United States Patent [19] .
`Keller et al.
`
`[11] Patent Number:
`[45] Date of Patent:
`
`4,846,920
`Jul. 11, 1989
`
`[75]
`
`[73] Assignee:
`
`[54] PLASMA AMPLIFIED PHOTOELECTRON
`PROCESS ENDPOINT DETECTION
`APPARATUS
`Inventors: John H. Keller, Poughkeepsie; Gary
`S. Selwyn; Jyothi Singh, both of
`Hopewell Junction, all of N.Y.
`International Business Machine
`Corporation, Armonk, N.Y.
`[21] Appl. No.: 130,573
`[22] Filed:
`Dec. 9, 1987
`[51]
`Int. Cl.4 .................... C23C 14/00; HOlL 21/306;
`B44C 1/22; C23F 1/02
`[52] U.S. Cl. .................................... 156/345; 156/643;
`156/626; 156/627; 204/192.33; 204/298;
`427/10; 427/34; 118/620; 118/665
`[58] Field of Search .............. 204/298, 192.33, 192.32;
`156/643, 646, 345, 626, 627; 118/665, 620;
`427/10, 34
`
`[56]
`
`References Cited •
`U.S. PATENT DOCUMENTS
`4,579,623 4/1986 Suzuki ................................. 156/626
`4,602,981 7/1986 Chen ................................... 156/627
`4,615,761 10/1986 Tada et al. ...................... 156/345 X
`4,664,769 5/1987 Cuomo ............................. 204/192.1
`4,675,072 6/1987 Bennett et al. ................. 204/298 X
`4,687,930 8/1987 Tamura ............................... 250/309
`
`FOREIGN PATENT DOCUMENTS
`686529 5/1964 Canada .
`0061036 4/1984 Japan.
`
`OTHER PUB LI CA TIO NS
`IBM TDB vol. 20, No. 2 Jul. 1977, Geipel-End-Point
`Detection for Reactive Ion Etching.
`Primary Examiner-David L. Lacey
`Assistant Examiner-Thi Dang
`
`Attorney, Agent, or Firm-William T. Ellis
`[57]
`ABSTRACT
`A plasma processing apparatus and process endpoint
`detection method including a plasma chamber for pro(cid:173)
`cessing an item that has a first portion of a first material
`and a second portion of a second material, with the first
`and second materials having different work functions,
`and a structure for generating a plasma in the plasma
`chamber, with the plasma generating structure includ(cid:173)
`ing at least a pair of RF-power electrodes with one of
`them being excited by an RF excitation frequency. The
`apparatus further includes a structure for generating
`and ejecting electrons from the second material only
`when the second material is exposed to the plasma, and
`a structure for increasing the energies of these gener(cid:173)
`ated electrons and accelerating these electrons into the
`etching plasma with sufficient energy to generate sec(cid:173)
`ondary electrons in the plasma. The apparatus further
`includes a structure for receiving a plasma discharge
`voltage signal, a structure for filtering the discharge
`electrical voltage signal to remove the RF excitation
`frequency and any DC components therein, and a struc(cid:173)
`ture for amplifying the natural frequencies of excitation
`and decay of the plasma discharge voltage perturbation
`signal, to thereby detect the processing endpoint.
`In a preferred embodiment, the electron energy increas(cid:173)
`ing and accelerating structure includes a structure for
`generating an electrode voltage sheath, and a structure
`for generating the electrons within this voltage sheath
`to thereby accelerate the electrons into the plasma. The
`electron generating structure includes a structure for
`directing a beam of photons in a selected energy range
`onto the item to be processed, which energy range is
`not sufficient to eject photoelectrons from the first ma(cid:173)
`terial, but is high enough to generate photoelectrons
`from areas of exposed second material.
`
`35 Claims, 2 Drawing Sheets
`
`41-1)_
`
`40
`
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`
`36
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`SIGNAL
`PROCESSING
`UNIT
`
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`SERVO
`CONTROL
`
`I
`I
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`IMPEDANCE I l
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`17
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`FILTER &
`MATCHING
`AMPLIFIER
`NETWORK
`I 1
`NETWORK
`I I
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`18
`-
`I I
`-------- ___7----------~~'-----~
`UV
`100
`LIGHT
`SOURCE
`OR LASER
`
`Page 1 of 11
`
`APPLIED MATERIALS EXHIBIT 1021
`
`

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`IMPEDANCE I 1
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`RF & DC
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`UNIT
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`[
`
`PROCESSING
`
`SIGNAL
`
`44
`
`Fl G.1
`
`Page 2 of 11
`
`

`

`U.S. Patent
`
`Jul. 11, 1989
`
`Sheet 2 of 2
`
`4,846,920
`
`FIG.2
`
`FIG.3
`
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`
`Page 3 of 11
`
`

`

`PLASMA AMPLIFIED PHOTOELECTRON
`PROCESS ENDPOINT DETECTION APPARATUS
`
`25
`
`5
`
`10
`
`BACKGROUND OF THE INVENTION
`The present invention relates generally to the field of
`material processing, and more particularly to a plasma
`apparatus and a method for detecting a process end-
`point.
`It is desirable to have a non-intrusive, sensitive etch
`endpoint apparatus and method to detect the exposure
`of a desired sublayer in an item being etched. Several
`techniques have been demonstrated for etch endpoint
`detection, including optical emission spectroscopy, 15
`plasma impedance monitoring, and laser interferometry.
`However, all of these techniques fail to provide suffi(cid:173)
`cient sensitivity when there is a very low pattern etch
`factor, i.e., a low percentage of the item's surface is
`exposed to the etching medium. Additionally, some of 20
`these techniques require considerable signal averaging
`to improve the signal-to-noise ratio. The use of these
`methods thus results in a slower response to etch plasma
`compositional changes and a slower response to end-
`point indicia in the plasma.
`The failure of the prior art techniques for detecting
`endpoint in the presence of very low pattern factors
`provide a significant impediment to the semiconductor
`industry drive for faster circuit devices. Such faster
`circuit devices require smaller component dimensions 30
`which often result in very low wafer pattern densities.
`At the same time, faster etch processes result in the need
`for more precise endpoint control with a fast endpoint
`detection response.
`Alternatively, it is desirable to be able to detect with 35
`precision the coverage of a low pattern factor area in a
`deposition process. Similar detection problems to those
`noted above are encountered in this type of processing.
`The invention as claimed is intended to remedy the
`above-described etch endpoint and deposition endpoint 40
`detection problems and limitations that arise when low
`pattern factors are present.
`The advantages offered by the present invention are
`that extremely low pattern factor endpoints can be
`detected with high resolution and a very fast response. 45
`This endpoint detection can be utilized when etching,
`for example, a top layer through to another layer there(cid:173)
`below, when those two layers have different work func(cid:173)
`tions. Likewise, this invention can be used when depos(cid:173)
`iting a top layer on to another layer, where those two 50
`layers have different work functions. Accordingly, this
`invention can be used to detect endpoint when etching
`or depositing a top layer of metal, semiconductor, or
`insulator material through or on to another layer there(cid:173)
`below of metal, semiconductor or insulator material 55
`which layer has a different work function. This inven(cid:173)
`tion is particularly advantageous in that it is essentially
`independent of the plasma composition, it has a high
`detection signal-noise ratio, and it is not highly wave-
`length sensitive.
`
`60
`
`SUMMARY OF THE INVENTION
`Briefly, one aspect of the invention comprises a
`plasma processing apparatus including
`a plasma chamber for processing an item that includes 65
`a first portion of a first material and a second por(cid:173)
`tion of a second material, with the first and second
`materials having different work functions;
`
`1
`
`4,846,920
`
`2
`means for generating a plasma in the plasma chamber,
`the plasma generating means including an RF-pow(cid:173)
`ered electrode excited by an RF excitation fre(cid:173)
`quency;
`means for generating and ejecting electrons only
`when the second material is exposed to the plasma;
`means for increasing the energies of the generated
`electrons and accelerating the electrons into the
`plasma, with sufficient energy to thereby generate
`a secondary electrons in the plasma;
`means for receiving a plasma RF discharge voltage
`signal;
`means for filtering the plasma RF discharge voltage
`signal to remove the RF excitation frequency
`therefrom; and
`means for amplifying the natural frequencies of the
`plasma discharge in response to the electron per(cid:173)
`turbation in the plasma discharge voltage signal to
`thereby detect the processing endpoint or a surface
`condition.
`In a preferred embodiment, the electron energy in(cid:173)
`creasing and accelerating means comprises means
`for generating an electrode voltage sheath, and
`means for generating the electrons within this volt(cid:173)
`age sheath to thereby accelerate the electrons into
`the plasma.
`In a further aspect of this embodiment, the electron
`generating means may comprise means for directing a
`beam of photons in a selected energy range onto the
`item, which energy range is not sufficient to eject pho(cid:173)
`toelectrons from the first material, but is high enough to
`generate photoelectrons from areas of exposed second
`material. This photon beam directing means may com(cid:173)
`prise means for generating laser pulses.
`In a further embodiment of the present invention, the
`filtering means may comprise a capacitor for blocking
`out any DC signal components, and notch filter means
`for removing the harmonics of the RF excitation signal.
`The present apparatus may further comprise means
`for integrating the filtered signal. In one embodiment,
`this integrating means may include means for detecting
`the filtered signal a predetermined time period after the
`occurrence of each laser pulse and integrating a plural(cid:173)
`ity of the detected filtered signals.
`In a further aspect of the present invention, a method
`is disclosed and claimed for detecting the endpoint in a
`plasma etching or deposition process. This method
`comprises the steps of
`disposing an item to be processed in a plasma cham(cid:173)
`ber, the item including a first portion of a first
`material and a second portion of a second material,
`with the first and second materials having different
`work functions;
`generating by means of an RF electrode excited by
`an RF excitation frequency a plasma in the
`plasma chamber to process the item;
`generating and ejecting electrons from the material
`only when the second material is exposed to the
`plasma;
`the
`the generated electrons into
`accelerating
`plasma with a sufficient energy to thereby gener(cid:173)
`ate secondary electrons in the plasma;
`receiving a plasma discharge voltage signal; and
`filtering and amplifying the plasma discharge volt(cid:173)
`age signal to monitor the natural frequencies of
`excitation and decay of the discharge plasma, to
`thereby determine the process endpoint or sur(cid:173)
`face condition.
`
`Page 4 of 11
`
`

`

`3
`BRIEF DESCRIPTION OF THE ORA WINGS
`FIG. 1 is a schematic block diagram of one embodi(cid:173)
`ment of the present invention.
`FIG. 2 is a schematic circuit diagram of a filter and
`amplifier network which may be utilized to implement
`the filter and amplifier block 42 of FIG. 1.
`FIG. 3 is a graphical representation of an integrated
`signal response obtained by utilizing the apparatus and
`method of the present invention.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`The present invention is based on the use of the pho(cid:173)
`toelectric effect, i.e., the fact that when an energy beam 15
`is directed at a material surface where the energy per
`quantum is greater than the work function for that ma(cid:173)
`terial, then electrons will be ejected from that surface. It
`was recognized that in an etching process for etching,
`for example, a top layer of a first material through to a 20
`second layer therebelow of a second material, the work
`functions of those two materials will differ in almost
`every case. Likewise, in a deposition process, it was
`recognized that in the deposition of a top layer of a first
`material on to a second layer of a second material, the 25
`work functions of these two materials will --differ in
`almost every case. The present invention utilizes the
`electron-ejection effect in combination with this realiza(cid:173)
`tion of the differing work functions for these two layers
`of material on the item being processed to form an oper- 30
`able endpoint detection apparatus and method. Addi(cid:173)
`tionally, the invention resides in the use of means to
`increase the energy of electrons ejected when a given
`material is exposed and to accelerate those electrons
`into the plasma with sufficient energy to generate de- 35
`tectable secondary electrons. Finally, the present inven(cid:173)
`tion resides in the discovery that the response to these
`secondary electrons in the etching plasma may be de(cid:173)
`tected at the natural frequencies of excitation and decay
`of the plasma discharge. Accordingly, the RF plasma 40
`excitation frequency and its harmonics, and the DC
`components in the excitation signal may be removed by
`appropriate filtering, while the band of frequencies
`containing the natural frequencies of excitation and
`decay of the plasma discharge is amplified to obtain a 45
`highly enhanced signal/noise ratio.
`The present invention will first be described in the
`context of an etching system. However, the invention
`applies equally to deposition and other processing sys(cid:173)
`tems. Referring now to FIG. 1, there is shown a stan- 50
`dard dry etching chamber 10 with an electrode 12 upon
`which an item 14 to be etched is disposed. This item 14
`being etched may comprise, by way of example, a top or
`a first layer 28 of a first material disposed over a second
`layer 30 of a second material, with the first and second 55
`materials having different work functions. (In FIG. 1,
`the second layer comprises the studs 30.) In the example
`shown in FIG. 1, this item to be etched may be a wafer
`14. By way of example, and not by way of limitation, a
`typical dry etching chamber that may be utilized to 60
`perform reactive ion etching is described in the refer(cid:173)
`ence L. M. Ephrath, "Dry Etching for VLSI-A Re(cid:173)
`view", in Semiconductor Silicon 1981, (eds. H. R. Huff,
`Y. Takeishi and R. J. Kriegler), The Electrochemical
`Society, Pennington, N.J., Vol. 81-5, pp. 627 (1981). 65
`Such a chamber would have gas inlets in order to pro(cid:173)
`vide an appropriate etching gas mixture for the chamber
`10.
`
`4,846,920
`
`4
`The RF electrode 12 in the chamber 10 is connected
`by means of an electrical line 17 to a standard RF source
`of energy 18. The RF energy source 18 provides an
`excitation frequency to excite the gases in the chamber
`5 to form an etching plasma therein. The RF excitation
`frequency from the RF excitation signal source 18 is
`provided to the electrode 12 by means of an impedance
`matching network 20. By way of example, and not by
`way of limitation, this impedance matching network 20
`10 may be implemented by a standard LC or Pi circuit of
`the type shown in the reference A. /. Diefenderfer,
`Principles of Electronic Instrumentation, W. B. Saun(cid:173)
`ders Co, Philadelphia, Pa. (1979). A second electrode 22
`is disposed on the opposite side of the chamber from the
`electrode 12 and is connected by means of a line 24 to a
`reference potential 26. The RIE etching plasma is gen-
`erated in the volume between the electrodes 12 and 22.
`The invention further comprises means for generat(cid:173)
`ing and ejecting electrons only when a selected material
`is exposed to the etching plasma. In one embodiment,
`the means for generating electrons comprises means for ·
`directing a beam of energy of either photons or particles
`in a selected energy range onto the surface of the item
`14 being etched. This energy range is not sufficient to
`eject electrons from one of the first material layer 28 or
`the second material layer 30 on the item 14 being
`etched, but is high enough to eject electrons from the
`other of the first material layer 28 or the second material
`layer 30, to thereby eject electrons when the other
`material is exposed.
`In the embodiment shown in FIG. 1, the energy beam
`directing means comprises an energy beam source 32,
`an energy beam 34 following a path 35, and a window
`36 into the chamber 10 to permit application of the
`energy beam onto the surface of the item 14 being
`etched. In this embodiment, the energy beam source
`may be comprised simply of a laser or a UV light
`source. An ultraviolet wavelength laser such as an ex(cid:173)
`cimer laser, or a frequency-quadrupled Nd:V AG laser,
`or a frequency-doubled tunable dye laser may also be
`utilized, for example. Conveniently, the energy beam
`source should be a pulsed source or a continuous wave
`source that is appropriately chopped. The energy beam
`path 32 may include one or more mirrors 38, as re(cid:173)
`quired, in order to direct the energy beam through the
`window 36 and into the chamber 10. This energy beam
`may be focussed or unfocused, depending on the
`amount of area that is to be impinged on the item 14
`being etched. It may be desirable to also include a win(cid:173)
`dow 40 in the chamber 10 and an energy beam stop 41
`to receive the energy beam after it is reflected off of the
`surface of the item 14 to prevent the beam from making
`uncontrolled reflections within the chamber 10. It
`should be noted that the energy beam may be directed
`normal to the item 14 being etched, or it may be di(cid:173)
`rected at an oblique angle to the item 14 being etched. It
`should also be noted that the more oblique the angle of
`incidence of the energy beam onto the surface of the
`item 14, the more generalized will be the measurement
`for the endpoint.
`In the example of FIG. 1, when the energy beam 34
`strikes a metal, semiconductor, or insulator surface, it
`will eject photoelectrons if the photon energy exceeds
`the work function, U, of the material. The ejected pho(cid:173)
`toelectrons will have an energy, KEe, equal
`to:
`KEe=hv-U, where hv is the energy of the incident
`light. However, if the photon energy in the energy
`beam is less than the work function for the material,
`
`Page 5 of 11
`
`

`

`4,846,920
`
`6
`5
`cies. Additionally, these high energy photoelectrons
`then no photoelectrons will be ejected, regardless of the
`intensity of the energy beam. Accordingly, the energy
`can strike the opposite electrode 16 and produce sec-
`ondary electrons from that surface. The net result of
`of the energy beam is chosen so that it does not eject
`this generation of secondary electrons is the amplifica-
`electrons from one of the first or second materials on
`the item 14, but does eject electrons from the other of 5 tion of the photoelectron ejection phenomena.
`the first or second materials. By way of example, as-
`If laser pulses are utilized as the energy beam source
`sume that the first layer 28 of first material comprises a
`32 to produce the primary photoelectrons, a repetitious
`layer of an insulator such as glass, polyimide, or silicon
`perturbation of the plasma discharge impedance in the
`dioxide, while the second layer 30 of second material
`chamber results from the pulsed influx of high energy
`comprises a metal. The use of a laser which generates a 10 electrons following each laser shot (assuming an appro-
`UV light in the range of 230-250 nm results in a photon
`priate work function material has been exposed). This
`energy of between 5.4 to 4.9 eV, respectively. A typical
`amplified repetitious perturbation of the plasma dis-
`metal work function is 4.3 to 4.5 eV, while a typical
`charge impedance and voltage is caused by the sudden
`change in the current at the RF electrode as the high
`work function for an insulator such as silicon dioxide is
`on the order of 9-10 eV. Thus the direction of an ultra- 15 energy electrons are ejected into and amplified (by an
`violet energy beam to strike the first layer 28 of silicon
`increase in secondary ejections) by the plasma. Since
`dioxide will not eject photoelectrons. However, when
`the RF electrode 12 and the plasma are electrically
`small areas of metal become exposed during the etching
`coupled, this perturbation results in an oscillation which
`dampens out in time. It has been discovered that this
`process, these exposed metal areas will eject photoelec-
`trons with an energy of between 0.6 to 0.8 eV, depend- 20 amplified repetitious perturbation of the plasma dis-
`ing on the wavelength of the light and the exact value of
`charge voltage may be monitored electronically with a
`high signal/noise ratio, by filtering out the RF excita-
`the work function for the material. These photoelec-
`tion frequency (usually 13.56 MHz) along with any RF
`trons thus are characterized by a low kinetic energy and
`insufficient energy to produce secondary ions by colli-
`excitation frequency harmonics and DC components of
`sional processes.
`25 the signal detected at the RF powered electrode 12,
`However, the present invention further includes
`while amplifying the frequencies of excitation and
`means for increasing the energies of these low kinetic
`decay of the plasma discharge perturbation.
`In order to detect and measure this plasma perturba-
`energy photoelectrons and accelerating them with a
`tion, the RF electrode 12 may be connected to a filter
`sufficient energy into the etching plasma to generate
`secondary electrons in the plasma. In a preferred em- 30 and amplifier network 42 to remove unwanted frequen-
`bodiment, this photoelectron energy increasing and
`cies and to amplify desired frequencies. In this regard,
`accelerating means comprises means for generating an
`applicants have discovered that the major response
`electrode voltage sheath, and means for generating
`from this plasma perturbation is in the natural frequen-
`cies of excitation and decay of the plasma discharge (the
`these low kinetic energy photoelectrons within this
`voltage sheath to thereby accelerate the photoelectrons 35 inverse of the decay time constant). Accordingly, a
`into the plasma. In the embodiment shown in FIG. 1,
`series ofbandpass and blocking filters may be utilized to
`the photoelectron energy increasing and accelerating
`remove the RF fundamental excitation frequency, asso-
`means is implemented by disposing the item 14 being
`ciated RF excitation frequency harmonics, and the DC
`etched on the RF cathode electrode 12 or the RF anode
`self-biased voltage of the cathode 12. Note that in some
`electrode 16 during the etching operation. The sheath 40 applications, a set of LC networks may be combined
`voltage for these electrodes is determined by the input
`with a low pass filter and a DC blocking capacitor in
`electrode power density and the gas composition and
`order to accomplish the desired filtering function. In
`pressure in the etching chamber 10. For example, the
`other applications with high RF power, commercially
`RF cathode electrode 12 will typically generate a
`available blocking networks may be required. Means
`sheath voltage of 100 eV to 1 KeV either in a batch RIE 45 are also provided for amplifying the natural frequencies
`tool using a 0.25 W /cm 2 electrode power density at a
`of the excitation and decay of the plasma discharge in
`pressure of 50 mTorr, or in a single wafer etch tool
`the plasma discharge voltage signal, i.e., amplifying the
`using a 1-2W /cm2 electrode power density and at a
`photoelectric signal by tuning the amplification re-
`pressure of 0.5-4 Torr. The anode electrode 16 will
`sponse of the filter to match the excitation and decay
`typically have a sheath voltage of on the order of 50 frequencies.
`30-500 volts for those excitation levels. Thus, in the
`After the removal of the undesirable DC and RF
`example shown in FIG. 1 with the item 14 disposed on
`components from the electrode signal and the amplifica-
`the cathode electrode 12, any low kinetic photoelec-
`tion of the natural frequencies of decay of the plasma
`trons produced are ejected within the cathode sheath
`discharge voltage perturbation, this filtered and ampli-
`voltage disposed around the cathode electrode 12. Ac- 55 tied signal is applied to a signal processing unit 44. In
`cordingly, these low kinetic energy ejected photoelec-
`one embodiment, this signal processing unit may simply
`trons are accelerated by the strong potential field in the
`comprise an oscilloscope. For a quantitative measure-
`cathode sheath. The photoelectrons are accelerated
`ment, this signal processing unit 44 may comprise means
`across the sheath, gaining considerable kinetic energy
`for integrating the filtered and amplified signal in syn-
`from the electrostatic interaction of the electrons with 60 chronization with the laser pulses from he energy beam
`source 32. This synchronization can be obtained by
`the sheath field so that the photoelectrons are acceler-
`means of a synchronization signal via the line 46. In
`ated close to the sheath potential, which, as noted previ-
`ously, ranges from 100 eV to 1 KeV. Accordingly,
`essence, the signal processing unit operates in accor-
`dance with the synchronization signal on line 46 to
`these low kinetic photoelectrons are converted to high
`energy electrons which are accelerated into the plasma 65 detect the filtered signal at a series of predetermined
`times after the occurrence of each laser pulse, and then
`between the electrodes 12 and 16. In the plasma, these
`high energy electrons have sufficient energy to induce
`to integrate these detected filtered signals over a plural-
`secondary electrons from collisions with gas phase spe-
`ity of laser pulses. A typical signal processing unit
`
`Page 6 of 11
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`4,846,920
`
`7
`which may be utilized to integrate the signal comprises
`a boxcar integrator circuit. Such a boxcar integrator
`could be set, for example, to detect the filtered and
`amplified signal over a series of selected time-windows
`occurring at a series of different selected times after a 5
`given laser pulse, and then to integrate each of these
`different time-window signals over a series of laser
`pulses. A standard time-window period might be, for
`example, 1 microsecond and the number of laser shots
`that may be integrated might be in the range of 5-100. 10
`Alternatively, the signal processing unit 44 may be
`implemented by means of a transient digitizer. In es(cid:173)
`sence, in this preferred embodiment the sudden appear(cid:173)
`ance of 100 KHz to 3 MHz damped oscillations in phase
`with the laser shots at the output of the signal process- 15
`ing unit 44 indicates that the endpoint has been reached
`and/or signals the appearance of the low work function
`material.
`The output from the signal processing unit 44 could
`be applied to an etch servo control unit 48 for control- 20
`ling an etching parameter (RF power, gas flow) in the
`chamber 10, or for stopping the etching process when a
`predetermined signal level is detected by the signal
`processing unit 44. Some form of threshold detection
`unit might be included in the control block 48 to facili- 25
`tate this operation. A similar servo control unit could be
`used to control a deposition parameter. Alternatively,
`the block 48 could simply comprise a chart recorder
`unit.
`Referring now to FIG. 3, there is shown a typical 30
`integrated plasma perturbation response as seen at the
`output of the signal processing unit 44 when low kinetic
`energy photoelectron pulses have been amplified by an
`etching plasma. It can be seen that in this graph, the
`time axis is in microseconds and the voltage axis is in 35
`millivolts. The points in the graph represent a series of
`integrated time-windows occurring after a series of
`laser pulses. 40 laser shots were integrated in order to
`form each point in the time graph.
`Referring now to FIG. 2, there is shown one example 40
`of a filter and amplifier network for removing various
`undesirable frequencies from the plasma discharge per(cid:173)
`turbation signal and for amplifying the frequencies of
`excitation and decay of the plasma discharge which
`may be utilized to implement the filter and amplifier 45
`network 40. In this embodiment, the electrode 12 is
`connected via line 16 to an optional capacitive divider
`network 50 for reducing the plasma discharge signal
`voltage to a desired voltage range. In the embodiment
`shown in FIG. 2, this divider network simply comprises 50
`the capacitors 52 and 54 connected in electrical series
`between the line 16 and a reference potential such as
`ground potential. A reduced voltage in the desired
`voltage range is taken from a node 56 disposed at the
`connecting point between the capacitors 52 and 54.
`The circuit further includes means for blocking any
`DC components in the plasma discharge signal. This
`DC blocking function is accomplished in FIG. 2 simply
`by connecting a DC blocking capacitor 58 to the node
`56 at one end thereof.
`The circuit further includes means for removing the
`fundamental RF excitation signal from the plasma dis(cid:173)
`charge impedance signal. In the embodiment shown in
`FIG. 2, this means is implemented simply by a notch
`filter 60 connected to the other end of the DC blocking 65
`capacitor 58 at node 59. The notch filter 60 comprises
`an inductor 62 connected in parallel to a capacitor 64,
`with the resulting notch filter designed to be in reso-
`
`55
`
`8
`nance with the RF drive frequency of approximately
`13.56 MHz. In the embodiment of the present invention
`shown in FIGS. 1 and 2, the notch filter 60, the DC
`blocking capacitor 58, the capacitive voltage divider
`network 50, the impedance matching network 20, and
`the RF signal source 18 are all disposed within a ground
`shield 100. Because of the potential for high RF volt(cid:173)
`ages at the notch filter 60, a wire wound inductor 62 is
`utilized in this filter.
`The circuit further includes means for amplifying the
`range of frequencies including the frequencies of decay
`of the plasma discharge perturbation. A variety of dif(cid:173)
`ferent amplifiers may be utilized to perform this amplifi(cid:173)
`cation function. In the embodiment shown in FIG. 2,
`this amplification function is accomplished by connect(cid:173)
`ing node 66, at the other end of the notch filter 60, to the
`reference potential via a capacitor 68. For frequency
`components in the filtered plasma discharge signal
`which are below the resonance frequency for the notch
`filter 60, the notch filter acts as an inductor. Accord(cid:173)
`ingly, the notch filter 60 in combination with the capaci(cid:173)
`tor 68 is designed to be in resonance for a band of these
`lower, frequencies to thereby increase or peak the am(cid:173)
`plitude of the signal in this frequency range. By way of
`example, the notch filter 60-capacitor 68 combination
`could be designed to amplify signal frequencies in the
`range of0.3-7 MHz, and preferably 1-5 MHz.
`In the alternative, if the drive frequency is below the
`perturbation frequencies to be amplified, then inductor
`elements could be substituted for the capacitors 68 and
`78 in FIG. 2 to effect signal amplification.
`The circuit may further include a second lower-volt(cid:173)
`age notch filter 70 disposed outside of the ground shield
`100 for removing any pick-up of the RF excitation fre(cid:173)
`quency fundamental in the low RF environment. This
`notch filter is connected at one end to the node 66, and
`again may comprjse a parallel-connected inductor 72
`and capacitor 74 designed to be in resonance at approxi(cid:173)
`mately 13.56 MHz. Again, the node 76 at the other end
`of the second notch filter 70 may be connected to the
`reference voltage via a capacitor 78 to form a second
`amplifier. The inductive notch filter 70 in combination
`with capacitor 78 again is designed to be in resonance
`for a band offrequencies below the 13.56 MHz notch of
`the filter 70 to thereby increase or peak the amplit