Flow verification for process gas in a wafer processing system, apparatus and method

Abstract: The disclosure relates to a semi-conductor wafer processing system (10) which includes a reaction chamber (12) for containing a wafer (12W) to be processed, pump (14) for reducing pressure within the chamber during processing and a plurality of process flow controllers (16) for providing a constant flow of process reaction gas into the chamber. The flow of process gas used is verified against the known flow of a verification gas using a derivative of the ideal gas law: n' pro = (P' pro /P' ver ) n' ver where n' pro is the flow rate of the process gas, n' ver is the flow rate of the verification gas, P' pro is the rate of change with respect to time of the process gas pressure entering the chamber, and P' ver is the rate of change with respect to time of the verification gas pressure entering the chamber. The volume and temperature are maintained constant during the verification procedure.

Description: This invention relates to verifying the flow rate of a process gas into the reaction chamber of a wafer processing system; and more particularly verifying by comparison with the known flow rate of a verification gas. During wafer fabrication, the deposition formation rate and etching removal rate depend on the input flow rate and the chamber pressure of the process gases. Changes in input flows create changes in chamber pressure; but, these changes do not directly correlate with changes in the deposited layer in a deposition system or to material removal in an etch system during a given exposure time. Heretofore, the determination of the process gas input flow rate was based on a best measurement of the ambient temperature of the reaction chamber, and a best estimate of the volume of the chamber at that ambient temperature. The measured temperature required an absolute baseline temperature determined through calibrated instruments. This baseline temperature was only as accurate as the instrument quality and condition, and the effectiveness of the calibration technique. Further, raising the reaction chamber to the operating temperature introduced a basic transition error. US-A-4901668 discloses a photo-CVD apparatus in which an article on which a thin film is to be grown is mounted on a holder in a main chamber. The holder is moved in a predetermined direction to coincide a predetermined point of the article with an imaging point of light radiated from a light source so that a thin film is grown on the predetermined point on the article. A process chamber for the article is formed within the main chamber and is separated by a seal wall. Process gas is supplied to the process chamber for the growing of the thin film on the article whilst inert gas is supplied to the remaining portion of the main chamber encircling the process chamber. A gas pressure in the main chamber is maintained slightly greater than that of the process chamber so that the process gas is prevented from flowing to the main chamber. It is an object of this invention to provide an improved method and apparatus for verification of the flow rate of the process gas. This invention provides a method of verifying the flow rate of a process gas entering the reaction chamber of a wafer processing system including the steps of: evacuating the chamber to a reduced pressure below the verification gas pressure of P1 ver in the absence of a supply of verification gas or a process gas, and then terminating gas flow from the chamber; providing a flow of verification gas having a known constant flow rate n' ver into the chamber; measuring a first pressure P1 ver of the verification gas within the chamber at a first verification time t1 ver ; measuring a second pressure P2 ver of the verification gas within the chamber at a second verification time t2 ver ; terminating the flow of verification gas; evacuating the chamber to a reduced pressure below the process gas pressure of P1 pro ; providing a flow of process gas having a constant flow rate of n' pro to be verified into the chamber; measuring a first pressure P1 pro of the process gas within the chamber at a first process time t1 pro ; measuring a second pressure P2 pro of the process gas within the chamber at a second process time t2 pro ; terminating the flow of process gas; maintaining the temperature and volume of the chamber constant during said steps of providing a flow of verification gas or of process gas into the chamber; determining the rate of change P' ver of the verification gas pressure with respect to time by the formula P' ver = (P2 ver -P1 ver ) / (t2 ver -t1 ver ) where P2 ver is the pressure of the verification gas in the chamber at time t2 ver and P1 ver is the pressure of the verification gas in the chamber at time t1 ver ; determining the rate of change P' pro of the process gas pressure with respect to time by the formula P' pro = (P2 pro -P1 pro ) / (t2 pro -t1 pro ) where P2 pro is the pressure of the process gas in the chamber at time t2 pro and P1 pro is the pressure of the process gas in the chamber at time t1 pro ' and determining the flow rate n' pro of the process gas into the chamber in accordance with the relationship: n' pro = (P pro '/P' ver ) (n' ver ); and performing the following further steps before the step of determining the flow rate n' pro : providing a flow of related gas, that is neither verification gas or process gas, at a flow rate of n' adj into the chamber; measuring a first pressure P1 adj of the related gas within the chamber at a first adjustment time t1 adj ; measuring a second pressure P2 adj of the related gas within the chamber at a second adjustment time t2 adj ; determining the rate of change P' adj of the related gas pressure with respect to time by the formula P' adj = (P2 adj - P1 adj ) / (t2 adj - t1 adj ) where P2 adj is the pressure of the related gas in the chamber at time t2 adj , and P1 adj is the pressure of the related gas in the chamber at time t1 adj ; determining the flow rate n' pro of the process gas into the chamber in accordance with the relationship: n' pro = (P' pro - P' adj ) (n' ver ) (P' ver - P' adj ). Further features of the present verification apparatus and the operation of the verification procedure will become apparent from the following detailed description and drawing in which: FIGURE 1 is a schematic diagram of a wafer processing system showing the verification apparatus; and FIGURE 2 is a chart of chamber pressure versus time showing the change of pressure for the verification sequence (curve VER), and the change of pressure for the process sequence (curve PRO); and FIGURE 3 is a chart of chamber pressure versus time showing the change of pressure for the related gas adjustment sequence (curve ADJ). The elements of the invention are designated by two digit reference numerals. The first digit indicates the Figure in which that element is first disclosed or is primarily described. The second digit indicates like features and structures throughout the Figures. Some reference numerals are followed by a letter which indicates a sub-portion or feature of that element. GENERAL EMBODIMENT (Figure 1) Semiconductor wafer processing system 10 includes a reaction chamber 12 for containing wafer 12W being processed, pump 14 for reducing the pressure within the chamber during processing, and a plurality of process flow controllers 16 for providing a constant flow of the process reaction gases into the chamber. The rate of flow of the process gas is periodically verified by comparison to a verification gas flowing at a known rate "n' ver " through verification flow controller 18 which is a standard source. The known flow rate n' ver (standard flow rate) permits calculation of a chamber operating coefficient K c for any operating temperature and volume of the chamber (see Ideal Gas Law section). Once K c has been determined for specific operating conditions, the flow rate "n' pro " of the process gas may be determined. The verification procedure requires two sequences. First, a verification gas sequence for determining K c based on the rate of pressure change of the verification gas "P' ver " as the verification gas flows into the evacuated chamber. The chamber pressure is measured by pressure monitor 14M at predetermined time intervals (see Figure 2) for determining the slope of the pressure change. Second, a process gas sequence for determining n' pro based on K c and the rate of pressure change of the process gas "P' pro ". The flow controllers may be any suitable regulation device for maintaining a constant flow of gas into the evacuated chamber, such as a thermo gas mass flow controller described in the publication "Mass Flow Controllers" (May 1987) prepared by Unit Instruments, Inc., 1247 West Grove Avenue, Orange, California 92665. Such mass flow controllers maintain a constant flow of mass therethrough over an operating range of upstream pressure and down stream pressure. During the gas sequences, a fixed number of molecules flow into the chamber per unit time. A process gas valve 16V is positioned in each of the plurality of process gas source lines between the process flow controller 16 and the reaction chamber for starting and stopping the flow of process gas. Valve 16V for the selected process gas establishes fluid communication with the chamber when opened during the process gas sequence. Similarly, verification gas valve 18V is positioned in the verification gas source line between verification flow controller 18 and the reaction chamber for establishing fluid communication when opened during the verification gas sequence. Pressure monitor 14M may be any suitable pressure measuring device such as a capacitance manometer described in the publication "MKS Baratron, Capacitance Manometers" (November 1987) prepared by MKS Instruments, Inc, Six Shattuck Road, Andover, Maine 01810. The MKS devices employ a prestressed metal strain gauge diaphragm. Computer 15 is a suitable calculator device such as a computer, which is responsive to the pressure measurements from the pressure monitor for determining K c and the process gas flow rate n' pro . Memory 15M for computer 15 receives the rate of pressure changes, coefficients, flow rates, and other calculations generated by the computer. Clock 15C for computer 15 provides the time base for establishing the time periods involved in the verification procedure and measures the elapsed time interval for calculation of the pressure rate of change. IDEAL GAS LAW The ideal gas law employed for calculating the conditional operating coefficient K c of the chamber and the process gas flow rate n' pro is based on Boyle's law of gases. The law concerns the compressibility of a fixed quantity of gas within a closed system, and is generally stated: PV = n(R u T) where P = pressure of gas within the closed system, V = volume of the closed system, n = quantity of the gas in moles, T = temperature of gas within the closed system, and R u = universal gas constant. The ideal gas law is particularly applicable to the low pressure, high temperature operating conditions found in typical wafer fabrication recipes. The low density and high velocity of the molecules of the gas within the chamber support a long mean free path between collisions. For the verification gas sequence (determining K c ) the ideal gas law is more conveniently stated as: K c = R u T/V = P ver /n ver where K c = a conditional operating coefficient of the chamber, a characteristic of each reaction chamber conditional for a particular volume and operating temperature, P ver = the pressure of the verification gas in the chamber and n ver = quantity of verification gas. The chamber conditions (temperature and volume) remain constant throughout the two gas sequences, and the coefficient K c determined during the verification gas sequence is valid for the process gas sequence. The temperature is held constant by regulated heating provided to the chamber and the chamber components. The volume is defined by the rigid side walls of the chamber and the "fixture" items that normally occupy the chamber during both gas sequences such as wafer holder 12H. Preferably, the wafer is not present in the reaction chamber during either gas sequence of the flow verification procedure. Taking the differential with respect to time for both the gas pressure P ver and the gas quantity n ver (in Eq-1) yields: K c = (dP ver /dt ver ) / (dn ver /dt ver ) = P' ver /n' ver The coefficient K c may therefore be calculated based on the flow rate n' ver of the verification gas (which is known), and the first derivative P' ver of the verification gas pressure (which can be accurately determined - see operation sections). For the process gas sequence (determining n' pro ), the ideal gas law is more conveniently stated as: n pro = P pro /(R u T/V) = P pro /K c where n pro = quantity of process gas, P pro = the pressure of the process gas in the chamber, and K c = the conditional operating coefficient of the chamber as defined during the verification gas sequence. Taking the differential with respect to time for both the gas pressure P pro and the gas quantity n pro , yields the flow rate of the process gas : n' pro = P' pro /K c or expressed in terms of the known verification gas flow rate n' pro = (P' pro /P' ver ) (n' ver ) The flow n' pro may therefore be calculated based on the flow rate n' ver of the verification gas (which is known), and by the first derivative P' ver and P' pro of the gases (which can be determined - see operation sections). OPERATION OF THE VERIFICATION GAS SEQUENCE (Figure 2) The determination of the conditional operating coefficient K c of the chamber is based on the known flow rate of the verification gas and the ideal gas law. Figure 2 shows the chamber pressure response versus time (curve VER) for the verification gas pressure during the verification sequence. Prior to the beginning of the procedure (at time = tB), isolation valve 14V between the chamber and the pump is open. Also prior to time tB, all process gas valves 16V and verification gas valve 18V are closed, and no verification gas or process gas is flowing. By time tB, the chamber has been evacuated to a reduced baseline chamber pressure. At tB the verification gas valve is opened and the verification gas flows into the chamber. The throttling transients are permitted to subside during a stabilization period (tB to tS1 as determined by clock 15C), and the verification gas flow stabilizes to chamber pressure P0. Computer 15 receives pressure measurements from pressure monitor 14M during a stabilization check period (tS1 to tS2) to determine that the gas flow has stabilized. At time equal zero (time = t0) the isolation valve is closed and the chamber pressure increases due to the input flow of verification gas n' ver . During valving period (t0 and t1 ver ), the transients associated with closing the isolation valve subside. At first verification time (t1 ver ), the pressure monitor provides a first verification pressure (P1 ver ) to computer 15. At a subsequent second verification time (t2 ver ), the pressure monitor provides a second verification pressure (P2 ver ) to computer 15. The computer calculates the rate of change of pressure (P' ver ) by dividing the change in pressure by the elapsed time interval: P' ver = (P2 ver - P1 ver ) / (t2 ver - t1 ver ). The rate of change of verification gas pressure P' ver is graphically illustrated by the slope of the pressure response curve VER of Figure 2 between t2 ver and t1 ver . The slope P' ver is recorded in memory 15M. The two measurement times are preferably selected so that the two pressure readings occur in the middle portion of the operating range of the monitor. The middle portion tends to be more linear than the lower and upper ends of the range, and therefore give a more accurate pressure reading. The conditional operating coefficient of the chamber is calculated based on the ideal gas law: K c = P' ver /n' ver and is independent of the absolute pressure of the gas within the reaction chamber. The effect of the constant chamber temperature and the constant chamber volume are included in the conditional operating coefficient. The same chamber coefficient is valid for both the verification gas sequence and the process gas sequence. The gas employed as the verification gas is preferably a suitable inert, non-toxic, dry, readily available gas such as nitrogen gas (N 2 ), oxygen gas (O 2 ), nitrous oxide gas (N 2 O) or argon gas (Ar), of appropriate chemical purity such as semiconductor fabrication grade. OPERATION OF THE PROCESS GAS SEQUENCE (Figure 2) The process gas sequence is similar to the verification gas sequence, except that prior to time = tB the selected process valve 16V is in the open position and verification valve 18V is closed. Process gas is flowing instead of verification gas. The rate of change of process gas pressure P' pro is determined following the same two point procedure shown in Figure 2. The computer provides the rate of change of the pressure (P' pro ) by dividing the change in pressure by the elapsed time interval: P' pro = (P2 pro - P1 pro ) / (t2 pro - t1 pro ) for substitution in Eq-6 to calculate the process gas flow rate n' pro . The rate of change of process gas pressure P' pro is graphically illustrated by the slope of the pressure response curve PRO of Figure 2 between t2 pro and t1 pro . The slope P' pro is recorded in memory 15M. In the embodiment shown in Figure 2, the process gas slope P' pro is slightly greater than the verification gas slope P' ver' indicating that the process gas flow (n' pro ) is slightly greater than the verification gas flow (n' ver ). The two process gas pressures are also preferably selected from the middle more linear portion of the operating range of the monitor. ADJUSTMENTS TO FLOW (Figure 3) A basic assumption inherent in the above discussion of the two gas sequences is that no other gases are flowing into the chamber except the verification gas or the process gas. Any unaccounted for gases flowing during the verification gas sequence which is not verification gas will increase P' ver resulting in a higher K c . Similarly, non process gases flowing during the process gas sequence will introduce an error in P' pro . Related gas flows which are not part of the verification procedure, but which occur during both the verification gas sequence and the process gas sequence may be subtracted from the gas law derived equations. A major related gas flow is N 2 purge gas flowing into the chamber from a turbo molecular pump used primarily in etch processes requiring particularly low chamber pressures. The turbo pump motor operates at a very high speed (about 27,000 rpm) and must be protected from the corrosive effects of the etch gas. The inward flow of the purge gas (n' pur ) prevents exposure of the pump bearings to the etch gas. Another source of related gas flow is leakage flow (n' lk ) into the chamber through seals and valves due to the reduced chamber pressure. The first derivative of pressure in the chamber coefficient relationship (Eq-3) may be adjusted to correct for this related flow by subtracting out the first pressure derivatives of each of the related pressures: K adj = [P' ver - (P' pur + P' lk )] / (n' ver ) or K adj = (P' ver - P' adj ) / (n' ver ) where P' pur = any intentional flow such a purge flow across bearing surfaces, P' lk = any incidental flow such as leakage flow into the low pressure region of the system, and P' adj = the total flow into the chamber during the verification procedure which is neither verification gas nor process gas. The adjusted conditional chamber coefficient is conditioned on the particular purge and leakage flows present at the time of the verification gas sequence. The first pressure derivatives of the related flows may be collectively determined by a separate related gas sequence shown in Figure 3 which is similar to the verification sequence. During the related gas sequence, both the verification gas valve 18V and the process gas valve 16V are closed. The only gas entering the chamber is the related gas under correction. The pressure slope P' adj between t1 adj and t2 adj represents the collective effect of the purge gas flow and the leakage flow. The purge component of the related gas is shown by slope P' pur (along curve PUR in dashed lines), and the leakage component is shown by slope P' lk (along curve LK also in dashed lines). The leakage component is typically much less then the purge component. The first derivative of pressure in the process gas relationship (Eq-6) may be adjusted to correct for this related flow by subtracting out the first pressure derivatives of each of the related pressures: n' pro = [P' pro - (P' pur + P' lk )] / k adj . Substituting for K adj (Eq-10): n' pro = [P' pro - (P' pur + P' lk )] (n' ver ) [P' ver - (P' pur + P' lk )] which may be stated more generally as: n' pro = (P' pro - P' adj ) (n' ver ) (P' ver - P' adj ) Thus the process gas flow rate can be adjusted for related gas flows based on rates of change of pressure. It is not necessary to know the actual purge flow or leakage flow for the verification procedure. CONCLUSION It will be apparent to those skilled in the art that the objects of this invention have been achieved as described hereinbefore. Clearly various changes may be made in the structure and embodiments shown herein without departing from the concept of the invention. Further, features of the embodiments shown in the various Figures may be employed with the embodiments of the other Figures. Therefore, the scope of the invention is to be determined by the terminology of the following claims.

Claims: A method of verifying the flow rate of a process gas entering the reaction chamber of a wafer processing system including the steps of: evacuating the chamber to a reduced pressure below the verification gas pressure of P1 ver in the absence of a supply of verification gas or a process gas, and then terminating gas flow from the chamber; providing a flow of verification gas having a known constant flow rate n' ver into the chamber; measuring a first pressure P1 ver of the verification gas within the chamber at a first verification time t1 ver ; measuring a second pressure P2 ver of the verification gas within the chamber at a second verification time t2 ver ; terminating the flow of verification gas; evacuating the chamber to a reduced pressure below the process gas pressure of P1 pro ; providing a flow of process gas having a constant flow rate of n' pro to be verified into the chamber; measuring a first pressure P1 pro of the process gas within the chamber at a first process time t1 pro ; measuring a second pressure P2 pro of the process gas within the chamber at a second process time t2 pro ; terminating the flow of process gas; maintaining the temperature and volume of the chamber constant during said steps of providing a flow of verification gas or of process gas into the chamber; determining the rate of change P' ver of the verification gas pressure with respect to time by the formula P' ver = (P2 ver -P1 ver ) / (t2 ver -t1 ver ) where P2 ver is the pressure of the verification gas in the chamber at time t2 ver and P1 ver is the pressure of the verification gas in the chamber at time t1 ver ; determining the rate of change P' pro of the process gas pressure with respect to time by the formula P' pro = (P2 pro -P1 pro ) / (t2 pro -t1 pro ) where P2 pro is the pressure of the process gas in the chamber at time t2 pro and P1 pro is the pressure of the process gas in the chamber at time t1 pro and determining the flow rate n' pro of the process gas into the chamber in accordance with the relationship: n' pro = (P pro '/P' ver ) (n' ver ) ; and performing the following further steps before the step of determining the flow rate n' pro : providing a flow of related gas, that is neither verification gas or process gas, at a flow rate of n' adj into the chamber; measuring a first pressure P1 adj of the related gas within the chamber at a first adjustment time t1 adj ; measuring a second pressure P2 adj of the related gas within the chamber at a second adjustment time t2 adj ; determining the rate of change P' adj of the related gas pressure with respect to time by the formula P' adj = (P2 adj - P1 adj ) / (t2 adj - t1 adj ) where P2 adj is the pressure of the related gas in the chamber at time t2 adj , and P1 adj is the pressure of the related gas in the chamber at time t1 adj ; determining the flow rate n' pro of the process gas into the chamber in accordance with the relationship: n' pro = (P' pro - P' adj ) (n' ver ) (P' ver - P' adj ). A method as claimed in claim 1, wherein after the step of determining the rate of change of P' ver the further step of determining a conditional operating coefficient Kc of the chamber in accordance with the relationship: Kc = P' ver /n' ver is performed. A method as claimed in claim 1 or claim 2, wherein the flow of the verification gas into the chamber is maintained at the known constant by a verification gas flow controller, and the flow of the process gas into the chamber is maintained constant by a process gas flow controller. A method as claimed in claim 3, wherein the flow of verification gas is provided by opening a verification flow valve located between the verification flow controller and the chamber, and the flow of process gas is provided by opening a process flow valve located between the process flow controller and the chamber. A method as claimed in claim 3, wherein the evacuation flow out of the chamber is maintained by a pump means, and the evacuation flow is terminated by closing an isolation valve located between the pump means and the chamber. A method as claimed in any of claims 3 to 5, wherein the pressure in the chamber is measured by a pressure monitoring means having a dynamic operating range with generally linear mid-portion. A method as claimed in claim 6, wherein the first and second pressure of the process gas in the chamber is measured employing the same generally linear mid-portion of the dynamic operating range of the pressure monitoring means as employed to measure the first and second pressure of the verification gas. A method as claimed in claim 3, and in the case were there is a leakage flow of gas into the chamber; wherein the flow rate n' pro of the process gas into the chamber is determined in accordance with the relationship: n' pro = [P' pro - (P' pur + P' lk )] (n' ver ) [P' ver - (P' pur + P' lk )] where P' pur = any intentional flow such as a purge flow across bearing surfaces, and P' lk = any incidental flow such as leakage flow into the low pressure region of the system. A method as claimed in claim 1, wherein the verification gas is a gas selected from the group consisting of nitrogen gas (N 2 ), oxygen gas (O 2 ), nitrous oxide gas (N 2 ) and argon gas (Ar). A method as claimed in claim 1, wherein the verification gas is nitrogen gas (N 2 ). A semiconductor wafer processing apparatus which calculates a processing gas flow according to the method of claim 1, comprising: a reaction chamber (12) for containing wafers (12W) during processing; a process gas source in fluid communication with the chamber (12) for providing a process gas required for wafer processing; process gas flow controller means (16) mounted between the process gas source and the chamber (12) for maintaining a constant flow of process gas during wafer processing; process gas valve means (16v) connected between the process gas flow controller (16) and the chamber (12) for starting and stopping the flow of process gas to the chamber; a verification gas source and verification gas flow controller means (18) in fluid communication with the chamber (12) for providing a predetermined constant flow of the verification gas to the chamber, verification gas valve means (18v) connected between the verification gas flow controller (18) and the chamber (12) for starting and stopping the flow of said verification gas; pump means (14) for evacuating the chamber to a reduced gas pressure below the pressure required during wafer processing and prior to gas flow verification; isolation valve means (14v) connected between the chamber and the pump means (14) for controlling the evacuation of the chamber; pressure monitoring means (14M) for measuring the reduced gas pressure within the chamber; characterised in that the verification gas valve means (18v) and the process gas valve means (16v) are selectively operable to supply one or the other gas to the reaction chamber, and computer means (15) are provided for responding to the values of pressure of the predetermined and constant flow of the verification gas pressure measurement from the pressure monitor means (14M) and to the values of the processing gas pressure over time to calculate the flow of process gas relative to the gas flow of verification gas while subtracting the effects of gas flows in the chamber other than those due to process gas flow and verification gas flow. A processing apparatus as claimed in claim 11, wherein the computer means further comprises a memory means for receiving and storing the gas flow rates n' pro and n' ver and rates of pressure change P' pro and P' ver . A processing apparatus as claimed in claim 12, wherein the computer means further comprises a clock means for providing the time base for determining the gas flow rates n' pro and rates of pressure change P' pro and P' ver . A processing apparatus as claimed in claim 13, wherein the computer means is arranged to calculate a conditional chamber coefficient for storage in the memory means, in accordance with the relationship: K c = P' ver / n' ver . A processing apparatus as claimed in claim 14, wherein the computer means is arranged to calculate the flow rate of the process gas, in accordance with the relationship: n' pro = P' pro / K c . A processing apparatus as claimed in any of claims 11 to 15, wherein the flow controller means are mass flow controllers for maintaining a constant flow of gas therethrough. A processing apparatus as claimed in any of claims 11 to 16, wherein the process gas source means comprises a plurality of gas sources for providing a plurality of process gases; the process gas valve means comprises a plurality of gas valves, one valve for each of the plurality of process gases; and the process mass flow controller means comprises a plurality of controllers, one for controlling each of the plurality of process gases. A processing apparatus as claimed in claim 11, wherein the pressure monitoring means is a capacitive manometer.

Inventors: FREERKS FRED W, MUH RICHARD C, FREERKS, FRED W., MUH, RICHARD C.
Applicants: APPLIED MATERIALS INC, APPLIED MATERIALS, INC.
Application year: 1992
Main classification: H01L21
IPC codes: H01L21, C30B25, C23C14, G01F1