PROCESS FOR THE CHEMICAL VAPOUR DEPOSITION OF A THIN FILM OF OXIDE ON A SILICON WAFER

Abstract: A process for depositing silicon dioxide on silicon wafers at low pressure from tetraethoxysilane includes a step of in­ troducing a constant flow of nitrogen into a vacuum pump (18) in parallel with exhaust flows from a deposition tube (16) containing the silicon wafers. Tetraethoxysilane is vapourised and flowed into the deposition tube (16) containing the silicon wafers. Tetraethoxysilane is vapourised and flowed into the deposition tube (16) where is pyrolyzes on the silicon wafers to deposit silicon dioxide thereon. Product gases and un­ expended tetraethoxysilane are drawn out of the deposition tube (16) by the vacuum pump (18). The constant flow of nit­ rogen provides a ballast effect to regulate the flow of tetra­ ethoxysilane.

Description: This invention relates to a process for the chemical vapour deposition of a thin film of oxide, particularly silicon dioxide, on a silicon wafer. The development of semiconductor devices, and in particular integrated circuits, has been notable for the continuing increase in performance of commercially available semiconductor devices. One of the reasons for this continuing increase in performance has been the ability of the industry to control to ever narrower tolerances the individual variables associated with the numerous processes required to manufacture integrated circuit devices. This invention relates to an improvement in a low pressure chemical vapour deposition (CVD) process for depositing silicon dioxide on silicon wafers from a vapourised liquid source reactant. One such source reactant is tetraethoxysilane [(C₂H₅0)₄Si)], which is also known as tetraethylorthosilicate. In the semiconductor field, this reactant is generically known by its acronym TEOS and a deposition process utilising TEOS as a reactant is known as a TEOS process. A low pressure CVD TEOS process is disclosed in U.S. Patent No. 3,934,060 to Burt et al. for METHOD FOR FORMING A DEPOSITED SILICON DIOXIDE LAYER ON A SEMICONDUCTOR WAFER. Similar processes are disclosed by A.C. Adams et al., J. Electrochemical Society, Vol. 126, No. 6, p. 1042 (June 1979) and Oroshnik et al., J. Electrochemical Society, Vol. 115, No. 6, p. 649 (June 1968). Burt et al. disclose that low pressure are advantageously utilised to obtain uniform film thicknesses and to achieve adherent dense films having relatively few impurities. One of the drawbacks of the process according to US-A-3 934 060, however, lies in the fact that TEOS is a liquid at room temperature (25 o C.) and pressure (760 torr). Accordingly, it is necessary, as Burt et al. disclose to vapourise the TEOS liquid by heating it. There are specially designed TEOS vapour generators commercially available for that purpose. The problem with such an arrangement is that the delivery of the TEOS vapour to a deposition chamber cannot be metered or controlled. It has also been found in the TEOS process according to US-A-3934060 that unsteady flows of TEOS vapour rather than a uniform vapour flow may be emitted from a TEOS generator. As vapour is generated, it carries away a quantity of thermal energy and the instantaneous generation of relatively large quantities of TEOS vapour causes the vapourisation of additional TEOS vapour from the liquid state to be reduced temporarily until additional thermal energy can be transferred to the remaining TEOS liquid from a surrounding heater. As a result of these variations in the rate of delivery of the TEOS vapour, wafer fabricators have had to be satisfied with thickness variations of deposited silicon dioxide films on silicon wafers of 8% to 9% along the length of a boat carrying the silicon wafers within the deposition chamber. And most importantly, none of the aforementioned prior art processes have proven consistent deposition rates run-to-run. It is conventional in the process art to control a flow of a process gas such as hydrogen, oxygen or nitrogen to a deposition chamber with a mass flow controller or other mass flow metering devices. It is impracticable to control the flow of TEOS vapour because flow controllers operating on a thermal bridge principle working in conjunction with a solenoid valve are not sensitive enough to properly respond to the available pressure drop, which is less than the difference between the TEOS vapour pressure at 90 o C and the operating pressure in the deposition chamber. Practical use of a controller would require the total pressure of the TEOS stream to be increased by using an inert carrier gas as is disclosed in U.S. Patent No. 4,517,220 to Rose, but this would dilute the reactant gas. Alternatively, it is theoretically possible, although undesirable, to increase the TEOS temperature to very high levels. This invention aims to provide an improved low pressure CVD process for consistently depositing on silicon wafers a uniform silicon dioxide film within closer tolerances from a liquid source reactant such as TEOS and to enable more consistent deposition rates to be achieved. The present invention provides a process for the chemical vapour deposition of a thin film of deposited oxide on at least one silicon wafer characterised in comprising the steps of: placing at least one silicon wafer in a deposition chamber; evacuating said deposition chamber to a subatmospheric pressure with a means for vacuum pumping; heating said wafer; flowing an inert gas into a means connecting said deposition chamber with said means for vacuum pumping for establishing a first vacuum level in said deposition chamber; vapourising a liquid source reactant; flowing said vapourised reactant into said deposition chamber while maintaining a second vacuum level in the deposition chamber; and contacting said silicon wafer with said vapourised source reactant to deposit a thin film of oxide on said silicon wafer. Preferably the inert gas is nitrogen and the liquid source reactant is tetraethoxysilane (TEOS). We have found the step of flowing inert gas into the process downstream of the silicon wafer(s) to be particularly useful in controlling the flow of the TEOS vapour. We have also found however that such a control scheme leads to gradually deteriorating process conditions over the life of the process equipment when a reactant such as TEOS is utilised. The invention also contemplates an improved process for calibrating the CVD system before depositing the film on the wafers for maintaining the same system operating parameters from run to run, particularly the operating pressure (i.e. the second vacuum level in the above process) and reactant gas flow rate, notwithstanding the deterioration of the process. This aspect of the process generally comprises the steps of evacuating the deposition chamber by the vacuum pump, flowing the inert gas into the vacuum pump at a first flow rate for establishing the preselected second vacuum level, determining a second inert gas flow rate from the difference between the first inert gas flow rate and the preselected reactant gas flow rate, flowing inert gas into the process downstream of the deposition chamber at the second inert gas flow rate, flowing reactant gas into the deposition chamber at the preselected flow rate, and then contacting the silicon wafers with reactant gas to deposit thin films on the wafers. Of course, the total process includes the conventional steps of purging, heating, backfilling and the like, although our invention per se does not include such steps. Recalibration of the system as described allows consistent operating conditions from run to run as long as the first inert gas rate exceeds the second inert gas rate. As the first inert gas rate begins to approach the reactant gas rate, the system must be taken off-line and cleaned out. The invention will be more particularly described with reference to the accompanying drawings which show a presently preferred embodiment thereof. In the accompanying drawings: Figure 1 is a schematic diagram of apparatus for carrying out the inventive low pressure CVD process utilising a liquid source such as TEOS; and Figure 2 is a graph disclosing a relationship of pumping pressure of a vacuum pump to gas flow rate through a system to which it is connected and the degradation in pumping capacity through a low pressure CVD apparatus used for depositing silicon dioxide. Referring now to the drawings, and especially Figure 1, the system for carrying out the inventive process comprises apparatus generally indicated by numeral 10 and includes a source of nitrogen 12, and a liquid source 70 of a reactant such as TEOS which are connected to a deposition chamber such as quartz deposition tube 16 in order to supply, respectively, nitrogen and TEOS vapour to the elongated horizontal quartz deposition tube 16. A conventional electric resistance heater 17 comprised of one or more heater sections is helically wrapped around the quartz deposition tube 16 and is connectable to a source of electric power for energisation thereby. A vacuum pump 18 is also connected to the quartz deposition tube 16 to reduce the pressure therein. The nitrogen source 12 is a pressurised nitrogen tank or the like which is connected to a nitrogen delivery line 20 which then branches into delivery lines 26, 34 and 56. Generally speaking, backfill line 34 and purge line 26 provide nitrogen to the system for purging air, reactants, byproduct gases and/or backfilling both before and after the deposition step. Ballast line 56 provides nitrogen for ballasting the CVD process during the deposition step. The backfill line 34 has a needle valve 30 connected thereto followed downstream by a normally open shutoff valve 32. Line 34 extends to door 36 at the load end 38 of the quartz deposition tube 16. The purge line 26 has a needle valve 22 connected thereto followed downstream by a normally closed shutoff valve 24. Line 26 extends to TEOS supply line 80 and has valve 28 near the connection. Line 80 extends to door 36 at the load end 38 of the quartz deposition tube 16. The nitrogen ballast line 56 has a mass flow controller 52 connected thereto followed downstream by a normally closed shutoff valve 54 for supplying dry nitrogen to the process downstream of the silicon wafers. As is illlustrated, the ballast line is connected to exhaust line 102 which connects the exhaust end 58 of the quartz deposition tube 16 to the vacuum pump 18. The mass flow controller may be any suitable commercially available controller. In the demonstration tests described herein, a one slm controller was advantageously used. A TEOS supply system, generally identified by numeral 70, is connected to the load end 38 of the quartz deposition tube 16. The TEOS delivery system 70 includes a TEOS vapouriser 72, of conventional bubbler design, having a heating element 73 wrapped around a vessel 74 containing TEOS liquid. The vessel 74 has an input port 76 and an output port 78 connected thereto. The input port 76 is only used if a carrier gas is supplied, which is not the case in the instant embodiment. The output port 78 is connected to a TEOS supply line 80 to deliver TEOS vapour thereto. The TEOS vapouriser has a temperature controller (not shown) for maintaining the TEOS in the TEOS vessel at a constant temperature. A heating tape 82 is wound around the TEOS supply line 80 to prevent the TEOS vapour flowing therein from condensing. A temperature controller 84 is connected to the heating tape 82 to maintain the TEOS supply line 80 at a constant preselected temperature. The temperature can be controlled at any suitable point. In the presently described apparatus, the temperature was controlled in a range of from about 40 o C to 80 o C (with the vapour pressure of TEOS varying from about 7 to 40 torr). A normally closed air valve 86 is connected in line 80 for isolating the TEOS system 70 from the diffusion tube 16. A TEOS flow control valve 88 is connected in line 80 downstream of shutoff valve 90. Preferably valve 90 is a plug valve. The valve comprises a valve body 90 and a motor driver 92 operatively connected to a pressure controller 94 which in turn is connected to a pressure sensor 96 in the door assembly 36 of the diffusion tube 16. The pressure sensor may be any suitable commercially available device such as, e.g., a capacitance manometer. Thus, in the embodiment illustrated by Figure 1, the pressure controller controls the pressure in the diffusion tube by controlling the TEOS flow through valve 88. Also the pressure controller 94 prevents the TEOS from being delivered to the quartz deposition tube 16 until the tube 16 has been evacuated to a subatmospheric pressure. A vacuum system 100 is connected to the exhaust end 58 of the quartz deposition tube 16 for maintaining the quartz deposition tube 16 at subatmospheric pressure during processing. The exhaust system 100 includes an exhaust line 102 connectively linking the exhaust end 58 of the quartz deposition tube 16 with a vacuum pump 18. Any suitable commercially available vacuum pump 18 may be used. In the demonstration tests described herein, a 50 cfm vacuum pump capable of pulling a vacuum of 10 millitorr was utilised. As is illustrated in Figure 1, a water cooled trap 116 having a water inlet 118 and outlet 120 may be connected in exhaust line 102 downstream of the diffusion tube 16 for preventing glassy materials and other debris from the TEOS diffusion step from reaching the vacuum pump 18. The trap 116 may be located at any location in line 102 but the illustrated location is preferred to prevent glassy materials from clogging the various valves and pressure sensors downstream of the deposition chamber 16. Downstream of trap 116 is an air operated full gate valve 104 in parallel with an air operated half gate valve 108. Generally speaking, the full gate valve 104 is used during the deposition and evacuation steps and the half gate valve 108 is initially used to permit a soft pump down of the deposition tube 16 to about one torr. Downstream of valve 104 are a pressure sensor 112, which may be a capacitance manometer for sensing the vacuum level in line 102, and the above-mentioned connection with nitrogen ballast line 56. The above described system enables the process operating parameters, including total deposition pressure, TEOS temperature and vapour flow rate, to be closely controlled so that the film deposited on the wafers is uniform. Also, the system may be periodically recalibrated to maintain uniformity from run to run. In the operation of the above described system to deposit a thin oxide film from TEOS vapour onto a silicon wafer, at least one silicon wafer and preferably one hundred or more silicon wafers, are placed in the deposition tube 16. The wafers are usually loaded on-end in a standard boat 140 with their faces substantially parallel to each other and perpendicular to the long axis of the boat 140. The loaded boat 140 is then placed into the deposition tube 16 which has been heated by heater 117 to a suitable temperature for decomposing the TEOS, which may be from about 700 o C to 740 o C in this process. The tube is then closed and evacuated by vacuum pump 18 to a pressure of about 10 millitorr. The half gate valve 108 is initially used to obtain a soft pump down and the full gate valve 104 is then used during the remainder of the operation. A low flow of dry nitrogen is then introduced through purge line 26 into the load end 36 of the deposition tube 16 for a short period of time sufficient to purge air and reactive gases from the system and bring the wafer boat and wafers up to the process temperature. The temperature in the diffusion tube 16 is usually ramped between about 5 o C to 30 o C to compensate for depletion effects of the TEOS vapour as it flows down the tube 16. After the loaded boat 140 has been placed in the deposition tube 16 and the other preliminary steps described above have been performed, the flow of dry nitrogen is stopped and the deposition tube 16 is evacuated by the vacuum pump 18 and the system is then pumped down to an initial level of about 10 millitorr. The deposition steps of the process may then be performed. Nitrogen valve 54 is opened to permit ballast nitrogen to flow through the mass flow controller 52 in line 56 into the process downstream of the deposition tube 16 for establishing a vacuum level in the deposition tube 16. As will be explained below, the pumping speed of pump 18 deteriorates with use and it is preferable to recalibrate the system before each run to account for any changes caused by the preceding runs. The TEOS system 70 is maintained at a suitable temperature for vapourising the TEOS liquid and preventing condensation of the TEOS vapour as it flows to the deposition tube 16. After the nitrogen ballast flow has been established, the TEOS shutoff valve 86 is opened and the vacuum level in the deposition tube causes a precalculated amount of the TEOS vapour to flow through control valve 88 into the deposition tube 16. The valve 88 controls the TEOS vapour flow to maintain a desired operating vacuum level in the deposition tube during the deposition step. After the deposition run has been completed the nitrogen ballast valve 54 and TEOS shutoff valve 86 and TEOS control valve 88 are shut off and nitrogen purge valve 24 is opened for purging the system of the residual TEOS vapour and other vapours and gases. Because the purging nitrogen should not entrain debris which might contaminate the wafers, the nitrogen pressure in the deposition chamber should not exceed about 500 millitorr. After several minutes, the purge valve 24 is closed and, optionally, the system may be again pumped down to a pressure of about 10 millitorr. The full gate valve 104 is then closed and the system is backfilled with nitrogen through backfill line 34 and the diffusion tube 16 is then opened and the boat removed. As stated above, and as can be best seen from Figure 2, it is important to periodically perform a system calibration because the pumping capacity (unit volume per unit time) of the system degrades. Most preferably, the system is recalibrated before each process run. Also the recalibration can be performed before or after the boat 140 is loaded into the deposition tube 16. Figure 2 shows that the pumping capacity of a system is a function of the vacuum level. Uppermost curve 200 shows the capacity/pressure relationship for a clean system. Vertical line 204 represents the desired system operating pressure P₁ during the deposition step. The intersection of curve 200 and line 204 at point f₁ determines the total pumping capacity at a pressure of P₁. The horizontal line 206 extending from point f₁ to the axis indicates that the total pumping capacity is F₁. The horizontal line 210 extending from point F₂ to point f₂ represents the desired TEOS vapour flow rate. The line may be either the actual TEOS vapour rate or an equivalent rate of nitrogen which reflects the particular operation's characteristics and the experience with it. The nitrogen ballast flow F₃ is then determined by the difference between the total capacity F₁ and the TEOS vapour rate F₂. After one or more runs, the system degrades and curve 200 no longer is representative of the pumping capacity of the system. Rather lower curve 202 characterises the system and the intersection with vertical line 204 at point f'₁ indicates that the capacity of the system at a desired pressure P₁ has decreased. Extending horizontal line 208 from point f'₁ to the axis indicates that the capacity of the system is F'₁. Because the TEOS vapour rate is a constant F₂, the ballast nitrogen flow rate must be reduced to a flow rate F'₃ which is the difference between the total flow rate F'₁ and the TEOS vapour rate F₂. The equipment described above was utilised in several runs of about 100 four inch silicon wafers per run which demonstrates the process and certain advantages over the Burt TEOS process. In a test of the Burt TEOS process wherein only the TEOS temperature was controlled (between 55 o C and 70 o C) the TEOS temperature varied ± 10 o C and the deposition rate varied from 15.8 nm/min (158 angstroms/minute) to 18.4 nm/min (184 angstroms/minute). In a first series of tests on four inch diameter wafers demonstrating the process illustrated in Figure 1, the TEOS temperature was controlled at about 65 o C and the pressure P₁ in the deposition chamber was calibrated at 800 millitorr by flowing 854 sccm of dry nitrogen. A dry nitrogen flow F₂ of 646 sccm representing the desired TEOS vapour flow was subtracted from the total flow F₁ to determine the desired flow rate of ballast nitrogen F₃ in line 56, which was 208 sccm and corresponded to a pressure P₃ of 320 millitorr. Having calibrated the system, the CVD process was run using 208 sccm nitrogen ballast in line 56 and 646 sccm of TEOS vapour while maintaining the total process pressure at 800 millitorr. The process, including the recalibration procedure, was repeated six times. The following calibration check before the second run indicated that there had been a slight degradation: 1st Run 2nd Run Total flow (SCCM) 854 844 TEOS flow (SCCM) 646 646 N₂ flow (SCCM) 208 198 Total pressure (millitorr) 800 800 Ballast N₂ pressure (millitorr) 320 318 The uniformity of the films deposited on wafers disposed in the boat on quarter inch centres down-the-boat and run-to-run was excellent, with deviations ranging ± 2.4% for all seven runs. The average deposition rate was 16.17 nm/min (161.7 angstroms/min). (±1.3%) for all seven runs.

Claims: A process for the chemical vapour deposition of a thin film of deposited oxide on at least one silicon wafer characerised in comprising the steps of: placing at least one silicon wafer in a deposition chamber (16); evacuating said deposition chamber to a subatmospheric pressure with a means (18) for vacuum pumping; heating said wafer; flowing an inert gas into a means (102) connecting said deposition chamber (16) with said means (18) for vacuum pumping for establishing a first vacuum level in said deposition chamber (16); vapourising a liquid source reactant; flowing said vapourised reactant into said deposition chamber (16) while maintaining a second vacuum level in the deposition chamber (16); and contacting said silicon wafer with said vapourised source reactant to deposit a thin film of oxide on said silicon wafer. A chemical vapour deposition process according to claim 1, wherein the second vacuum level in the deposition chamber is maintained by controlling the flow of vapourised reactant. A chemical vapour deposition process according to claim 1 or 2, wherein the liquid source reactant is tetraethoxysilane. A chemical vapour deposition process according to claim 3, wherein the tetraethoxysilane temperature is from 40 o C to 80 o C. A process according to any one of the preceding claims, wherein a plurality of silicon wafers are placed in an elongated tubular deposition chamber (16), each of said silicon wafers having a pair of opposed faces, said faces of said silicon wafers being aligned substantially parallel to one another and substantially perpendicular to a long axis of said deposition chamber; evacuating said deposition chamber to a subatmospheric pressure of less than 1 torr by means of a vacuum pump (18); heating said wafers; flowing nitrogen into a means (102) for connecting said deposition chamber (16) with said vacuum pump (18); vapourising a quantity of tetraethoxysilane; flowing said vapourised tetraethoxysilane into said deposition chamber (16) while maintaining a vacuum level in said deposition chamber; and contacting said silicon wafers with said vapourised tetraethoxysilane to deposit a thin film of silicon dioxide thereon. A chemical vapour deposition process according to claim 1 for depositing a thin oxide film on at least one silicon wafer in the deposition chamber at the preselected second vacuum level and reactant gas flow rate, comprising the steps of: evacuating the deposition chamber (16) by the vacuum pumping means (18); flowing the inert gas into the vacuum pumping means (18) at a first flow rate for establishing the preselected second vacuum level; determining a second inert gas flow rate from the difference between the first inert gas flow rate and the preselected reactant gas flow rate; flowing inert gas into the process downstream of the deposition chamber (16) at the second inert gas flow rate; flowing reactant gas into the deposition chamber (16) at the preselected flow rate; and contacting the silicon wafer with reactant gas to deposit a thin film of oxide thereon. A chemical vapour deposition process according to claim 6, wherein said inert gas is nitrogen. A chemical vapour deposition process according to claim 6 or 7, wherein said reactant gas is tetraethoxysilane. A chemical vapour deposition process according to claim 6, 7 or 8, wherein the preselected operating pressure is maintained by controlling the flow of reactant gas into the deposition chamber (16). A chemical vapour deposition process according to claim 9, wherein the reactant gas flow rate is controlled by a motorised plug valve (90) in response to the pressure in the deposition chamber (16).

Inventors: FISK, MICHAEL J., GOLDMAN, JON C., SARGENT, DIANE J.
Applicants: SILICON VALLEY GROUP, INC.
Application year: 1986
Main classification: H01L21
IPC codes: H01L21, C23C16