Plasma reactors for processing semiconductor wafers

Abstract: The disclosure relates to a plasma chamber RF excitation system including a high frequency RF power source (12a) having a fixed RF match circuit (30) at its output and sensing and control apparatus (32) for sensing the amount of RF power delivered by the RF power source and for regulating the output power level of the RF power source so as to maintain the RF power delivered by the RF power source at a desired level, and an RF plasma chamber including an RF radiator. The power source is mounted proximate or directly on the plasma chamber (10) so that the distance between them is much less than an eighth of a wavelength at the frequency of the RF source. The system may further include an endpoint detector (12c) for a plasma etch process or a chamber cleaning process which halts the process when the VSWR or reflected power ceases to change in response to the progress of the etch process.

Description: The invention is related to high frequency capacitively coupled radio frequency (RF) plasma reactors for semiconductor wafers and in particular to improvements in the control of the RF source power level and to protection of the wafer pedestal from damage during chamber cleaning operations. Referring to Figure 1A, there is shown a known apparatus in which a plasma chamber 10 operating at a high RF frequency (e.g. 13.56 MHz) holds a semiconductor wafer or substrate (not shown) and performs a desired process on the wafer, such as plasma etching, plasma-enhanced chemical vapour deposition, or sputtering . In order to maintain a plasma within the chamber 10 RF power is furnished to the chamber 10 by a conventional 13.56 MHz RF generator 12 via a transmission line 14 (such as a 50 ohm coaxial cable) and a variable RF match circuit 16. The convention al RF generator 12 includes a 13.56 MHz RF power source 12a, an output stage 12b, and a conventional dual directional coupler 12c. The output stge 12b matches the non-standard output impedance of the RF power circuit 12a to the industry standard characteristic impedance of the coaxial cable of 50 ohms. The dual directional coupler 12c provides outputs 18a, 18b indicative of the forward and reflected RF power at the output of the output stage 12b. A conventional feedback control circuit 20 governs the variable RF match circuit in such a manner as to minimize the VSWR, that is, to minimize the reflected RF power sensed at the reflected power output 18b of the dual directional coupler. Specifically, the control circuit adjusts the variable RF match network so as to transform the impedance of the plasma chamber to the 50 ohm characteristic impedance of the coaxial cable. Since the plasma chamber impedance tends to vary during wafer processing, the variable RF match circuit 16 maintains an RF match by changing its impedance to compensate for fluctuations in the plasma chamber impedance. EP-A-0058820 discloses such an RF generator system for supporting ionizing power to gases in a dry process wafer etching system. The generator provides power to the load via a transmission line and load impedance is automatically matched to the surge impedance of the transmission to provide maximum transfer of energy from the generator to the load. Voltages representative of input power and reflected power are utilized to control a match circuit to cause load impedance to equal transmission time impedance. Feedback loop means are also provided to maintain net load power at an extendably commanded level. JP-A-58 073848 also discloses an arrangement for ensuring a constant power consumption irrespective of variation in local impedance by maintaining a constant difference between an incident wave power to the load and the reflected power from the load. The variable RF match circuit and its feedback control circuit to maintain a low VSWR are essential for three reasons: (1) It provides a constant load impedance to the output stage of the RF generator, so that the generator will supply a constant RF power level to the plasma chamber. Fluctuations in the RF power level would cause instability and unpredictability in the semiconductor processing performed by the plasma chamber. (2) It avoids the high peak voltages associated with a high VSWR on the transmission line 14, which high voltages could damage the transmission line. (3) It avoids the high peak currents associated with a high VSWR on the transmission line, which currents could cause overheating of the transmission line which could change its characteristic impedance, thereby causing nonuniform chamber process conditions depending on the temperature history of the transmission line. One problem with the system of Figure 1 is that the variable RF match circuit 16 is expensive and bulky. Furthermore, variable RF match networks which employ moving parts tend to be unreliable. However, at the high RF power levels of 1200 Watts required for chemical vapor deposition processes or 2000 Watts required for etch or chamber cleaning operations, the variable RF match network 14 has seemed indispensable at high RF frequencies on the order of 13.56 MHz. Referring to Figure 1B, a low frequency (e.g., 350 kHz) RF generator 12' has a low frequency power circuit 12a' in lieu of the high frequency power circuit 12a of Figure 1A. The low frequency RF generator 12' is coupled through a fixed RF match network 30 to the plasma chamber 10. Because the RF frequency is so low, the transmission line between the RF generator and the plasma chamber generally is much shorter than a quarter wavelength, so that no standing wave pattern occurs in the transmission line. Therefore the above-mentioned problems (2) and (3) of excessive peak voltages and peak currents will not occur even if there is a mismatch between the plasma chamber impedance and the transmission line. However, item (1), the effect of plasma chamber load impedance changes on RF power source output level, still must be addressed. In one known low frequency (350 kHz) RF generator design shown in Figure 1B, a control circuit 32 dynamically adjusts the power level of the RF power circuit 12a to maintain the delivered RF power (i.e. the difference between the forward power 18a and reflected power 18b) at a constant level. Since this control circuit makes the RF power level independent of moderate changes in load impedance, this design omits the variable RF match circuit and its feedback control 20, and substitutes a fixed RF match network 30. The fixed RF match network 30 has a fixed output impedance which is selected to match the average impedance of the plasma inside the chamber 10 over time. Since the plasma impedance will necessarily fluctuate, there will be RF mismatching and loss of RF power due to power reflection back to the RF generator 12. This contrasts with the approach of Figure 1A in which the power level of the RF circuit 12a is maintained at a constant level and fluctuations in the plasma impedance are compensated by changing the impedance of the variable match circuit 16 to minimize the reflected RF power. Since the transmission line 14 is short compared to a quarter wavelength, it does not matter which end of the transmission line the match network is connected to, so for convenience the fixed RF match circuit 30 can be included as a component within the RF power generator 12'. The control circuit 32 includes a first subtractor node 34 which senses delivered RF power (i.e., the difference between the forward power and reflected power outputs 18a, 18b of the dual directional coupler 12c) and a second subtractor node 36 which senses the difference between the delivered RF power and a desired RF power set by the user. (In the diagram of Figure 2, the desired power level is depicted as a variable analog power source 38 applied to one input of the subtractor node 36, although the source 38 may be digital and the comparison may be carried out digitally by the subtraction node 36.) The output of the second subtracter node is applied as an error signal to a control input of the RF power circuit 12a. The RF power circuit 12a responds to the error signal by either increasing or decreasing the output power level, depending upon the sign of the error signal. This servos the power output level of the RF power circuit 12a to minimize or null the error signal so that the delivered power is maintained at the desired power level. The error signal applied to the control input of the RF power circuit 12a is zero in the absence of fluctuations in the plasma impedance. The low frequency RF plasma system of Figure 1B advantageously avoids the need for an expensive, potentially unreliable, variable RF match circuit. However, it has not seemed possible to avoid the use of a variable RF match circuit at higher RF frequencies, such as the commonly used frequency of 13.56 MHz. This invention provides a high frequency RF plasma system for processing a semiconductor substrate, comprising a plasma chamber adapted to receive a semiconductor substrate and a process gas therein, a coupling device for coupling RF energy to the process gas inside the chamber, and an RF power generator mounted adjacent to the plasma chamber, including an RF power source for producing RF energy, an RF impedance matching circuit connected between the RF power source and the coupling device, a power sensing coupler connected in series with the match circuit between the RF power source and the coupling device, said power sensing coupler having a measurement output indicating the amount of RF power delivered through the coupler, and a control circuit coupled to said measurement output of said power sensing coupler and connected to said RF power circuit (12a) to govern the amount of RF power produced by said RF power circuit so as to maintain the delivered power measured by the coupler at a predetermined level, wherein said RF power source produces RF energy having a frequency of at least 1 MHz, the power source having an output located at a distance from the coupling device of substantially less than an eighth of a wavelength at said frequency; and in that the RF impedance matching circuit has a fixed impedance. In a further aspect the invention provides a plasma reactor system according to claim 9. Preferably, the RF power source and fixed match circuit are mounted directly on the plasma chamber to minimize such distance. Optionally, the invention also includes apparatus for detecting the endpoint of an etch or cleaning process by detecting when the reflected power or VSWR ceases to change as a function of the removal of the substance being etched or cleaned. The following is a description of some specific embodiments of the invention, reference being made to the accompanying drawings, in which: Figure 1A is a simplified block diagram of a high frequency plasma reactor and RF control system of the prior art. Figure 1B is a simplified block diagram of a low frequency plasma reactor and RF control system of the prior art. Figure 2 is a block diagram of a plasma reactor and RF control system of a first embodiment of the invention. Figure 3 is a Smith chart corresponding to the embodiment of Figure 2. Figure 4 is a block diagram of a plasma reactor and RF control system of a second embodiment of the invention. Plasma Excitation Circuit with Fixed RF Match The present invention may be used with any plasma chamber for processing semiconductor wafers or substrates, such as plasma etch chambers and plasma-enhanced deposition chambers. The chamber may using any conventional device for electrically coupling RF power into the chamber, such as electrodes for capacitive coupling, a coil for inductive coupling, or an antenna for electromagnetic coupling of RF power into the chamber. Referring to Figure 2, the invention employs a novel high frequency RF generator 12" having the same topology as the prior art low frequency RF generator 12' of Figure 1B. A high frequency is considered a frequency of at least 1 MHz. Like the prior art low frequency generator, but unlike the prior art high frequency RF generator, our design uses a fixed RF match and includes a control circuit 32 which maintains delivered RF power at a desired level. Our design has a high frequency power source 12a in lieu of the low frequency power source 12' of Figure 1B. Importantly, our design avoids the need for a variable RF match circuit by locating the RF generator a very short distance from the electrode, inductor, antenna, or other RF coupling device in the plasma chamber 10 to which the RF power is being delivered, specifically, a distance which is much less than an eighth of a wavelength at the frequency of the RF generator. At a typical high RF frequency of 13 MHz, the RF signal has a wavelength in a typical coaxial transmission line of about 15 meters, so that an eighth of a wavelength would be aboue 2 meters. Preferably, the RF generator 12" is mounted directly on the plasma chamber 10 so that there is negligible distance separating the two components. The elimination of a long transmission line between the RF generator and the plasma chamber allows us to substitute a relatively inexpensive, compact, and reliable fixed RF match circuit 30 in place of the much more expensive and often less reliable RF match circuit 16 used in conventional high frequency RF power systems. The use of a fixed RF match means the RF generator will experience all fluctuations in the load impedance of the plasma chamber. The controller 32 maintains a constant delivered power (forward power minus reflected power) in spite of changes in the plasma chamber load impedance during processes performed within the chamber. The principles for designing an RF match circuit to match the output impedance of an RF generator to the impedance of a load are well known. The RF match contains inductors (L) and/or capacitors (C) whose values are chosen according to straightforward design equations to match given input and output impedances at a given frequency. For example, the design equations are described in the following reference books, the complete contents of which are hereby incorporated by reference into this patent application: "Reflections-Transmission Lines and Antennas" by Walter Maxwell (1990); and "Radio Frequency Transistors - Principles and Practical Applications" by Norm Dye and Helge Granberg (1993). The RF match circuit should be optimised to maintain the VSWR, caused by the impedance mismatch between the output impedance of the RF generator and the impedance of the plasma reactor load, below a value at which the RF generator can safely supply the required delivered power. Most RF generators can tolerate a higher VSWR when operating below their rated maximum output power. Therefore, the RF match L and C component values should be chosen to provide a relatively low VSWR during the plasma processes that require the highest RF power. The Smith chart of Figure 3 illustrates the method used to optimise the RF match circuit 30 in our invention. The small squares on the chart represent the load impedance of the plasma chamber during different phases of a typical plasma-enhanced chemical vapour deposition (PECVD) process performed within the chamber. The small crosses on the chart represent the chamber load impedance during different phases of a typical plasma etch process used to clean the chamber between PECVD processes. The exemplary CVD process requires 1200 watts of RF power, and the etch process requires 2000 watts of RF power, both at a frequency of 13.56 MHz. The commercially available RF generator used in our example can deliver 2000 watts to a load if the VSWR is 2 or less, and can deliver 1200 watts if the VSWR is 3 or less. If the L and C components of the RF match are chosen to match the 40 ohm RF generator output impedance to the chamber "center" impedance indicated by the large dot in Figure 3, then all the crosses fall within the VSWR - 2 circle, and all the squares fall within the VSWR - 3 circle. Therefore, designing the RF match to this "center" impedance accomplishes our objectives. The power level regulator circuit in the design shown in Figure 2 is identical to the regulator circuit of the prior art system shown in Figure 1B. A conventional 50 ohm dual directional coupler 12c provides the requisite measurements of forward and reflected power 18a, 18b to a control circuit 32. The first comparator or subtracter circuit 34 determines the power delivered to the load (the plasma chamber) by subtracting the rectified and low pass filtered forward power signal 18a from the reflected power signal 18b. A second comparator or subtracter circuit 36 compares the delivered power level as so determined with a value 38 representing the desired delivered power. The second comparator adjusts a control sent to the power source 12a according to whether the delivered power is too high or too low. The circuitry described may be implemented by conventional, straightforward techniques in analog or digital form, or by programming a general purpose microcomputer. A variation on the design shown in Figure 2 would be to simplify the RF generator by eliminating the output stage 12b which converts the output impedance of the RF power source 12a to the 50 ohm industry standard impedance. The L and C components of the fixed RF match circuit then would be chosen according to well known design equations to match the non-standard output impedance of the RF power source instead of matching 50 ohms. The complication presented by this variation is that the dual directional coupler also would have to be designed for this non-standard impedance. Dual directional coupler design principles are also extremely well known, so designing one for a non-standard impedance presents no conceptual difficulty, but it precludes using an "off the shelf" 50 ohm dual directional coupler. Figure 4 illustrates a further variation of the invention in which the output stage 12b is eliminated as just described, and furthermore the respective positions within the topology of the RF generator circuit 12 of the dual directional coupler and the RF match are swapped. An advantage of the embodiment of Figure 4 is that the measurements of forward and reflected power provided by the dual directional coupler 54 more accurately indicate the actual power delivered to the plasma chamber. In the previously described embodiments in which the fixed RF match network 30 is located between the dual directional coupler 12c and the reactor chamber 10 as shown in Figure 2, the dual directional coupler is measuring the total power delivered to the match network 30 and the plasma chamber 10. However, five to ten percent of the delivered power typically will be dissipated in the match network 30; consequently, the power actually delivered to the plasma chamber will be 90-95% of the power indicated by the dual directional coupler as being delivered. The amount of power will vary within this range as the impedance presented by the plasma chamber varies under different plasma process conditions. Therefore, there may be up to a 5% unpredictability in the RF power delivered to the chamber. By locating the dual directional coupler 54 between the match network 52 and the chamber as shown in Figure 4, the coupler 54 will measure the power actually delivered to the plasma chamber. Therefore, the power delivered to the chamber will be more accurately controlled by the control circuit 32, thereby permitting more uniform and predictable processing of the semiconductor substrate in the chamber. In the Figure 4 embodiment, because there is no RF match circuit between the dual directional coupler and the plasma chamber, the dual directional coupler 54 must be designed to operate at the load impedance of the plasma reactor rather than the output impedance of the RF power source. Process Endpoint Detection As described above, the control circuits of the embodiments of Figures 2 and 4 permit the VSWR, and hence the reflected power to fluctuate rather than attempting to null the reflected power in response to fluctuations in the plasma impedance. In accordance with another aspect of the invention, this feature is exploited during etching or chamber cleaning operations to determine when the substance being etched has been completely removed, or when the chamber has been thoroughly cleaned, so that the etching or cleaning process may be terminated. In a conventional process for cleaning a deposition chamber, any production wafer is removed from the chamber, and then a strong etchant gas is introduced into the chamber while a high level of RF power is applied to the chamber to excite the etchant gas into a plasma state. As a result, various unwanted deposits are removed from the chamber side walls and pedestal. If for example the reactor is normally used to perform chemical vapor deposition of silicon nitride on production wafers, then silicon nitride deposits accumulate over many deposition cycles to the extent that they interfere with the chemical vapor deposition process during subsequent production cycles and must be removed before the reactor can be used again. However, the length of time that the chamber cleaning step must be performed to remove all unwanted deposits is difficult to determine, and so it is customary to perform the cleaning step for an excessive amount of time to guarantee removal of all deposits. Unfortunately, after the unwanted deposits are removed, continuing the chamber cleaning process will etch any exposed metal components within the chamber interior, such as the aluminum chamber walls, the aluminum gas inlet, and the aluminum wafer support pedestal. This etching eventually damages the chamber components to the point where they must be replaced. Therefore, the life of the chamber components could be extended if the etching could be stopped as soon as all the unwanted deposits were removed. Our solution to this problem is to sense the change in plasma chamber load impedance during the chamber cleaning operation. Such a change in plasma impedance indicates the removal of deposits from the chamber interior surfaces, because the thickness of deposits on the chamber walls affects the impedance between the metal chamber components and the plasma body. Specifically, our endpoint detection design senses changes in the plasma impedance by sensing changes in the VSWR or the reflected power, as measured by the dual directional coupler 12c (Figure 2 or 4). Initially during roughly the first ten seconds of a typical chamber cleaning process, there is no perceptible change in the plasma impedance because the incremental amount of deposits being removed represents a relatively small fraction of the total deposits. Then, after the remaining layer of deposits becomes appreciably thinner, the plasma chamber load impedance begins changing noticeably as the layer of deposits continues to diminish. Once all deposits or contamination have been removed from the chamber interior, the plasma impedance stops changing (or, the rate of change of the plasma impedance falls below a predetermined threshold). Thus, the completion of the chamber cleaning operation is signaled by the reflected RF power reaching a constant level (or by a reduction in the rate of change of the reflected power below the predetermined threshold). At this time, the chamber cleaning process may be halted. For this purpose, an alarm signal to halt the chamber cleaning operation is generated by a microprocessor 70 shown in Figure 4. The microprocessor 70 is programmed to wait (during the first ten seconds of the chamber cleaning operation) until the plasma impedance first begins to change. Thereafter, the processor continuously determines whether the reflected RF power is changing at a rate of change exceeding a predetermined threshold rate (which, for example, may be zero). Once the rate of change is below the predetermined threshold for a minimum amount of time (e.g., one second), the alarm signal indicating completion of the chamber cleaning operation is transmitted. In one embodiment, the microprocessor 70 has an input port coupled through an analog-to-digital converter 72 to the reflected power output signal 18b from the dual directional coupler 54. The microprocessor 70 samples the reflected power signal periodically and compares it with the value sampled during the previous processing cycle. If the difference falls below a predetermined value for a predetermined number of processing cycles, then the microprocessor 70 outputs a signal indicating to the user that the chamber cleaning operation is complete and should be halted to avoid excess wear on the chamber interior surfaces. Either the user must halt the chamber cleaning operation or else this signal may be employed to automatically halt the chamber cleaning operation as well as signalling the user. While a digital device (the microprocessor 70 and analog-to-digital converter 72) has been disclosed as providing the "completion-of-chamber clean" signal, the microprocessor 70 may be replaced by a dedicated logic circuit or else an analog circuit. The same endpoint detection process also can be used in a plasma etch chamber to detect when a film being etched from a semiconductor substrate has been completely removed. The RF excitation circuit having a fixed RF match network can be use equally well with any means for coupling RF power to a plasma, such as a pair of electrodes for capacitively coupling power to the plasma, or an inductor or antenna for inductively coupling power to the plasma. Numerous designs for such power coupling devices are known in the art. While the invention has been described in detail by specific reference to preferred embodiments, it is understood that variations and modifications thereof may be made without departing from the scope of the invention as defined by the appended claims.

Claims: A high frequency RF plasma system for processing a semiconductor substrate, comprising: a plasma chamber (10) adapted to receive a semiconductor substrate and a process gas therein; a coupling device for coupling RF energy to the process gas inside the chamber; and an RF power generator (12") mounted adjacent to the plasma chamber, including an RF power source (12a) for producing RF energy; an RF impedance matching circuit (30) connected between the RF power source and the coupling device; a power sensing coupler (12c) connected in series with the match circuit between the RF power source and the coupling device, said power sensing coupler having a measurement output indicating the amount of RF power delivered through the coupler; and a control circuit (32) coupled to said measurement output of said power sensing coupler and connected to said RF power circuit (12a) to govern the amount of RF power produced by said RF power circuit so as to maintain the delivered power measured by the coupler at a predetermined level; characterised in that said RF power source produces RF energy having a frequency of at least 1 MHz, the power source having an output located at a distance from the coupling device of substantially less than an eighth of a wavelength at said frequency; and in that the RF impedance matching circuit has a fixed impedance. A reactor system as claimed in Claim 1, characterised in that said power sensing coupler comprises a dual directional coupler (12) whose measurement output includes: a forward signal output (18a) indicating the amount of power transmitted forward through the coupler and, a reflected signal output (18b) indicating the amount of power reflected back through the coupler, so that the difference between the forward and reflected signal outputs indicates the amount of RF power delivered through the coupler. A reactor system as claimed in Claim 1, characterised in that said power sensing coupler (12c) is connected in series between the power source (12a) and the matching circuit (30) whereby the measurement output of the coupler indicates the total RF power delivered to the matching circuit and the chamber. A reactor system as claimed in Claim 1, characterised in that said power sensing coupler (12c) is connected in series between the matching circuit (30) and the chamber (10), whereby the measurement output of the coupler indicates the RF power delivered to the chamber. A reactor system as claimed in Claim 3, further comprising an output stage (12b) connected between said RF power circuit (12a) and said dual directional coupler (12c), said RF power circuit, said output stage and said dual directional coupler being within an RF generator and said fixed RF match circuit (30) being mounted directly on said RF generator. A reactor system as claimed in Claim 5, characterised in that said output stage (12b) provides a predetermined output impedance, and in that said fixed RF match circuit (30) has a reactance providing an RF match between said predetermined output impedance and an expected load impedance lying in a range of load impedances presented by a plasma in said reactor chamber. A reactor system as claimed in Claim 4, characterised in that said fixed RF match circuit (30) has a reactance providing an RF match between an output impedance of said RF power circuit and an expected load impedance lying in a range of load impedances presented by a plasma in said reactor chamber. A reactor system as claimed in Claim 1, further comprising a sensor (70, 72) connected to said control circuit for sensing during a chamber cleaning operating whether said power reflected back has reached a stable level, indicative of a complete removal of contamination from said reactor chamber. A plasma reactor system, comprising: an RF plasma chamber (10) including an RF coupling device for coupling RF power into a chamber; an RF power source (12a) on said RF plasma chamber, comprising: an RF power circuit (12a) providing an RF signal of a high frequency greater than 1 MHz; an RF match circuit (30); and sensing and control means (12c, 32) for sensing the amount of RF power delivered by said RF power source and for regulating the output power level of said RF power circuit so as to maintain said RF power delivered at a desired level; characterised in that the RF power source is a high frequency source and in that the RF match circuit is a fixed RF match circuit cablelessly connected between said RF power circuit and said RF coupling device. A plasma reactor system as claimed in Claim 9, characterised in that said high frequency is of the order of about 13 MHz. A plasma reactor system as claimed in Claim 9, characterised in that said sensing and control means comprises: means (12c) for sensing forward power and reflected power at said fixed RF match circuit; means (34) for computing delivered Rf power as a difference between said forward power and reflected power; means (36) for comparing said delivered RF power to a desired power level to produce an error signal and for changing the power produced by said RF power source (12a) so as to null said error signal. A plasma reactor system as claimed in Claim 11, characterised in that said means for sensing (12c) is connected to an RF input of said fixed RF match circuit (30). A plasma reactor system as claimed in Claim 11, characterised in that said means for sensing (54) is connected to an RF output of said fixed RF match circuit (52). A plasma reactor as claimed in Claim 9, characterised in that said fixed RF match circuit (30) provides an approximate RF match between an output impedance of said reactor chamber. A plasma reactor as claimed in Claim 14, characterised in that said nominal impedance is a load impedance presented by a plasma within said reactor chamber (10) and lies in a range of load impedances presented by said plasma under different operating conditions of said reactor chamber. A plasma reactor as claimed in Claim 15, characterised in that said range corresponds to a voltage standing wave ratio between forward and reflected power not exceeding about 3:1. A reactor system as claimed in claim 11, further comprising a sensor (70, 72) connected to said control circuit (32) for sensing during a chamber cleaning operation whether said power reflected back has reached a stable level, indicative of a complete removal of contamination from said reactor chamber (10).

Inventors: MCNUTT GERALD, SMYTH KENNETH, TAYLOR WILLIAM NIXON JR, WOLFF STEFAN, ZHAO JUN, MCNUTT, GERALD, SMYTH, KENNETH, TAYLOR, WILLIAM NIXON, JR., WOLFF, STEFAN, ZHAO, JUN
Applicants: APPLIED MATERIALS INC, APPLIED MATERIALS, INC.
Application year: 1995
Main classification: H01J37
IPC codes: H01J37, H05H1, H01L21