GAS ETCHING SYSTEM INCLUDING END POINT DETECTION

Abstract: A gas etching system includes a plasma etching chamber (1), a pressure sensor (9) adapted to sense the pressure within the chamber (1) and a constant displacement vacuum pump (2) adapted to extract gases from the chamber (1). The etchant gases entering the chamber (1) have their flow regulated by a feedback loop which includes the pressure sensor (9). Changes in the chemical composition within the chamber (1) which occur in time proximity to the end point of etching affect the chamber pressure and are detected as variations in the gas flow rates. These variations are depicted visually on a strip chart recorder (12). The system detects the end point of etching with high sensitivity.

Description: GAS ETCHING SYSTEM INCLUDING END POINT DETECTION Technical Field This invention relates to gas etching systems of the kind including a gas etching chamber, first sen- sing means adapted to sense the molar content within said chamber, gas supply means adapted to regulate the supply of a gaseous etchant into said chamber, and gas extraction means adapted to regulate the extraction of gases from said chamber. The invention also relates to methods for detecting the end point of etching in a gas etching system. Background Art Gaseous etching processes such as plasma etching have gained great importance in recent years as substitutes for wet chemical etching processes in the fabrication of integrated circuits (iCs) . The present invention deals with one aspect of the processes in¬ volving this technology, namely, the precise and re- producible determination of when gaseous etching should be terminated, commonly known as end point detection. Recognizing the extensive availability of literature dealing with plasma etching techniques and equipment, one skilled in the art is undoubtedly already aware of the importance attributed to end point detec¬ tion. An illustrative recent article, summarizing the recognized technology of end point detection, is printed in the April 1981 issue of Solid State Technology, pages 115-122, entitled "Methods of End Point Detection for Plasma Etching" and authored by P. J. Marcoux et al. At present, the emission spectroscopy method of end point detection has the greatest following of users. Recent plasma etching process developments have, unfortunately, refined and altered recognized, procedures to the point where conventional optical methods are no longer capable of providing the precision sought under controlled laboratory conditions, and are clearly in¬ adequate for a volume manufacturing environment. In part, -this is attributable to the increased concentration of oxygen in the etchant gas, displacing more of the fluorine-based gas upon which the optical methods strong¬ ly rely. A review of the literature and hardware asso¬ ciated with implementing most optical methods also sug¬ gests a dire need for greater simplicity and a lower cost. An etching system of the kind specified is known from the article "A Simple Method of End-Point Determination for Plasma Etching", Journal of Vacuum Science Technology, November/December 1980, pages 1378- 1381. According to this known system, a pressure gauge responsive to the pressure within a plasma etching chamber is utilized to sense pressure changes within the chamber. The signal from the pressure gauge is moni¬ tored by a pen recorder and the end point of etching is determined by a detected pressure change . The known system has the disadvantage of limited sensitivity. Thus, in the known system neither ■ the gas flow rates nor the chamber pressure are regu¬ lated parameters. Consequently, a pressure variation in the chamber, e.g., the response to a change in the chemical reactions, also causes a gas flow variation. Interaction between gas flow and pressure distributes the perceivable magnitude of the change between the two variable and related parameters. Disclosure of the Invention It is an object of the present invention to provide a gas etching system of the kind specified having high sensitivity. Therefore, according to the present invention, there is provided a gas etching system of the kind specified, characterized by control means responsive to said first sensing means and adapted to control one of said gas supply and gas extraction means, the other of said gas supply and gas extraction means maintaining a substantially constant gas flow rate, and second sensing means adapted to sense flow rate changes in the controlled one of said gas supply and gas extraction means, whereby the end point of etching is detected by a sensed flow rate change . It will be appreciated that a gas etching system according to the invention embodies a closed loop system which permits the regulation of the gas flow rate to maintain a constant level of pressure within the chamber. Therefore, all the chemical reaction induced variations occurring within the chamber appear as changes in a single parameter, gas flow. Thus, the sensitivity is increased by the focus of all the chamber variations to one, measurable parameter. A further advantage as compared with the known system is that contamination of the sensing element which can occur if the sensing element utilized for end point detection directly senses the pressure within the etching chamber is avoid¬ ed by sensing gas flow rate changes into or out of the chamber. Furthermore the system is of simple, low-cost construction. According to another aspect of the invention, there is provided a method for detecting the end point of etching in a gas etching system, including the step of coupling a gas etching chamber to a source of vacuum and a source of etchant gas, characterized by the steps of regulating either the gas supply flow rate or the gas flow rate to said source of vacuum while maintaining the other flow rate substantially contrast, and sensing the time related change of the regulated gas flow to detect transitions between substantially steady levels. In one form of the invention to be described hereinafter inner and outer feedback control loops together regulate the proportioned flow rates of multiple etchant gases in response to the chamber pressure. The flow rates of the gases are regulated to maintain a fixed ratio therebetween, while the combined flow is responsive to chamber pressure. The chemical composi- tion of the gases within the chamber is indicative of the etching progress and is evidenced parametrically by changes in the molar content of the chamber. A vacuum pump, operated to remove chamber gases at a constant rate of volumetric flow, couples the parameters repre- senting chamber pressure to those representing the molar content within the chamber when the input gas flow is regulated to maintain a constant pressure. In ef¬ fect, end point related changes in the chemical compo¬ sition of the chamber gas appear as variations in the flow rate of the gases entering the chamber. One embodiment includes apparatus and methods by which the pressure controller regulating the input gases prescribes a specified ratio between the flow rates of the two gases. Furthermore, in addition to regulating the flow rate of the composite gas into the chamber, each individual gas includes an. inner feedback loop coupling the respective gas servo valve with a corresponding flow meter. Again, variations in the conditions within the chamber are perceived as pressure changes and eventually converted to proportional adjust¬ ments in the flow rates of the two gases. Brief Description of the Drawings Fig. 1 contains a schematic of the composite sys em. Fig. 2 presents a block diagram schematically showing the feedback loops characterizing the embodiment. Fig. 3 is a schematic diagram of the interface circuit used with the system in Fig. 1. Figs. 4A and 4B are strip chart presentations of gas flow rates for etch sequences involving poly- crystalline silicon layers. Figs. 5A and 5B are strip chart presentations of gas flow rates for etch sequences involving a photo¬ resist layer and a sandwich layer of tungsten suicide/ polycrystalline silicon, respectively. Best Mode for Carrying Out the Invention The invention will first be presented from a theoretical perspective, then shown in terms of hardware configurations, and finally in the context of test results for an exemplary group of embodiments. In terms of supporting theoretical development, one may begin with the classic ideal gas law, PV = nRT, (1) and proceed to recognize that the plasma etching chamber and ancillary control systems constrain the volume, V, and the gas constant, R, to substantially constant values. Though the temperature increases slightly during the etching process, the effects are sufficiently minor to also consider the temperature, T, as a constant in the mathematical relationship. Another relationship can be established be¬ tween pressure, P, within the etching chamber, the volumetric flow, Q, into the chamber and the speed, S, of a vacuum pump attached to the chamber, wherein, Q = PS. (2) Absent external influences, the relationship in equation (2) merely defines that if pumping speed is increased, the pressure will decrease, or if flow is increased, the pressure will increase proportionally. For purposes of the present invention, the pump speed remains essentially constant during the period of concern. Now, substituting the relationship defined by equation (2) into equation (1) and inserting subscripted letters C for variables heretofore constrained to be constants, one arrives at the following relationship: Q -V = n C c C . (3) This relationship shows that the volumetric flow, Q, and the number of moles of atoms, n, are directly propor¬ tional during the course of a plasma etch sequence. The number of moles within the chamber system changes during the progression of the etching process by virtue of chemical reactions. For example, if the etchant gas SF_ is introduced into the system, the radio frequency (RF) energy in the plasma produces a chemical . reaction defined by the following transformation re- lationship: SFg + radio freq. energy ^ SF X + (6-X)F. (4) From relationship (4), it is apparent that the number of moles n increases as an outcome of the chemical reaction induced by RF energy. Recall, however, that the inven- tion contemplates a system in which chamber pressure is regulated to remain substantially constant by closed loop adjustments in the flow rates of the input gases. Since the vacuum pump withdraws gas from the etching chamber at a constant volumetric flow rate, and the gases entering the chamber change their composition according to equation (4), relationships (1) and (3) prescribe that the flow rate of the input gases must be decreased to maintain constant pressure. Note that the above analysis provides for only one reaction within the chamber, namely, the RF energy with SF, gas. In the operation of an actual etching chamber, other chemical reactions are occurring simul¬ taneously. A representative example is the etching of a polycrystalline silicon surface using the F ions gener- ated during transformation (4). That chemical reaction is defined by trans ormation relationships (5) or (6). Si + 4F ^ S F _ ' (5) or Si ÷ 2F^ ?-SiF 4 (more likely). (6) One no doubt recognizes that either transformation (5) or (6) produces a decrease in the number of moles within the chamber. To compensate for such a change in the chamber conditions, again recognizing the fixed pressure requirement, the flow rate of input gases must increase. In the case of etching polyerystalline silicon, chemical reactions (5) or (6) account for the increase in flow rate accompanying the onset of plasma etching in the context of the contemplated system. • In one manner of practicing the invention, the completion of etching is evidenced by an abrupt and significant change in the gas flow rate from its etching equilibrium to a succeeding equilibrium level. The conceptual foundation is again linked to the molar constancy of the vacuum pump and the fixed level of chamber pressure. For example, upon the exhaustion of the polycrystalline silicon being etched, the RF gener¬ ated fluorine radicals in the chamber cease undergoing the chemical reactions defined in transformations (5) or (6). The consequential rise in molar density produces a corresponding rise in the chamber pressure, which in turn decreases the input gas flow rate in response to signals conveyed by the feedback loop of the flow con¬ trol. Note that this change in flow rate is substan- tially coincident with the completion of the polycry¬ stalline silicon etch sequence. Though the chemical reactions differ, the fundamental principles described above apply in like manner to other etchant gases and etched materials. For instance, in the case of etching photoresist materials, tests have shown analogous end point characteristics when etching three different positive material types and one negative material type. The etching or stripping of photoresist layers using a plasma does differ in one respect, in that the gas flow increases upon reaching the end point of. etching. This occurs because the resist, at the onset of etching, reacts quite readily and forms a large number of moles within the chamber. As more of the resist is etched away, fewer moles are formed, chamber pressure decreases, and gas flow in¬ creases to maintain the regulated level of pressure. Given the foregoing theoretical foundation, it still remains necessary to define practical organization of methods and structrual apparatus suitable for detec¬ ting the end point. As specifically embodied herein, the end point is detected as a change in the electrical signal generated by a gas flow meter situated in one path leading to the etching chamber. Refer to Fig. 1 for a schematic illustration of the composite system. . As shown, chamber 1 is a barrel type plasma etcher. The chamber is evacuated using a pump, 2, which draws at a constant rate of volumetric flow, while etchant gases entering the chamber are supplied through servo valves 3 and 4, providing SF, and 0„, respectively. Flow meters 6 and 7 are coupled to ratio and flow controller 8. At the left end of chamber 1 is a capa- citance manometer type pressure sensor, 9, which is directly coupled to controller 8. In the present manner of practicing the invention, oxygen flow is measured electrically using flow meter 7, whereafter the signal is processed by interface circuit 11 and visually de— picted by strip chart reporter 12. Within the chamber shown, there appear a group of silicon wafers 13. The practice of the invention according to Fig. 1 includes an arrangement of commercially available apparatus. The etching chamber is an IPC Model 2000, the controller is an KKS Model 254, both servo valves are Brooks/MKS Model 251-200, both flow meters are Tylan/MKS. Model FM 360, while the capacitance mano¬ meter is a Baratron/MKS Model 222A. The vacuum pump, 2, is a conventional unit. The interface circuit, 11, will be shown and described at a later point in the development. The remaining item in the system, the strip chart recorder, is also a conventional laboratory instrument with high input impedance and an input signal range extending from 0 to approximately 1 volt. Before embarking upon an analysis of exemplary etch sequences, it would be desirable to recognize the nuances of the composite system. For purposes of such considerations, note Fig. 2 of the drawings showing the mechanical, electrical, chemical and pneumatic couplings of the apparatus in Fig. 1. The presentation in Fig. 2 is in the format of a simplified feedback control system. The blocks represent transfer functions, while the lines of flow depict the manner of coupling. Together, they conceptually illustrate the complete system and accen¬ tuate the distinguishing features. No attempt will be made to mathematically describe the transfer functions represented in each block, since it is the organization that has the focus of attention. However, when a block serves a unique purpose, or contributes an important effect, it will be highlighted. Begin at the left with signal P . Signal P is defined as being the commanded pressure. The ' error between the commanded and the sensed feedback pressure, P , is amplified by the controller gain and provided to a pair of parallel inner loops individually regulating the flow rates of oxygen and SF fi gas. Since the command to each of the inner loops is identical, except for multiplication factors K n and K , the steady state U 2 h~ 6 ratio of gas flow is identical to the ratio of these constant factors. A comparison of Figs. 1 and 2 shows the correspondence of node 14, the point for monitoring gas flow. The embodiment in Fig. 1 shows that the gas from each inner loop is pneumatically combined before entering the etching chamber.- The simplified feedback control system presented in Fig. 2 omits the cross- coupling effects between the ' two gas loops, simplifying the presentation and recognizing that their effects are secondary. Generally, end point detection is an attempt to correlate the conditions on a wafer to the chemical composition of the gases within the etching chamber. However, the numerous nonlinear effects attributable to the plasma etching chamber, to variables such as the gas flow rates, the RF energy levels, the diversity of gases undergoing chemical decomposition, and to the rates and types of chemical reactions taking place between chamber gases and exposed wafer materials, severely complicate the detection process . Interestingly, the present invention makes no attempt to separate the nonlinear effects as they appear within the chamber, but rather, monitors the effects on a functionally cascaded element within a closed loop. As shown, the negative contribution of the vacuum pump to the molar content of the chamber, where the content is represented by the symbol M, is substan¬ tially constant in magnitude. Consequently, the afore¬ mentioned relationship between pressure and molar con- tent insures that the outer feedback -loop, sensing and regulating chamber pressure, coincidentally regulates the molar content of the gas within the chamber. When the controller gain is adequate, the molar content of the chamber is maintained by the balance between the commanded pressure, P , and the feedback pressure, P . Though the foregoing relationships presuppose a static mode of operation, the dynamics of the system para¬ meters, after the start-up transients have subsided, are sufficiently low, stable and damped, that the static relationships are not materially altered. When viewing the composite system in the context of a feedback control, one now recognizes that the nonlinear effects in the direct path, i.e., those created within the etching chamber, are substantially offset by equal and opposite changes in the gas flows. If feedback control of the chamber pressure in the manner shown maintains a constant molar content therein. all chemical changes within the etching chamber which affect the molar content, whether those changes are linear or nonlinear, appear as variations in the gas flow rate. Contrast this with an open loop configura- tion, in which the changes internal to the chamber appear as dispersed effects in the optical, flow or pressure characteristics. Node 14 was selected as a monitoring point from the perspective of convenience, since the change in oxygen flow rate is greater both in absolute value of gas and in electrical signal. Once the effects of the closed loops are recognized, other suitable detection points for sensing changes in gas flow can be located and suitably sensed. For instance, a gas other than oxygen or a combination of the various gases. The interface circuit presented in Fig. 3, cor¬ responding to block 11 in Fig. 1, is comparatively rudimentary, introducing a nominal electrical gain and limited noise filtering. As to the latter effect, the 0.1 second time constant proved to be adequate for the specific embodiment. The frequencies filtered suppres¬ sed extraneous noise without distorting the dynamic characteristics of the gas flow rates . One no doubt recognizes that the filter time constant is readily var- iable and should be consistent with the response charac¬ teristics of the remaining hardware . Test results have confirmed that the end point detection systems and processes described herein are not only viable in the context shown, but are suitable for use with a broad range of IC materials capable of undergoing plasma etching. For instance, successful results have been shown with polycrystalline silicon, a variety of different photoresists, tungsten suicide, a tungsten silicide/polycrystalline silicon sandwich, and poly- crystalline silicon etched with a low concentration of oxygen. In each case, the end point was reliably and repeatedly detected. To expand one's understanding of the system and processes, the drawings have been supplemented with four plots illustrating the shapes of representative outputs generated on a strip chart recorder. Since the amplitudes of the waveforms shown provide no meaningful information, aside from relative changes with respect to the plots themselves, scale markings have been deleted. A one minute increment of time is shown in each pair of plots to provide one with a feel for the relative dura- tions. Figs. 4A and 4B show two polycrystalline silicon etch sequences, with the former having a chamber load of ten wafers and the latter loaded at five wafers. Fig. 5A corresponds to a photoresist strip etching sequence, with the chamber carrying a. six wafer load. The last example, illustrated in Fig. 5B, represents an etch of nine wafers, in which the etched material was a tungsten suicide/polycrystalline silicon sandwich. First Example Fig. 4A shows a time plot of oxygen flow obtained from recorder 12 when etching polycrystalline silicon using the embodiment shown in Fig. 1. The oscil¬ latory behavior of the gas flow immediately after the onset of the flow and the RF plasma merely illustrates the existence of an initial period of unstable flow rates. Within approximately one minute, the oscillatory operations subside and oxygen gas flow rises smoothly to a substantially steady level. At a later point in time, approximately t , the polycrystalline silicon layer undergoing etching shows signs of being depleted. In the manner described hereinbefore, the depletion initiates a rise in chamber pressure, or as depicted here, a drop in the flow rate of oxygen gas. Time t„ corresponds to the end point of etching, clearly indi¬ cated in the plot by the presence of a stable new level. In the practical setting of an automated or operator-controlled process, time ^ is readily discer¬ nible because conventional etching procedures extend the etching sequence by an additional overetch period of approximately ten percent to compensate for normal layer depth tolerances. Overetching ensures complete removal of the etched layer material, -which is typically dis- tributed with a +_ 10% nonuniformity in thickness. One undoubtedly recognizes that such periods of overetching ease the burden of determining the end point by pro¬ viding a relatively stable period against which to compare any changes in the flow rate. Second Example As was true of the first example, the poly¬ crystalline silicon etch sequence in Fig. 4B commences with a brief period of instability. In this case, how¬ ever, the leading edge of the plot differs in that the rise to a stable level is delayed until time t_ . The phenomenon illustrated is generally known as "scumming," and represents the etching of a scum layer of photo¬ resist material covering the wafer at the time plasma etching is commenced. Note from the analysis contained hereinbefore, etching of photoresist increases, rather than decreases, the molar content in the chamber. Con¬ sequently, the oxygen flow rate is shown to increase at the completion of scumming and the onset of polycrystal¬ line silicon etching. The end point of etching is again clearly evident from the plot at time t.. As is standard prac¬ tice, an overetch interval follows. Third Example Fig. 5A illustrates the typical results ob- tained when etching a layer of photoresist. After the initial oscillatory period, the oxygen gas flow remains low as a result of the increased molar content in the chamber. Again, the end point of etching is evidenced by an abrupt change followed by a stable new level of etchant gas flow. In this case, the end point corres¬ ponds to time t j .. OMPI Fourth Example The curvature in the gas flow plot of Fig. 5B illustrates the effects of etching a compound layer including tungsten suicide and polycrystalline silicon. Nevertheless, the end point of the etch sequence remains remarkably distinct at point t fi . Given the development hereinbefore, one un¬ doubtedly recognizes that the fundamental concepts presented are broader than the embodiments by which they have been illustrated. For instance, the reactions within the chamber, their effects on the molar quanti¬ ties, the relationship to end point, and their inter¬ action with the pressure and gas flow, suggest other system organizations and structural implementations than those appearing in Figs. 1 and 2. Therefore, it is feasible, though not presently practical, to set the input gas flow rates to fixed values and close the feedback loop by regulating the output side gas flow rate by changing the vacuum pump speed. If a variable speed pump is not available, it is also conceivable that the output gas flow rate could be regulated with a variable orifice so as to maintain the chamber pressure at a fixed level. In the first of the above cases, changes in pump speed would be used to detect the end point, while in the latter case, end point would be derived from changes in the orifice setting. It should be recognized that in all the fore¬ going cases end point is characterized by the inter¬ action of chamber pressure and flow rate when one para— meter is regulated with a feedback loop while the other responds to changes in the chemical makeup within the chamber. From the foregoing it is also clear that the invention contemplates and encompasses the plasma etch- ing of numerous types of diversely mixed materials. Furthermore, the invention contemplates variations in the chamber structure, organization and character of the apparatus, the number and compositions of the etchant gases, techniques for regulating gas flow rates, and other known or readily perceived alterations of the systems shown and described herein. OMPI

Claims: CLAIMS : 1. A gas etching system including a gas etching chamber (1), first sensing means (9) adapted to sense the molar content within said chamber (1), gas supply means (4) adapted to regulate the supply of a gaseous etchant into said chamber (1), and gas extrac¬ tion means (2) adapted to regulate the extraction of gases from said chamber (1), characterized by control means (8) responsive to said first sensing means (9) and adapted to control one of said gas supply and gas extraction means (4, 2), the other of said gas supply and gas extraction means (4, 2) maintaining a substan¬ tially constant gas flow rate, and second sensing means (7) adapted to sense flow rate changes in the controlled one of said gas supply and gas extraction means (4, 2), whereby the end point of etching is detected by a sensed flow rate change. 2. A gas etching system according to claim 1, characterized in that said control means (8) is adapted to control the controlled one of said gas supply and gas extraction means (4, 2) such that a substan— tially constant molar content is maintained in said chamber (1) . 3. A gas etching syste according to claim 2, characterized in that said first sensing means in¬ cludes a pressure sensor (9). 4. A gas etching system according to claim 3, characterized in that the controlled one of said gas supply and gas extraction means (4, 2) is said gas supply means (4) . 5. A gas etching system according to claim 4, characterized in that said gaseous etchant includes 5 . ( concluded ) first and second etchant gases, and in that said control means (8) is adapted to control said gas supply means (3, 4), such that changes detected by said pressure sensor (9) are substantially compensated by changes in the flow rates of said first and second etchant gases. 6. A gas etching system according to claim 5, characterized in that said control means (8") is adapted to regulate the flow rates of said first and second gases such as to maintain a fixed proportion by volume. 7. A gas etching system according to claim 4 or 6, characterized in that said gas extraction means includes a constant displacement vacuum pump (2). 8. A gas etching system according to claim 7, characterized in that said second sensing means includes a gas flow meter (7). 9. A gas etching system according to claim 8, characterized in that said gas flow meter (7) is coupled to time plot generating means (12) adapted to generate a time plot of the sensed flow rate. 10. A gas etching system according to claim 1, characterized in that said gas etching chamber is a plasma gas etching chamber. 11. A method for detecting the end point of etching in a gas etching system, including the step of coupling a gas etching chamber (1) to a source of vacuum and a source of etchant gas, characterized by the steps of regulating either the gas supply flow rate or the gas flow rate to said source of vacuum while maintaining the other flow rate substantially contrast, and sensing 11 . ( concluded) the time related change of the regulated gas flow to detect transitions between substantially steady levels. 12. A method according to claim 11, charac¬ terized in that said step of regulating is effective to maintain a fixed molar content within said chamber (1). 13. A method according to claim 12, charac¬ terized in that said fixed molar content is represented by the pressure within said chamber (1).

Inventors: COE MARY ELLEN BURROUGHS, COE, MARY, ELLEN, BURROUGHS
Applicants: NCR CO, NCR CORPORATION
Application year: 1982
Main classification: B44C1
IPC codes: H01J37, H01L21