Abstract:
A method for the in situ cleaning of a semiconductor deposition chamber utilized for the deposition of a semiconductor material such as titanium or titanium nitride comprising, between wafers, introducing chlorine gas into the chamber at elevated temperature, purging the chamber with an inert gas and evacuating it before introduction of the next wafer. A two-stage between wafer cleaning process is carried out by introducing chlorine into the chamber at elevated temperature, thereafter initiating a plasma without removing the chlorine, purging the chamber with an inert gas and evacuating it before introduction of the next wafer. In a preferred embodiment, a thin protective film of titanium is deposited on the inner sur aces of the chamber prior to utilizing the chamber for he deposition of such material. The protective layer is replenished following each two-stage cleaning.

Description:
This application is a divisional of copending U.S. patent application Ser. No. 09/163,711, filed Sep. 30, 1998, now U.S. Pat. No. 6,242,347, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE DISCLOSURE 
     1. Field of the Invention 
     This invention relates to a method for the in situ cleaning of a process chamber. Specifically, a chlorine-based cleaning is performed in a chemical vapor deposition (“CVD”) chamber to improve deposition process conditions and wafer-to-wafer repeatability. 
     2. Description of the Background Art 
     Semiconductor processing typically is carried out in specialized apparatus comprised of multiple chambers wherein wafers are processed by the deposition and various treatments of multiple layers of semiconductor material in a single environment. In such tools, the plurality of processing chambers and preparatory chambers are arranged in clusters, each served by robotic transfer means. Hence, such tools are commonly referred to as cluster tools. Such tools process semiconductor wafers through a plurality of sequential steps to produce integrated circuits. 
     Regardless of whether semiconductor processing apparatus is configured as described above or in other arrangements known to those skilled in the art, it will be appreciated that the objective is simply to process the greatest number of wafers per unit of time in the most efficient manner. However, it is characteristic of all such apparatus that active chambers, i.e. those wherein semiconductor material is deposited or treated in some other manner, such as etching, must periodically be cleaned of the residues inherently formed during such procedures. It will further be appreciated that it is desirable to have as high a throughput as possible without sacrifice of the quality of the operation carried out in a deposition chamber before it must be cleaned. 
     One particularly desirable feature of cleaning a deposition chamber is to perform the cleaning in situ, i.e. without the necessity of removing the chamber from the cluster tool or exposing the chamber to oxidizing elements present in the atmosphere. In situ cleaning would mean that the cluster tool would not have to be turned off while one or more chambers are cleaned. This is particularly advantageous when it is considered that chambers where different semiconductor processes are carried out become unsuitable for efficient processing at different rates and, hence, do not require cleaning at the same time. Therefore, a process whereby one or more chambers in a multiple chamber tool can be cleaned while others continue to perform their function in the process is particularly advantageous. A cleaning process that meets the needs described above is provided in accordance with the present invention. 
     SUMMARY OF THE INVENTION 
     The disadvantages associated with the prior art are overcome by the present invention of a method of cleaning a process chamber. In accordance with the present invention, wafer-to-wafer repeatability for wafers individually undergoing chemical vapor deposition of metals such as titanium or materials such as titanium nitride, is enhanced by a brief thermal cleaning step between wafers utilizing chlorine as the cleaning gas. This cleaning step significantly extends the interval, in number of wafers processed, before the deposition chamber must be cleaned. In a second aspect of the invention, the deposition chamber may be cleaned in situ by performing a first stage thermal clean utilizing chlorine as the cleaning gas and then carrying out a second stage clean by exciting the cleaning gas with a plasma. The method may also comprise the step of depositing a protective coating of material in the chamber following the two-stage deposition chamber clean. Such a protective coating is titanium that is deposited on the interior surfaces of the chamber by chemical vapor deposition and can be between about 50 Å and 1μ thick. 
     Additionally, in a system for controlling the process sequence of wafers through a semiconductor wafer processing tool containing a plurality of chambers, at least two of which are performing the deposition of titanium or titanium nitride, a general purpose computer system that operates as a special purpose controller when executing a cleaning program for such chambers performs a process including the steps of cleaning the chamber between wafers by thermally energized chlorine, monitoring the conditions in the chamber, invoking a decision to clean the chamber, performing a two-stage clean of the chamber consisting of a thermal chlorine clean followed by a plasma chlorine clean and returning the chamber to the processing sequence being carried out. The computer readable medium may further perform the additional step after the two-stage clean and prior to returning the chamber to the processing sequence of depositing a protective coating of titanium on the interior surfaces of the chamber. 
     With the method as described herein, wafer-to-wafer repeatability is enhanced and throughput of a wafer processing system is optimized as there is little or reduced down time for the system as a whole. The vacuum environment of the system is not broken thereby allowing wafers to be processed in another chamber while the cleaning operation is being performed in a selected chamber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates a sequence of steps of the thermal cleaning process to be carried out between wafers being processed in accordance with the present invention; 
     FIG. 2 illustrates the two-stage cleaning process as contemplated in the present invention; 
     FIG. 3 illustrates a block diagram of a control system that is employed in accordance with the present invention to control the operation and cleaning of a deposition chamber that is used for depositing titanium by CVD in accordance with the present invention; and 
     FIG. 4 depicts an alternate embodiment of the two-stage cleaning process of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed to an improvement in the cleaning of deposition chambers in multistep semiconductor processing apparatus where deposition of certain materials, particularly titanium or titanium nitride, is performed by chemical vapor deposition (“CVD”). Typically, in CVD deposition of titanium or titanium nitride, a precursor gas is admitted to the chamber and thermally decomposed to deposit the material on a target wafer that is suitably biased. The deposition of titanium is usually plasma enhanced in that an RF plasma is created in the chamber to further bias the wafer and enhance decomposition of the precursor. 
     Although various organic titanium precursor compounds, including certain organic compounds, are utilized to deposit titanium films by CVD, it is preferred for a number of reasons to utilize titanium tetrachloride (TiCl 4 ) as the precursor compound. TiCl 4 is admitted to the chamber, typically in combination with a carrier gas such as argon and a reactant gas such as hydrogen. The decomposition of the TiCl 4  results in the deposition of titanium on the target wafer and the formation of HCl and other by-products. Most of these by-products are evacuated to prevent contamination of the deposited film formed on the wafer. There may or may not be an argon plasma added to enhance the overall effect of the deposition process. Although the described deposition is a CVD of titanium, this does not preclude the deposition of other material such as CVD TiN and the like. Such a process is described in commonly assigned application Ser. No. 08/982,872, filed Dec. 2, 1997 and herein incorporated by reference. An example of a suitable chamber and cluster tool capable of performing CVD in the manner described is the TixZ chamber and the Centura 5200 mainframe both manufactured and sold by Applied Materials, Inc. of Santa Clara, Calif. 
     The cleaning process of the present invention comprises both a single stage and a two-stage cleaning, the latter being markedly more aggressive. The cleaning process of the present invention materially improves wafer-to-wafer repeatability and permits a single deposition chamber to process a significant number of wafers with CVD titanium before a periodic cleaning is required. Those skilled in the art appreciate that problems in automated semiconductor processing systems are frequently associated with residual particles in the deposition chamber. The fact that the cleaning process of the present invention reduces such problems represents a significant increase in both wafer-to-wafer repeatability and throughput which is of considerable commercial interest. As will be readily apparent to those skilled in the art, throughput for a given apparatus will vary depending primarily on the processes being performed therein. While the cleaning process of the present invention may be utilized in a number of processes, it is most useful in connection with a CVD titanium or titanium nitride process, particularly one utilizing titanium tetrachloride as the precursor gas. 
     In the TiCl 4  deposition process, as is the case with any such process, the build-up of impurities, i.e., the aforementioned by-products, as well as the unintended deposition of material on certain surfaces, such as a substrate heater, walls and faceplate or lid of the deposition chamber must be periodically removed in order to maintain the repeatability of the deposition process. Repeatability in the context of the present process is defined as preventing or minimizing a decrease in the deposition rate in a CVD process and/or a material change in a process parameter. The parameter utilized to measure repeatability is the sheet resistance of the layer being deposited by CVD since both a loss of deposition rate and a material change in a process parameter will be manifest as a change is sheet resistance. in accordance with the present invention and referring to FIG. 1, a thermal chlorine clean, depicted as a sequence of steps  100 , is performed between processing of individual wafers in a deposition chamber. The method starts at step  102  which is withdrawal of a wafer which has completed processing, i.e. chemical vapor deposition, in the chamber. Optionally, in step  102 , a placebo wafer is inserted into the chamber. The placebo wafer is used to protect the substrate heater during the cleaning process. In a preferred embodiment of the invention, the placebo wafer is a quartz disk having the same size and shape as a semiconductor wafer. In step  103 , a pre-clean purge is performed in the chamber. Specifically, an inert gas such as argon, helium or nitrogen is pumped into the chamber and exhausted to purge the chamber of deposition byproducts. In step  104 , the chlorine gas is admitted to the chamber under elevated temperature, approximately 500-700° C., at a pressure between approximately 0.5-50 Torr and preferably 1 Torr. While pure chlorine can be utilized in the cleaning process of the invention, it is preferably mixed with up to about 99.9% by volume of an inert diluent gas such as argon, nitrogen or helium, preferably argon. The thermal chlorine clean is carried out for from about 5 to 60 seconds, preferably about 10 seconds. 
     Following the thermal chlorine clean step  104 , the flow of chlorine is stopped and the chamber is purged in step  106  with the inert diluent gas. Specifically, the inert diluent gas, preferably argon, is pumped into the chamber to a pressure of from about 0.5-50 Torr and preferably 3 Torr for from about 1 second to 10 seconds, preferably about 5 seconds. In step  108 , the chamber is pumped down to a final pressure of from about 5to 150 mtorr, preferably from about 40-50 mTorr. Finally, in step  110 , a new wafer is introduced into the chamber for processing. 
     While it is understood that the above-discussed thermal cleaning sequence  100  is performed between processing of individual wafers, the sequence can also be run on a periodic basis between a group of wafers. For example, process parameters are monitored to make a determination as to when the periodic sequence is initiated. Such parameters include but are not limited to wafer count, sheet resistance of a deposited film or the like. Upon such determination, the periodic sequence is performed according to the same steps as those for the “between wafer sequence”  100  with slight variation. 
     Specifically, after a wafer count of approximately 200-250 wafers, the method starts at step  102  with withdrawal of a wafer which has completed processing, i.e. chemical vapor deposition, in the chamber and optional insertion of a placebo wafer. In step  103 , a pre-clean purge is performed in the chamber. Specifically, an inert gas such as argon, helium or nitrogen is pumped into the chamber and exhausted to purge the chamber of deposition byproducts. In step  104 , the chlorine is admitted to the chamber under elevated temperature, approximately 500-700° C., at a pressure between approximately 0.5-50 Torr and preferably 20 Torr. While pure chlorine can be utilized in the cleaning process of the invention, it is preferably mixed with up to about 99.9% by volume of an inert diluent gas such as argon, nitrogen or helium, preferably argon. The thermal chlorine clean is performed for approximately 200-1000 seconds and preferably 500 seconds. 
     Following the thermal chlorine clean step  104 , the flow of chlorine is stopped and the chamber is purged in step  106  with the inert diluent gas. Specifically, the inert diluent gas, preferably argon, is pumped into the chamber to a pressure of from about 0.5-50 Torr and preferably 3 Torr. The chamber is then pumped down to a pressure of from about 5-150 mTorr. This pump up and pump down cycle is repeated for from about 1-20 cycles and preferably 10-15 cycles. This repeated cycling of inert, diluent gas removes particles and residual gases from the chamber. In step  108 , the chamber is pumped down to a final pressure of from about 5 to 150 mTorr. Finally, in step  110 , a new wafer is introduced into the chamber for processing. 
     An alternate method for cleaning a process chamber is shown in FIG.  2 . Specifically, a two-stage cleaning process is executed as a “between wafer sequence”  200 . That is, the process is run after every time a wafer is processed in the chamber. As in FIG. 1, the process illustrated in FIG. 2, begins with withdrawal of the wafer being processed and optional insertion of a placebo wafer in step  202 . In step  203 , a pre-clean purge is performed in the chamber. 
     Specifically, an inert gas such as argon, helium or nitrogen is pumped into the chamber and exhausted to purge the chamber of deposition byproducts. The first stage clean is performed by introducing a flow of pure chlorine gas or a mixture of up to about 99.9% by volume of an inert gas as described above with the chlorine gas into the chamber under high temperature (in the range of approximately 500-700° C. and preferably 650° C.) in step  204 . The chamber pressure is regulated to be in the range of approximately 0.5-50 Torr and preferably 20 Torr. This first stage clean is carried out for from about 5 to 50 seconds, preferably about 20 seconds. 
     In the second stage clean, a different form of energy, i.e. a plasma, is utilized. Without withdrawing the chlorine from the chamber, RF power is applied to the chamber to initiate a chlorine plasma  206  under the same temperature conditions as the thermal clean step  204 . The chamber pressure is regulated to be in the range of approximately 0.5-10 Torr and preferably 0.8 Torr. The RF power is in the range of approximately 50-900 watts, preferably 100 to 200 watts. The plasma clean step  206  is preferably shorter than the thermal clean step  204  and is performed for from about 2 to 20 seconds, preferably about 5 seconds. Following the plasma clean  206 , the deposition chamber is purged at step  208  with inert gas, as described above with reference to the “between wafer sequence”  100  shown in FIG.  1 . In the purge step  208 , the flow of pure chlorine gas or mixture thereof is halted and the power is reduced to about one-half of the level utilized for the plasma step  206 . This reduces the pressure in the chamber from about 0-1 Torr and preferably 0.5 Torr. In this way, the plasma is spread out in the chamber yet maintains suspended particles so that they can be pumped out of the chamber. The chamber is next pumped down at step  210  for about 20 seconds to a final pressure of from about 5 to 150 mTorr. Finally, in step  212 , a new wafer is introduced into the chamber for processing. 
     The combination of the thermal chlorine clean followed by the shorter plasma clean provides excellent control of the cleaning operation with a minimum of risk to the deposition chamber components. The high efficiency of the two-stage chlorine clean provided in accordance with the process of the invention enables cleaning of deposition chambers to be carried out in situ after every wafer thereby minimizing downtime for the deposition semiconductor processing apparatus. Specifically, it has been realized that running process  200  after every wafer is so effective that a periodic or extended clean step or some other similar maintenance step is not required to maintain chamber conditions. 
     In a further alternate embodiment of the present invention, the internal surfaces of the deposition chamber are coated with a protective coating, preferably titanium or titanium nitride, to provide protection against the subsequent cleaning stages. This coating protects chamber components, i.e. the substrate heater, pedestal, walls, roof and the like during the cleaning process. The protective coating is applied following the pump down step  210  of the two-stage cleaning process illustrated in FIG. 2 shown as optional step  220  and is performed simply by conducting the CVD deposition process without a wafer on the pedestal and without biasing the pedestal so a deposited material forming the protective coating may be deposited on all of the exposed surfaces of the deposition chamber. 
     The protective coating must be sufficiently thick to protect the deposition chamber components during the cleaning stages, yet not so thick that it would in any way interfere with the normal deposition process. In general, it is preferred that the deposition material is titanium and preferably, the protective coating has a thickness of between about 50 Å and 1μ thick. Those of ordinary skill in the art will readily be able to deposit a titanium coating of the requisite thickness by reference to the deposition capability of the specific apparatus being used. 
     It will be appreciated that, in the embodiment of the present invention utilizing a protective coating, the initial processing of wafers is begun in a deposition chamber having a protective coating in place and, therefore, the application of the protective coating described above is, in reality, a replenishment. The protective coating is particularly useful in protecting the heating element of the deposition chamber which is conventionally aluminum nitride. Without the coating, chlorine will attack (i.e., etch) the surface of the heating element which will subsequently create contaminants that condense and redeposit as aluminum chloride on the cooler chamber surfaces as wafers are processed. The degree of deposition of aluminum chloride may be a factor in reaching a decision to initiate step  220  of the two-stage cleaning illustrated in FIG.  2 . Preferably, however, the decision may be triggered by reaching a predetermined number of wafers processed in the deposition chamber since the last cleaning, or be the result of periodic monitoring of one or more chamber parameters, or by direct inquiry of an operator or the like. If it is determined that the chamber does not require cleaning, the deposition chamber continues to process wafers. 
     FIG. 4 depicts an alternate embodiment of the two-stage cleaning process  400 . Essentially, process  400  is identical to process  200  in terms of the parameters established for each of the steps. The only significant difference between these two methods is the order of the cleaning stages. Specifically: steps  402  and  403  are identical to steps  202  and  203  respectively; a plasma clean step  404  and a thermal clean step  406  of process  400  are identical to steps  206  and  204  respectively of process  200  except that they are interchanged. That is, the plasma is initiated first to plasma clean the chamber, then the plasma is halted and the thermal clean is performed. Steps  408 ,  410 ,  412  and optional step  420  are identical to steps  208 ,  210 ,  212  and optional step  220  respectively. It is offered that the overall two stage cleaning process is effective regardless of the order of the individual cleaning stages. 
     The cleaning process provided herein is particularly advantageous when utilized in semiconductor processing tools comprised of multiple chambers wherein wafers are prepared and processed by the deposition of and treatment of multiple layers of semiconductor material in a single apparatus served by robotic transfer mechanisms. Such a tool is described in commonly assigned U.S. Pat. No. 5,186,718 issued Feb. 16, 1993 to Tepman et al. In such tools where extra chambers may be available, or the same process step may be carried out in more than one chamber, it is possible to utilize the in situ clean of the invention without any downtime for the apparatus. This is clearly a primary advantage of the subject process. 
     The above-described process steps, as illustrated in FIGS. 1,  2  and  4  are advantageously performed in a system that is controlled by a processor-based control unit. FIG. 3 shows a block diagram of a deposition system  300  having a control unit  302  that can be employed in such a capacity. The control unit  302  includes a processor unit  304 , a memory  306 , a mass storage device  308 , an input control unit  314 , and a display unit  310  which are all coupled to a control unit bus  312 . 
     The processor unit  304  forms a general purpose computer that becomes a specific purpose computer when executing programs such as a program for implementing deposition and cleaning process of the present invention. Although the invention is described herein as being implemented in software and executed upon a general purpose computer, those skilled in the art will realize that the method of the present invention could be operated using hardware such as an application specific integrated circuit (ASIC) or other hardware circuitry. As such, the invention should be understood as being able to be implemented, in whole or in part, in software, hardware or both. 
     The processor unit  304  is either a microprocessor or other engine that is capable of executing instructions stored in a memory. The memory  306  can be comprised of a hard disk drive, random access memory (“RAM”), read only memory (“ROM”), a combination of RAM and ROM, or another processor readable storage medium. The memory  306  contains instructions that the processor unit  304  executes to facilitate the performance of the above mentioned process steps. The instructions in the memory  306  are in the form of program code. The program code may conform to any one of a number of different programming languages. For example, the program code can be written in c+, C++, BASIC, Pascal, or a number of other languages. 
     The display unit  310  provides information to a chamber operator in the form of graphical displays and alphanumeric characters under control of the processor unit  304 . The input control unit  314  couples a data input device, such as a keyboard, mouse, or light pen, to the control unit  302  to provide for the receipt of input from an operator or from the computer system operating a complex processing tool. 
     The control unit bus  312  provides for the transfer of data and control signals between all of the devices that are coupled thereto. Although the control unit bus  312  is displayed as a single bus that directly connects the devices in the control unit  302 , the control unit bus  312  can also be a collection of busses. For example, the display unit  310 , input control unit  314  and mass storage device  308  can be coupled to an input-output peripheral bus, while the processor unit  304  and memory  306  are coupled to a local processor bus. The local processor bus and input-output peripheral bus are coupled together to form the control unit bus  312 . 
     The control unit  302  is coupled to the chamber  332  via a plurality of elements of the deposition system  300 , employed in depositing a protective coating on the exposed surfaces of the chamber, depositing material on wafers being individually passed through the chamber and cleaning of the chamber, all in accordance with the present invention. Although each of these elements are not physically contained within the chamber  332 , they are connected to the chamber to alter the chamber environment to create the desired conditions (e.g., temperature, pressure and gas flow rate for two-stage cleaning as described above). For example, gas control panel  316 , vaporizer  326  and flow controllers  330  are connected to the chamber  332  via one or more fluid transfer lines (not shown). Likewise, RF plasma control  324  and signal source  322  are connected to the chamber  332  via one or more electrical lines (not shown). Each of the elements is coupled to the control unit bus  312  to facilitate communication between the control unit  302  and the elements. The elements include the gas control panel  316 , the heating element  318 , the pressure control unit  320 , the signal source  322 , the RF plasma control  324 , vaporizers  326 , the mixer block  328  and flow controllers  330 . The control unit  302  provides signals to the chamber elements that cause the elements to perform the operations required for the process steps described above. In operation, the processor unit  302  directs the operation of the chamber elements in response to the program code instructions that it retrieves from the memory unit  306 . In response to these instructions, the chamber elements are directed to perform the process steps described above. 
     In addition, the general purpose computer control system utilized in semiconductor processing tools typically has the capability to determine when a particular chamber being used to perform a step in a multiple step sequence must be taken off-line for cleaning. This capability is provided through real-time analytical determinations or by relying on predetermined restrictions on the number of wafers that can be processed without significant risk of inability to meet specifications. 
     The present invention is advantageous for such a control system where more than one chamber is being utilized to carry out titanium deposition. For example, the computer can take one such chamber off-line, perform the two stage chlorine cleaning and optionally carry out a titanium deposition as described above prior to wafer processing (i.e., Ti deposition). There is no down time for the apparatus as a whole since wafers to be processed while the cleaning operation is being performed in a given chamber are diverted to another chamber. 
     Although the present invention has been described in terms of particular embodiments, numerous changes can be made thereto as will be known to those skilled in the art. The invention is only meant to be limited in accordance with the limitations of the appended claims.