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`(cid:44)(cid:49)(cid:55)(cid:40)(cid:47) EXHIBIT 10(cid:21)1
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`US. Patent
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`Jul. 11, 1989
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`Sheet 1 of2
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`4,846,920
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`US. Patent
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`Jul. 11,1989
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`Sheet 2 of2
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`4,846,920
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`H02
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`SO‘L;J
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`4,846,920
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`means for generating a plasma in the plasma chamber,
`the plasma generating means including an RF-pow-
`ered electrode excited by an RF excitation fre-
`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-
`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-
`creasing and accelerating means comprises means
`for generating an electrode voltage sheath, and
`means for generating the electrons within this volt-
`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-
`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-
`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-
`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-
`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;
`into the
`accelerating the generated electrons
`plasma with a sufficient energy to thereby gener-
`ate secondary electrons in the plasma;
`receiving a plasma discharge voltage signal; and
`filtering and amplifying the plasma discharge volt-
`age signal to monitor the natural frequencies of
`excitation and decay of the discharge plasma, to
`thereby determine the process endpoint or sur-
`face condition.
`
`PLASMA AMPLIFIED PHOTOELECTRON
`PROCESS ENDPOINT DETECTION APPARATUS
`
`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,
`plasma impedance monitoring, and laser interferometry.
`However, all of these techniques fail to provide suffi-
`cient sensitivity when there is a very low pattern etch
`factor, Le, a low percentage of the item’s surface is
`exposed to the etching medium. Additionally, some of
`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
`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
`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
`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.
`This endpoint detection can be utilized when etching,
`for example, a top layer through to another layer there-
`below, when those two layers have different work func-
`tions. Likewise, this invention can be used when depos-
`iting a top layer on to another layer, where those two
`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-
`below of metal, semiconductor or insulator material
`which layer has a different work function. This inven-
`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.
`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
`a first portion of a first material and a second por-
`tion of a second material, with the first and second
`materials having different work functions;
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`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
`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
`may be implemented by a standard LC or Pi circuit of
`the type shown in the reference A. J. Diefenderfer,
`Principles of Electronic Instrumentation, W. B. Saun-
`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-
`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-
`cimer laser, or a frequency-quadrupled NszAG 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-
`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-
`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-
`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-
`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,
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a schematic block diagram of one embodi-
`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-
`toelectric effect, i.e., the fact that when an energy beam
`is directed at a material surface where the energy per
`quantum is greater than the work function for that ma—
`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
`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
`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-
`tion of the differing work functions for these two layers
`of material on the item being processed to form an oper-
`able endpoint detection apparatus and method. Addi-
`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-
`tectable secondary electrons. Finally, the present inven-
`tion resides in the discovery that the response to these
`secondary electrons in the etching plasma may be de-
`tected at the natural frequencies ofexcitation and decay
`of the plasma discharge. Accordingly, the RF plasma
`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
`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-
`tems. Referring now to FIG. 1, there is shown a stan-
`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
`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
`perform reactive ion etching is described in the refer-
`ence L. M. Ephrath, “Dry Etching for VLSI—A Re-
`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).
`Such a chamber would have gas inlets in order to pro-
`vide an appropriate etching gas mixture for the chamber
`10.
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`then no photoelectrons will be ejected, regardless of the
`intensity of the energy beam. Accordingly, the energy
`of the energy beam is chosen so that it does not eject
`electrons from one of the first or second materials on
`the item 14, but does eject electrons from the other of 5
`the first or second materials. By way of example, as-
`sume that the first layer 28 of first material comprises a
`layer of an insulator such as glass, polyimide, or silicon
`dioxide, while the second layer 30 of second material
`comprises a metal. The use of a laser which generates a 10
`UV light in the range of 230-250 nm results in a photon
`energy of between 5.4 to 4.9 eV, respectively. A typical
`metal work function is 4.3 to 4.5 eV, while a typical
`work function for an insulator such as silicon dioxide is
`on the order of 9—10 eV. Thus the direction of an ultra— 15
`violet energy beam to strike the first layer 28 of silicon
`dioxide will not eject photoelectrons. However, when
`small areas of metal become exposed during the etching
`process, these exposed metal areas will eject photoelec-
`trons with an energy of between 0.6 to 0.8 eV, depend- 20
`ing on the wavelength of the light and the exact value of
`the work function for the material. These photoelec-
`trons thus are characterized by a low kinetic energy and
`insufficient energy to produce secondary ions by colli-
`sional processes.
`invention further includes
`However,
`the present
`means for increasing the energies of these low kinetic
`energy photoelectrons and accelerating them with a
`sufficient energy into the etching plasma to generate
`secondary electrons in the plasma. In a preferred em- 30
`bodiment,
`this photoelectron energy increasing and
`accelerating means comprises means for generating an
`electrode voltage sheath, and means for generating
`these low kinetic energy photoelectrons within this
`voltage sheath to thereby accelerate the photoelectrons 35
`into the plasma. In the embodiment shown in FIG. 1,
`the photoelectron energy increasing and accelerating
`means is implemented by disposing the item 14 being
`etched on the RF cathode electrode 12 or the RF anode
`electrode 16 during the etching operation. The sheath 4O
`voltage for these electrodes is determined by the input
`electrode power density and the gas composition and
`pressure in the etching chamber 10. For example, the
`RF cathode electrode 12 will
`typically generate a
`sheath voltage of 100 eV to l KeV either in a batch R113 45
`tool using a 0.25 W/cm 2 electrode power density at a
`pressure of 50 mTorr, or in a single wafer etch tool
`using a l—2W/crn2 electrode power density and at a
`pressure of 0.5—4 Torr. The anode electrode 16 will
`typically have a sheath voltage of on the order of 50
`30—500 volts for those excitation levels. Thus,
`in the
`example shown in FIG. 1 with the item 14 disposed on
`the cathode electrode 12, any low kinetic photoelec-
`trons produced are ejected within the cathode sheath
`voltage disposed around the cathode electrode 12. Ac- 55
`cordingly, these low kinetic energy ejected photoelec-
`trons are accelerated by the strong potential field in the
`cathode sheath. The photoelectrons are accelerated
`across the sheath, gaining considerable kinetic energy
`from the electrostatic interaction of the electrons with 60
`the sheath field so that the photoelectrons are acceler-
`ated close to the sheath potential, which, as noted previ-
`ously, ranges from 100 eV to l KeV. Accordingly,
`these low kinetic photoelectrons are converted to high
`energy electrons which are accelerated into the plasma 65
`between the electrodes 12 and 16. In the plasma, these
`high energy electrons have sufficient energy to induce
`secondary electrons from collisions with gas phase spe-
`
`4,846,920
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`6
`these high energy photoelectrons
`cies. Additionally,
`can strike the opposite electrode 16 and produce sec-
`ondary electrons from that surface. The net result of
`this generation of secondary electrons is the amplifica-
`tion of the photoelectron ejection phenomena.
`If laser pulses are utilized as the energy beam source
`32 to produce the primary photoelectrons, a repetitious
`perturbation of the plasma discharge impedance in the
`chamber results from the pulsed influx of high energy
`electrons following each laser shot (assuming an appro-
`priate work function material has been exposed). This
`amplified repetitious perturbation of the plasma dis-
`charge impedance and voltage is caused by the sudden
`change in the current at the RF electrode as the high
`energy electrons are ejected into and amplified (by an
`increase in secondary ejections) by the plasma. Since
`the RF electrode 12 and the plasma are electrically
`coupled, this perturbation results in an oscillation which
`dampens out in time. It has been discovered that this
`amplified repetitious perturbation of the plasma dis-
`charge voltage may be monitored electronically with a
`high signal/noise ratio, by filtering out the RF excita-
`tion frequency (usually 13.56 MHz) along with any RF
`excitation frequency harmonics and DC components of
`the signal detected at the RF powered electrode 12,
`while amplifying the frequencies of excitation and
`decay of the plasma discharge perturbation.
`In order to detect and measure this plasma perturba-
`tion, the RF electrode 12 may be connected to a filter
`and amplifier network 42 to remove unwanted frequen-
`cies and to amplify desired frequencies. In this regard,
`applicants have discovered that the major response
`from this plasma perturbation is in the natural frequen-
`cies of excitation and decay of the plasma discharge (the
`inverse of the decay time constant). Accordingly, a
`series of bandpass and blocking filters may be utilized to
`remove the RF fundamental excitation frequency, asso-
`ciated RF excitation frequency harmonics, and the DC
`self-biased voltage of the cathode 12. Note that in some
`applications, a set of LC networks may be combined
`with a low pass filter and a DC blocking capacitor in
`order to accomplish the desired filtering function. In
`other applications with high RF power, commercially
`available blocking networks may be required. Means
`are also provided for amplifying the natural frequencies
`of the excitation and decay of the plasma discharge in
`the plasma discharge voltage signal, i.e., amplifying the
`photoelectric signal by tuning the amplification re-
`sponse of the filter to match the excitation and decay
`frequencies.
`After the removal of the undesirable DC and RF
`components from the electrode signal and the amplifica-
`tion of the natural frequencies of decay of the plasma
`discharge voltage perturbation, this filtered and ampli-
`fied signal is applied to a signal processing unit 44. In
`one embodiment, this signal processing unit may simply
`comprise an oscilloscope. For a quantitative measure-
`ment, this signal processing unit 44 may comprise means
`for integrating the filtered and amplified signal in syn-
`chronization with the laser pulses from he energy beam
`source 32. This synchronization can be obtained by
`means of a synchronization signal via the line 46. In
`essence, the signal processing unit operates in accor-
`dance with the synchronization signal on line 46 to
`detect the filtered signal at a series of predetermined
`times after the occurrence of each laser pulse, and then
`to integrate these detected filtered signals over a_ plural-
`ity of laser pulses. A typical signal processing unit
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`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-
`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-
`ferent amplifiers may be utilized to perform this amplifi-
`cation function. In the embodiment shown in FIG. 2,
`this amplification function is accomplished by connect-
`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-
`ingly, the notch filter 60 in combination with the capaci-
`tor 68 is designed to be in resonance for a band of these
`lower, frequencies to thereby increase or peak the am-
`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 of 0.3—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-
`age notch filter 70 disposed outside of the ground shield
`100 for removing any pick-up of the RF excitation fre-
`quency fundamental in the low RF environment. This
`notch filter is connected at one end to the node 66, and
`again may comprise a parallel-connected inductor 72
`and capacitor 74 designed to be in resonance at approxi-
`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 of frequencies below the 13.56 MHz notch of
`the filter 70 to thereby increase or peak the amplitude of
`the signal in this frequency range.
`The circuit further includes means 80 connected to
`node 76 for filtering out any harmonics of the RF exci-
`tation frequency fundamental. In the example embodi-
`ment shown in FIG. 2, the harmonic filtering means
`comprises a resistor 82 connected between node 76 and
`a node 84, and a capacitor 86 connected between the
`. node 84 and the reference potential.
`Finally, the circuit may include an optional fuse Cir-
`cuit 88.
`Photoelectrons will be ejected from a given substrate
`material if the incident light energy exceeds the work
`function of the substrate material. Accordingly, a laser
`beam energy may be tailored to a wide variety of first
`and second materials on the item being etched. The only
`requirement is that the two materials have different
`work functions. Typical examples in which this process
`may be utilized comprise the etching of an insulator
`material such as glass, quartz, or polyimide with typi-
`cally high work functions of on the order of 9—10 eV,
`with a second layer therebelow of a lower work func-
`tion material such as a metal (with a work function in
`the range of 4.3—4.5 eV), or a semiconductor (with a
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`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
`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.
`Alternatively,
`the signal processing unit 44 may be
`implemented by means of a transient digitizer. In es-
`sence, in this preferred embodiment the sudden appear-
`ance of 100 KHz to 3 MHz damped oscillations in phase
`with the laser shots at the output of the signal process-
`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-
`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-
`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
`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
`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
`of a filter and amplifier network for removing Various
`undesirable frequencies from the plasma discharge per-
`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
`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
`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-
`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
`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 resov
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`5
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`9
`work function in the range of 4.2). When an energy
`beam with a selected energy which is less than the work
`function for the insulator material, but greater than the
`work function energy for the material layer therebelow
`is incident on the insulators, then the energy beam will
`not eject photoelectrons at the outset of the etching
`process. However, when the energy beam is incident on
`exposed metal, (e.g., AlCu, Au, or W) or incident on a
`silicon surface, then photoelectrons will be ejected from
`the surface and accelerated by the sheath field to pro- 10
`duce a plasma perturbation.
`In the alternative,
`the low energy work function
`material may be disposed as the top layer over the sec-
`ond layer of a higher work function material. In this
`instance, a plasma perturbation signal would be re- 15
`ceived until the lower work function material has been
`etched away. This plasma perturbation would then
`significantly decrease and this change in the plasma
`perturbation could be monitored and used to determine
`endpoint.
`Additionally, the present apparatus and technique is
`sensitive to silicon and may be used for the endpoint
`detection of heavily doped silicon regions over or under
`materials such a polysilicon. In this regard, N+ silicon
`disposed below polysilicon is generally very difficult to 25
`detect by other diagnostic techniques, especially where
`low pattern factors of the type found with advanced
`transistors are present. However, since the Fermi levels
`are much different between N+ silicon and polysilicon,
`the work functions for these materials also differ, with 30
`the work function of the N+ silicon being lower. Ac-
`cordingly, the use of a frequency-doubled tunable dye
`laser may be used to emit photoelectrons from the N+
`silicon but not from the polysilicon, thereby providing
`an endpoint detection facility.
`As a further point, it has been determined that the
`present apparatus and method is highly sensitive to the
`surface composition f silicon during plasma etching. In
`particular,
`the present apparatus and technique can
`detect in-situ, during plasma processing, the presence of 40
`surface contaminants or extremely thin deposited layers
`disposed over silicon. Such layers form an effective
`barrier to photoelectron penetration, and thus result in a
`change in the observed photoelectric signal. Since other
`in situ techniques, such as laser interferometry, require 45
`much greater film thicknesses before detection is possi-
`ble, the present apparatus and method offer consider-
`able advantage in sensitivity for use in an in—situ surface
`analysis technique. Conventional surface monitoring
`techniques, such as X-ray photoelectron spectroscopy, 50
`or auger spectroscopy are highly surface sensitive, but
`the use of these techniques requires the transfer of the
`wafer to an ultra-high vacuum environment and so may
`only be employed for post-process analysis after the
`etching process is complete. Accordingly, the present 55
`apparatus and method is advantageous for the measure-
`ment and detection of surface contaminants or sputter
`deposited films during the actual plasma exposure per-
`iod.
`Note that the present structure can be used generally 60
`to detect the uniform deposition of a layer over an elec-
`tron emitting layer. In this case, there would be an
`initial signal generated by the generation of secondary
`electrons in the plasma. This signal would disappear
`when a uniform layer was deposited over the electron 65
`emitting material.
`-
`The present etching apparatus has been discussed in
`the context of an item with a first layer of a first material
`
`10
`disposed in a vertical relationship with a second layer of
`a second material. However, the invention is not limited
`for use in etching items with this configuration. In this
`regard, a first portion of a first material and a second
`portion of a second material may be disposed in differ-
`ent lateral locations on the item to be etched, and not
`directly over each other. This first and second material
`relationship could be used to determine endpoint.
`The present inventio