to about 5 minutes. The first etching process is followed by a second etching process including the acidic etching mixture according to the preferred embodiments. The second etching process includes dipping and optional ultrasonic scrubbing carried out at temperatures of about 60° C. to about 90° C., more preferably about 75° C., for about 20 to about 30 minutes. Following the second etching process, a deionized water rinse and a conventional dry process completes the wet etching process for forming the cobalt salicide. It will be appreciated that the acidic etching mixture including the sequential etching process according to the present invention may be advantageously carried out on titanium metal and titanium nitride to form titanium salicides or local interconnects, for example overlying the cobalt salicide. 
   In using the acidic etching mixture including the sequential etching process according to the present invention, it has been found that sheet resistance (Rs) values of the salicides, for example the cobalt silicide, is greatly improved. For example, salicides formed over N doped silicon and polysilicon showed reduced sheet resistances of about 30 percent compared to the prior art. By comparison, salicides formed over P doped silicon and polysilicon showed reduced sheet resistances of about 10 percent compared to the prior art. In addition, the distribution of sheet resistance values over measured areas of the wafer showed a substantially reduced distribution tail as indicated in a Weibull or cumulative distribution analysis, as is common in the art to represent a large number of measurements over various areas of a process wafer. For example, referring to  FIGS. 2A and 2B , are shown exemplary cumulative distribution graphs of a series of sheet resistance measurements taken over an exemplary wafer according to methods commonly used in the art, for example according to a Van der Pauw four probe method. The vertical axis represents the cumulative percent of sheet resistance measurements below a sheet resistance value represented on the horizontal axis in ohms. In  FIG. 2A  are represented sheet resistance distributions of a cobalt silicide formed over N doped polysilicon using a wet etching method according to the prior art, for example SC-1 and SC-2 wet etching solutions. The area of resistance values is contained within area A 1 . It is seen that a significant distribution tail occurs for resistances greater than about 10 ohms, whereas sheet resistance values less than about 10 ohms occurs for only about 25 percent of the cumulative measurements. In contrast, in  FIG. 2B  is represented a cobalt silicide produced over N doped polysilicon using the wet etching solution and etching process according to a preferred embodiment of the invention, showing a significantly reduce sheet resistance distribution tail of resistance values included in area A 2  where about 85 percent of the cumulative measurements are less than about 10 ohms. As a result, the wet etching method according to preferred embodiments provides more reliable and uniform etching giving lower sheet resistances over a larger portion of the process wafer. 
   Thus, a method has been presented for reliably and selectively wet etching unsilicided portions of a cobalt or titanium metal layer in the formation of a salicide. The wet etching process of the present invention allows achievement of lower sheet resistances and convergent values of sheet resistance over a process wafer with a significantly reduced distribution of sheet resistances. In addition, gate oxide integrity is preserved by providing a more selective wet etching process while reducing junction leakage. The method is especially useful in forming low resistance cobalt salicides over sub-quarter micron semiconductor devices with reliable electrical behavior. 
   The preferred embodiments, aspects, and features of the invention having been described, it will be apparent to those skilled in the art that numerous variations, modifications, and substitutions may be made without departing from the spirit of the invention as disclosed and further claimed below.