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`Samsung Electronics Co., Ltd. v. Demaray LLC
`Samsung Electronic's Exhibit 1021
`Exhibit 1021, Page 1
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`

`

`
`US. Patent
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`Jul. 11, 1989
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`Sheet 1 of 2
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`4,846,920
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`
`LHON
`
`Bo~|#9Ly92SO~~xyaW29|_CF|ee=
`08ZVIOLa“™~XXveQZTASeviaa”NN
`
`cofyoo~OF
`MASV1MO|—2JONNOS
`
`____
`SONVddI
`INIHOLVA
`MYOMLAN
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`opecemnecroberoecsmeentiontt
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`0¢
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`
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`cv
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`MMW®MALI
`
`YMOMLIN
`
`
`SNISSIOONd
`
`By—
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`
`
`LINA
`
`__Hols
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`OANAS
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`TOMLNOOD
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`
`Ex. 1021, Page 2
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`Ex. 1021, Page 2
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`

`

`US. Patent
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`Jul. 11,1989
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`Sheet2 of2
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`4,846,920
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`rIG.2
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`Ex. 1021, Page 3
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`Ex. 1021, Page 3
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`

`

`1
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`4,846,920
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`PLASMA AMPLIFIED PHOTOELECTRON
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`PROCESS ENDPOINT DETECTION APPARATUS
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`5
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`BACKGROUND OF THE INVENTION
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`The present invention relates generally to the field of
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`material processing, and more particularly to a plasma
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`apparatus and a method for detecting a process end-
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`point.
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`It is desirable to have a non-intrusive, sensitive etch
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`endpoint apparatus and methodto detect the exposure
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`of a desired sublayer in an item being etched. Several
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`techniques have been demonstrated for etch endpoint
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`detection,
`including optical emission spectroscopy,
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`plasma impedance monitoring,and laser interferometry.
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`However,all of these techniques fail to provide suffi-
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`cient sensitivity when there is a very low pattern etch
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`factor, ie., a low percentage of the item’s surface is
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`exposed to the etching medium. Additionally, some of
`20
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`these techniques require considerable signal averaging
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`to improve the signal-to-noise ratio. The use of these
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`methodsthusresults in a slower response to etch plasma
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`compositional changes and a slower response to end-
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`point indicia in the plasma.
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`The failure of the prior art techniques for detecting
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`endpoint in the presence of very low pattern factors
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`provide a significant impediment to the semiconductor
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`industry drive for faster circuit devices. Such faster
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`circuit devices require smaller component dimensions
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`which often result in very low wafer pattern densities.
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`At the sametime,faster etch processes result in the need
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`for more precise endpoint control with a fast endpoint
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`detection response.
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`Alternatively, it is desirable to be able to detect with
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`precision the coverage of a low pattern factor area in a
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`deposition process. Similar detection problems to those
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`noted above are encountered in this type of processing.
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`The invention as claimed is intended to remedy the
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`above-described etch endpoint and deposition endpoint
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`detection problemsand limitations that arise when low
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`pattern factors are present.
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`The advantages offered by the present invention are
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`that extremely low pattern factor endpoints can be
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`45
`detected with high resolution and a very fast response.
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`This endpoint detection can be utilized when etching,
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`for example, a top layer through to another layer there-
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`below, when those twolayers have different work func-
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`tions. Likewise, this invention can be used when depos-
`50
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`iting a top layer on to another layer, where those two
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`layers have different work functions. Accordingly, this
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`invention can be used to detect endpoint when etching
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`or depositing a top layer of metal, semiconductor, or
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`insulator material through or on to another layer there-
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`below of metal, semiconductor or insulator material
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`which layer has a different work function. This inven-
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`tion is particularly advantageousin that it is essentially
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`independent of the plasma composition, it has a high
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`detection signal-noise ratio, and it is not highly wave-
`60
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`length sensitive.
`SUMMARY OF THE INVENTION
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`Briefly, one aspect of the invention comprises a
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`plasma processing apparatus including
`65
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`a plasma chamberfor processing an item that includes
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`a first portion of a first material and a second por-
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`tion of a second material, with thefirst and second
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`materials having different work functions;
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`2
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`meansfor generating a plasma in the plasma chamber,
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`the plasma generating means including an RF-pow-
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`ered electrode excited by an RF excitation fre-
`quency;
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`means for generating and ejecting electrons only
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`when the second material is exposed to the plasma;
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`means for increasing the energies of the generated
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`electrons and accelerating the electrons into the
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`plasma, with sufficient energy to thereby generate
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`a secondary electrons in the plasma;
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`means for receiving a plasma RF discharge voltage
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`signal;
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`meansfor filtering the plasma RF discharge voltage
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`signal
`to remove the RF excitation frequency
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`therefrom; and
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`means for amplifying the natural frequencies of the
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`plasma discharge in response to the electron per-
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`turbation in the plasma discharge voltage signal to
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`thereby detect the processing endpoint or a surface
`condition.
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`In a preferred embodiment, the electron energy in-
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`creasing and accelerating means comprises means
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`for generating an electrode voltage sheath, and
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`means for generating the electrons within this volt-
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`age sheath to thereby accelerate the electrons into
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`the plasma.
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`In a further aspect of this embodiment, the electron
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`generating means may comprise means for directing a
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`beam of photons in a selected energy range onto the
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`item, which enérgy range is not sufficient to eject pho-
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`toelectrons from thefirst material, but is high enough to
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`generate photoelectrons from areas of exposed second
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`material. This photon beam directing means may com-
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`prise means for generating laser pulses.
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`In a further embodimentof the present invention, the
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`filtering means may comprise a capacitor for blocking
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`out any DCsignal components, and notch filter means
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`for removing the harmonics of the RF excitation signal.
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`The present apparatus may further comprise means
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`for integrating the filtered signal. In one embodiment,
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`this integrating means may include meansfor detecting
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`the filtered signal a predetermined time period after the
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`occurrenceof each laser pulse and integrating a plural-
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`ity of the detected filtered signals.
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`In a further aspect of the present invention, a method
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`is disclosed and claimed for detecting the endpoint in a
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`plasma etching or deposition process. This method
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`comprises the steps of
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`disposing an item to be processed in a plasma cham-
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`ber, the item including a first portion of a first
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`material and a second portion of a second material,
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`with the first and second materials having different
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`work functions;
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`generating by means of an RF electrode excited by
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`an RF excitation frequency a plasma in the
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`plasma chamberto process the item;
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`generating and ejecting electrons from the material
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`only when the second material is exposed to the
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`plasma;
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`accelerating the generated electrons
`into the
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`plasma with a sufficient energy to thereby gener-
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`ate secondaryelectrons in the plasma;
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`receiving a plasma discharge voltage signal; and
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`filtering and amplifying the plasma discharge voit-
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`age signal to monitor the natural frequencies of
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`excitation and decay of the discharge plasma, to
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`thereby determine the process endpoint or sur-
`face condition.
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`Ex. 1021, Page 4
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`Ex. 1021, Page 4
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`

`

`4,846,920
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`10
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`55
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`3
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`BRIEF DESCRIPTION OF THE DRAWINGS
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`FIG.1 is a schematic block diagram of one embodi-
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`ment of the present invention.
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`FIG.2 is a schematic circuit diagram ofa filter and
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`amplifier network which maybeutilized to implement
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`the filter and amplifier block 42 of FIG.1.
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`FIG.3 is a graphical representation of an integrated
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`signal response obtained byutilizing the apparatus and
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`method of the present invention.
`DETAILED DESCRIPTION OF THE
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`
`PREFERRED EMBODIMENT
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`The present invention is based on the use of the pho-
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`toelectric effect, i.e., the fact that when an energy beam
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`is directed at a material surface where the energy per
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`quantum is greater than the work function for that ma-
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`terial, then electrons will be ejected from that surface.It
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`was recognized that in an etching process for etching,
`20
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`for example,a top layerof a first material through to a
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`second layer therebelow of a second material, the work
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`functions of those two materials will differ in almost
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`every case. Likewise, in a deposition process, it was
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`recognized that in the deposition of a top layerofa first
`25
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`material on to a second layer of a second material, the
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`work functions. of these two materials will “differ in
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`almost every case. The present invention utilizes the
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`electron-ejection effect in combination with this realiza-
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`tion of the differing work functions for these two layers
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`of material on the item being processed to form an oper-
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`able endpoint detection apparatus and method. Addi-
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`tionally, the invention resides in the use of means to
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`increase the energy of electrons ejected when a given
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`material is exposed and to accelerate those electrons
`35
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`into the plasma with sufficient energy to generate de-
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`tectable secondary electrons. Finally, the present inven-
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`tion resides in the discovery that the response to these
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`secondary electrons in the etching plasma may be de-
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`tected at the natural frequencies ofexcitation and decay
`40
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`of the plasma discharge. Accordingly, the RF plasma
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`excitation frequency and its harmonics, and the DC
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`components in the excitation signal may be removed by
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`appropriate filtering, while the band of frequencies
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`containing the natural frequencies of excitation and
`45
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`decay of the plasma discharge is amplified to obtain a
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`highly enhanced signal/noise ratio.
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`The present invention will first be described in the
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`context of an etching system. However, the invention
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`applies equally to deposition and other processing sys-
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`tems. Referring now to FIG.1, there is showna stan-
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`dard dry etching chamber 10 with an electrode 12 upon
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`which an item 14 to be etched is disposed. This item 14
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`being etched may comprise, by way of example, a top or
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`a first layer 28 of a first material disposed over a second
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`layer 30 of a second material, with the first and second
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`materials having different work functions. (In FIG.1,
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`the second layer comprises the studs 30.) In the example
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`shownin FIG.1, this item to be etched may be a wafer
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`14. By way of example, and not by wayoflimitation, a
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`typical dry etching chamber that may beutilized to
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`perform reactive ion etching is described in the refer-
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`ence L. M. Ephrath, “Dry Etching for VLSI—A Re-
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`view”, in SemiconductorSilicon 1981, (eds. H. R. Huff,
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`Y. Takeishi and R. J. Kriegler), The Electrochemical
`65
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`Society, Pennington, N.J., Vol. 81-5, pp. 627 (1981).
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`Such a chamber would havegasinlets in order to pro-
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`vide an appropriate etching gas mixture for the chamber
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`4
`The RFelectrode 12 in the chamber10 is connected
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`by meansofan electrical line 17 to a standard RF source
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`of energy 18. The RF energy source 18 provides an
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`excitation frequency to excite the gases in the chamber
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`to form an etching plasma therein. The RF excitation
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`frequency from the RF excitation signal source 18 is
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`provided to the electrode 12 by means of an impedance
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`matching network 20. By way of example, and not by
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`wayoflimitation, this impedance matching network 20
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`may be implemented by a standard LC orPicircuit of
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`the type shown in the reference A. J. Diefenderfer,
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`Principles of Electronic Instrumentation, W. B. Saun-
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`ders Co, Philadelphia, Pa. (1979). A second electrode 22
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`is disposed on the opposite side of the chamberfrom the
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`electrode 12 and is connected by meansofa line 24 to a
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`reference potential 26. The RIE etching plasma is gen-
`erated in the volume between the electrodes 12 and 22.
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`The invention further comprises means for generat-
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`ing and ejecting electrons only when a selected material
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`is exposed to the etching plasma. In one embodiment,
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`the means for generating electrons comprises meansfor’
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`directing a beam of energyofeither photonsorparticles
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`in a selected energy range onto the surface of the item
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`14 being etched. This energy rangeis not sufficient to
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`eject electrons from one of the first material layer 28 or
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`the second material
`layer 30 on the item 14 being
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`etched, but is high enough to eject electrons from the
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`otherofthefirst material layer 28 or the second material
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`layer 30, to thereby eject electrons when the other
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`material is exposed.
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`In the embodiment shownin FIG.1, the energy beam
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`directing means comprises an energy beam source 32,
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`an energy beam 34 following a path 35, and a window
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`36 into the chamber 10 to permit application of the
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`energy beam onto the surface of the item 14 being
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`etched. In this embodiment, the energy beam source
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`may be comprised simply of a laser or a UV light
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`source. An ultraviolet wavelength laser such as an ex-
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`cimerlaser, or a frequency-quadrupled Nd:VAGlaser,
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`or a frequency-doubled tunable dye laser may also be
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`utilized, for example. Conveniently, the energy beam
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`source should be a pulsed source or a continuous wave
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`source that is appropriately chopped. The energy beam
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`path 32 may include one or more mirrors 38, as re-
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`quired, in order to direct the energy beam through the
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`window 36 and into the chamber 10. This energy beam
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`may be focussed or unfocused, depending on the
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`amountof area that is to be impinged on the item 14
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`being etched. It may be desirable to also include a win-
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`dow 40 in the chamber 10 and an energy beam stop 41
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`to receive the energy beam afterit is reflected off of the
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`surface ofthe item 14 to prevent the beam from making
`uncontrolled reflections within the chamber 10.
`It
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`should be noted that the energy beam maybe directed
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`normal to the item 14 being etched, or it may be di-
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`rected at an oblique angle to the item 14 being etched.It
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`should also be noted that the more oblique the angle of
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`incidence of the energy beam onto the surface of the
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`item 14, the more generalized will be the measurement
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`for the endpoint.
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`In the example of FIG. 1, when the energy beam 34
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`strikes a metal, semiconductor, or insulator surface, it
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`will eject photoelectrons if the photon energy exceeds
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`the work function, U, of the material. The ejected pho-
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`toelectrons will have an energy, KE,, equal
`to:
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`KE.=hv-U, where hv is the energy of the incident
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`light. However, if the photon energy in the energy
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`beam is less than the work function for the material,
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`Ex. 1021, Page 5
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`Ex. 1021, Page 5
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`

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`4,846,920
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`5
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`then no photoelectrons will be ejected, regardless of the
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`intensity of the energy beam. Accordingly, the energy
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`of the energy beam is chosen so that it does not eject
`electrons from one ofthe first or second materials on
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`5
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`the item 14, but does eject electrons from the other of
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`the first or second materials. By way of example, as-
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`sume thatthe first layer 28 of first material comprises a
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`layer of an insulator such as glass, polyimide,orsilicon
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`dioxide, while the second layer 30 of second material
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`comprises a metal. The use of a laser which generates a
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`UVlight in the range of 230-250 nm results in a photon
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`energy of between 5.4 to 4.9 eV, respectively. A typical
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`metal work function is 4.3 to 4.5 eV, while a typical
`workfunction for an insulator such assilicon dioxideis
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`on the order of 9-10 eV. Thus the direction of an ultra-
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`violet energy beam to strike the first layer 28 of silicon
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`dioxide will not eject photoelectrons. However, when
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`small areas of metal become exposed during the etching
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`process, these exposed metal areas will eject photoelec-
`20
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`trons with an energy of between 0.6 to 0.8 eV, depend-
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`ing on the wavelength of the light and the exact value of
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`the work function for the material. These photoelec-
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`trons thus are characterized by a low kinetic energy and
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`insufficient energy to produce secondaryions bycolli-
`25
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`sional processes.
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`invention further includes
`However,
`the present
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`means for increasing the energies of these low kinetic
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`energy photoelectrons and accelerating them with a
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`sufficient energy into the etching plasma to generate
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`secondary electrons in the plasma. In a preferred em-
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`this photoelectron energy increasing and
`bodiment,
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`accelerating means comprises means for generating an
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`electrode voltage sheath, and means for generating
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`these low kinetic energy photoelectrons within this
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`voltage sheath to thereby accelerate the photoelectrons
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`into the plasma. In the embodiment shown in FIG.1,
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`the photoelectron energy increasing and accelerating
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`means is implemented by disposing the item 14 being
`etched on the RF cathode electrode 12 or the RF anode
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`40
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`electrode 16 during the etching operation. The sheath
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`voltage for these electrodes is determined by the input
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`electrode power density and the gas composition and
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`pressure in the etching chamber 10. For example, the
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`RF cathode electrode 12 will
`typically generate a
`45
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`sheath voltage of 100 eV to 1 KeV either in a batch RIE
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`tool using a 0.25 W/cm 2 electrode powerdensity at a
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`pressure of 50 mTorr, or in a single wafer etch tool
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`using a 1-2W/cm2 electrode power density and at a
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`pressure of 0.5-4 Torr. The anode electrode 16 will
`350
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`typically have a sheath voltage of on the order of
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`30-500 volts for those excitation levels. Thus,
`in the
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`example shown in FIG. 1 with the item 14 disposed on
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`the cathode electrode 12, any low kinetic photoelec-
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`trons produced are ejected within the cathode sheath
`55
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`voltage disposed around the cathode electrode 12. Ac-
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`cordingly, these low kinetic energy ejected photoelec-
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`trons are accelerated by the strong potential field in the
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`cathode sheath. The photoelectrons are accelerated
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`across the sheath, gaining considerable kinetic energy
`from the electrostatic interaction of the electrons with
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`the sheath field so that the photoelectrons are acceler-
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`ated close to the sheath potential, which, as noted previ-
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`ously, ranges from 100 eV to 1 KeV. Accordingly,
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`these low kinetic photoelectrons are converted to high
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`energy electrons which are accelerated into the plasma
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`between the electrodes 12 and 16. In the plasma, these
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`high energy electrons have sufficient energy to induce
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`secondary electrons from collisions with gas phase spe-
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`6
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`these high energy photoelectrons
`cies. Additionally,
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`can strike the opposite electrode 16 and producesec-
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`ondary electrons from that surface. The net result of
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`this generation of secondary electrons is the amplifica-
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`tion of the photoelectron ejection phenomena.
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`If laser pulses are utilized as the energy beam source
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`32 to produce the primary photoelectrons, a repetitious
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`perturbation of the plasma discharge impedancein the
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`chamberresults from the pulsed influx of high energy
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`electrons following each laser shot (assuming an appro-
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`priate work function material has been exposed). This
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`amplified repetitious perturbation of the plasma dis-
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`charge impedance and voltage is caused by the sudden
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`change in the current at the RF electrode as the high
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`energy electrons are ejected into and amplified (by an
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`increase in secondary ejections) by the plasma. Since
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`the RF electrode 12 and the plasmaare electrically
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`coupled,this perturbation results in an oscillation which
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`dampensout in time. It has been discovered that this
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`amplified repetitious perturbation of the plasma dis-
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`charge voltage may be monitored electronically with a
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`high signal/noise ratio, by filtering out the RF excita-
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`tion frequency (usually 13.56 MHz) along with any RF
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`excitation frequency harmonics and DC components of
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`the signal detected at the RF powered electrode 12,
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`while amplifying the frequencies of excitation and
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`decay of the plasma discharge perturbation.
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`In order to detect and measurethis plasma perturba-
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`tion, the RF electrode 12 may be connectedtoa filter
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`and amplifier network 42 to remove unwanted frequen-
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`cies and to amplify desired frequencies. In this regard,
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`applicants have discovered that the major response
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`from this plasma perturbation is in the natural frequen-
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`cies of excitation and decay of the plasma discharge (the
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`inverse of the decay time constant). Accordingly, a
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`series of bandpass and blocking filters may beutilized to
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`remove the RF fundamental excitation frequency, asso-
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`ciated RF excitation frequency harmonics, and the DC
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`self-biased voltage of the cathode 12. Note that in some
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`applications, a set of LC networks may be combined
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`with a low pass filter and a DC blocking capacitor in
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`order to accomplish the desired filtering function. In
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`other applications with high RF power, commercially
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`available blocking networks may be required. Means
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`are also provided for amplifying the natural frequencies
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`of the excitation and decay of the plasma discharge in
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`the plasma discharge voltagesignal, i.e., amplifying the
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`photoelectric signal by tuning the amplification re-
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`sponse of the filter to match the excitation and decay
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`frequencies.
`After the removal of the undesirable DC and RF
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`components from the electrode signal and the amplifica-
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`tion of the natural frequencies of decay of the plasma
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`discharge voltage perturbation, this filtered and ampli-
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`fied signal is applied to a signal processing unit 44. In
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`one embodiment,this signal processing unit may simply
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`comprise an oscilloscope. For a quantitative measure-
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`ment, this signal processing unit 44 may comprise means
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`for integrating the filtered and amplified signal in syn-
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`chronization with the laser pulses from he energy beam
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`source 32. This synchronization can be obtained. by
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`means of a synchronization signal via the line 46. In
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`essence, the signal processing unit operates in accor-
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`dance with the synchronization signal on line 46 to
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`detect the filtered signal at a series of predetermined
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`times after the occurrenceof each laser pulse, and then
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`to integrate these detected filtered signals over aplural-
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`ity of laser pulses. A typical signal processing unit
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`Ex. 1021, Page 6
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`Ex. 1021, Page 6
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`7
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`which maybeutilized to integrate the signal comprises
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`a boxcar integrator circuit. Such a boxcar integrator
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`could be set, for example, to detect the filtered and
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`amplified signal over a series of selected time-windows
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`occurring at a series of different selected times after a
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`given laser pulse, and then to integrate each of these
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`different time-window signals over a series of laser
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`pulses. A standard time-window period might be, for
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`example, 1 microsecond and the numberof laser shots
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`that may be integrated might be in the range of 5-100.
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`Alternatively,
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`implemented by means of a transient digitizer. In es-
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`sence, in this preferred embodiment the sudden appear-
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`ance of 100 KHz to 3 MHz dampedoscillations in phase
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`with the laser shots at the output of the signal process-
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`ing unit 44 indicates that the endpoint has been reached
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`and/orsignals the appearance of the low workfunction
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`Theoutput from the signal processing unit 44 could
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`be applied to an etch servo control unit 48 for control-
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`ling an etching parameter (RF power, gas flow) in the
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`chamber10, or for stopping the etching process when a
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`predetermined signal level is detected by the signal
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`processing unit 44. Some form of threshold detection
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`unit might be included in the control block 48 to facili-
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`tate this operation. A similar servo control unit could be
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`used to control a deposition parameter. Alternatively,
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`the block 48 could simply comprise a chart recorder
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`Referring now to FIG. 3, there is shown a typical
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`integrated plasma perturbation response as seen at the
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`outputof the signal processing unit 44 when low kinetic
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`energy photoelectron pulses have been amplified by an
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`etching plasma. It can be seen that in this graph, the
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`time axis is in microseconds and the voltage axis is in
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`millivolts. The points in the graph represent a series of
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`integrated time-windows occurring after a series of
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`laser pulses. 40 laser shots were integrated in order to
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`form each point in the time graph.
`40
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`Referring now to FIG.2, there is shown one example
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`of a filter and amplifier network for removing various
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`undesirable frequencies from the plasma discharge per-
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`turbation signal and for amplifying the frequencies of
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`excitation and decay of the plasma discharge which
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`may be utilized to implement the filter and amplifier
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`network 40. In this embodiment, the electrode 12 is
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`connected via line 16 to an optional capacitive divider
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`network 50 for reducing the plasma discharge signal
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`voltage to a desired voltage range. In the embodiment
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`shownin FIG.2, this divider network simply comprises
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`the capacitors 52 and 54 connected in electrical series
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`between the line 16 and a reference potential such as
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`ground potential. A reduced voltage in the desired
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`voltage range is taken from a node 56 disposed at the
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`connecting point between the capacitors 52 and 54.
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`The circuit further includes means for blocking any
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`DC components in the plasma discharge signal. This
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`DC blocking function is accom