United States Patent [19]
`Sawin et al.
`
`IIIIIIIIIIIIIIIIIIIIIIIIUIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII
`USOO5450205A
`[ii] Patent Number:
`5,450,205
`[45] Date of Patent:
`Sep. 12, 1995
`
`[75]
`
`[56]
`
`[54] APPARATUS AND METHOD FOR
`REAL-TIME MEASUREMENT OF THIN
`FILM LAYER THICKNESS AND CHANGES
`THEREOF
`Inventors: Herbert H. Sawin, Arlington;
`William T. Conner, Somerville;
`Timothy J. Dalton, N. Reading;
`Emanuel M. Sachs, Somerville, all of
`Mass.
`[73] Assignee: Massachusetts Institute of
`Technology, Cambridge, Mass.
`[21] Appl. No.: 70,118
`[22] Filed:
`May 28,1993
`[51] Int. C1.6
`G01B 11/06; H01L 21/00
`[52] U.S. Q
`356/382; 356/357;
`156/626.1
`[58] Field of Search
`356/381, 382, 355, 357,
`356/345, 445; 250/560; 156/601, 612, 626, 627
`References Cited
`PATENT DOCUMENTS
`U.S.
`3,892,490 7/1975 Uetsuki et al
`4,053,234 10/1977 McFarlane
`4,198,261 4/1980 Busta et al
`4,454,001 6/1984 Sternheim et al
`4,713,140 12/1987 Tien
`4,841,156 6/1989 May et al
`5,034,617 7/1991 Isobe
`5,042,949 8/1991 Greenberg et al
`5,056,923 10/1991 Ebisawa et al
`OTHER PUBLICATIONS
`J. W. Coburn, "Summary Abstract: Diagnostics in
`Plasma Processing," J. Vac. ScL Technol. A 4(3), 1830
`(May/Jun. 1986).
`A. G. Nagy, "Radial Etch Rate Nonuniformity in Reac(cid:173)
`tive Ion Etching," J. Ekctrochem. Soc. 131(8), 1871
`(Aug. 1984).
`Alan S. Kao & Harvey G. Stenger, Jr., "Analysis of
`Nonuniformities in the Plasma Etching of Silicon," /.
`Ekctrochem. Soc. 137(3), 954 (Mar. 1990).
`Paul J. Marcoux & Pang Dow Foo, "Methods of End
`Point Detection for Plasma Etching," Solid State Tech(cid:173)
`nology, 24(4), 115 (Apr. 1981).
`Sternheim, W. van Gelder & A. W. Hartman, "A Laser
`
`Interferometer System to Monitor Dry Etching of Pat(cid:173)
`terned Silicon," J. Ekctrochem. Soc. 130(3), 665 (Mar.
`1983).
`F. Heinrich, H.-P. Still, and H.-C. Scheer, "New and
`Simple Optical Method to in situ etch rate determina(cid:173)
`tion and endpoint detection," Appl. Phys. Lett. 55(14),
`1474 (Jul. 1989).
`S. A. Henck, "In situ Real-Time Ellipsometry for Film
`Thickness Measurement and Control," /. Vac. ScL
`Technol. A 10(4), 934 (Jul/Aug. 1992).
`D. Angell & G. S. Oehrlein, "Grazing Angle Optical
`Emission Interferometry for End-Point Detection,"
`Appl. Phys. Lett 58(3), 240 (Jan. 1991).
`J. T. Davies, T. Metz, R. N. Savage & H. Simmons,
`"Real-time, in situ Measurement of File Thickness and
`Uniformity During Plasma Ashing of Photoresist,"
`Proc. SPIE, 1392, 551 (1990).
`D. Economou, E. Aydil & G. Bama, "In situ Monitor(cid:173)
`ing of Etching Uniformity in Plasma Reactors," Solid
`State Technology, 34(4), 107 (Apr. 1991).
`(List continued on next page.)
`
`Primary Examiner—Hoa Q. Pham
`Attorney, Agent, or Firm—Steven J. Weissburg
`
`[57]
`ABSTRACT
`A new technique has been developed to measure etch(cid:173)
`ing or deposition rate uniformity in situ using a CCD
`camera which views the wafer during plasma process(cid:173)
`ing. The technique records the temporal modulation of
`plasma emission or laser illumination reflected from the
`wafer; this modulation is caused by interferometry as
`thin films are etched or deposited. The measured etch(cid:173)
`ing rates compare very well with those determined by
`Helium-Neon laser interference. This technique is capa(cid:173)
`ble of measuring etching rates across 100-mm or larger
`wafers. It can resolve etch rate variations across a wafer
`or within a die. The invention can also be used to make
`endpoint determinations in etching operations as well as
`measuring the absolute thickness of thin films.
`
`29 Claims, 13 Drawing Sheets
`
`356/382
`356/381
`356/382
`356/382
`356/381
`356/381
`356/357
`356/382
`356/382
`
`LAM Exh 1004-pg 1
`
`

`
`5,450,205
`
`Page 2
`
`OTHER PUBLICATIONS
`D. A. Banner, M. Dalvie, and D. W. Hess, "Plasma
`Etching of Aluminum: A Comparison of Chlorinated
`Etchants," Electrochem. Soc. 134(3), 669 (Mar. 1987).
`Konstantino P. Giapis, Geoffrey R. Scheller, Richard
`A. Gottscho, William S. Hobson & Yong H. Lee, "Mi(cid:173)
`croscopic and Macroscopic Uniformity Control
`in
`Plasma Etching," AppL Phys. Lett. 57(10), 983 (Sep.
`1990).
`IBM Technical Disclosure Bulletin, vol. 28, No. 5, Oct.
`1985, "Etching End-Point Detector Employing Laser
`Interferometer and Video Display".
`Babic, D., Trynolds, T, Hu, E and Bowers, J., "In situ
`characterization of sputtered thin films using a normal
`incidence laser reflectormeter," J. Vac. Sci. Technol. A
`10(4) Jul./Aug. 1992, pp. 939-944.
`
`Kamins, T. I., and Dell'Oca, C. J., "In-Process Thick(cid:173)
`ness Monitor for Polycrystalline Silicon Deposition," J.
`Electrochem. Soc: Solid-State Science and Technol(cid:173)
`ogy, Jan. 1972, pp. 112-114.
`Park, K. O., and Rock, F. C, "End Point Detection for
`Reactive Ion Etching of Aluminum," GTE Laborato(cid:173)
`ries Technical Notes, vol. 131, No. 1 (Jul. 1983).
`Danner, D. A., and Hess, D. W., "The Effect of Tem(cid:173)
`perature and Flow Rate on Aluminum Etch Rates in
`RF Plasmas," J. Electrochem. Soc: Solid-State Sci(cid:173)
`ence and Technology, vol. 133, No. 1, pp. 151-155 (Jan.
`1986).
`Economou, D. J., Park, S., and Williams, G. D., "Uni(cid:173)
`formity of Etching in Parallel Plate Plasma Reactors,"
`J. Electrochem. Soc, vol. 136, No. 1, Jan. 1989, pp.
`188-198.
`
`LAM Exh 1004-pg 2
`
`

`
`U.S. Patent
`
`Sep. 12,1995
`
`sheet 1 of 13
`
`5,450,205
`
`130
`
`d-
`
`V
`
`122
`
`V
`
`120
`
`230
`
`"~v~
`
`FIG.l
`
`(PRIOR ART)
`
`J
`
`He He LASER
`
`r 202
`_z>-14
`
`216
`
`208
`&///M/M//<>"'>'>»»<\-
`
`-202
`
`FIG. 2
`
`(PRIOR ART)
`
`LAM Exh 1004-pg 3
`
`

`
`U.S. Patent
`
`Sep. 12,1995
`
`sheet 2 of 13
`
`5,450,205
`
`A-206
`
`230
`
`hv
`
`hv
`
`•214
`
`-216
`
`V;M»»//»»>/WM;Mi<
`
`-208
`
`-202
`
`FIG. 3
`
`(PRIOR ART)
`
`•346
`
`FIG. 4
`
`LAM Exh 1004-pg 4
`
`

`
`U.S. Patent
`
`Sep. 12, 1995
`
`Sheet 3 of 13
`
`5,450,205
`
`• ^4
`
`300-y
`
`200-
`
`60
`TIME (SECONDS)
`
`SINGLE PIXEL 9PIXEL M. FIG. 5
`
`-e-
`
`^ 100-
`
`0.0
`
`0.02
`
`0.08
`0.06
`0.04
`DIHENSIONLESS FREQUENCY
`
`FIG. 6B
`
`0.10
`
`r
`0.4
`
`0.3
`0.2
`DIHENSIONLESS FREQUENCY
`
`FIG. 6A
`
`LAM Exh 1004-pg 5
`
`

`
`U.S. Patent
`
`Sep. 12, 1995
`
`Sheet 4 of 13
`
`5,450,205
`
`40
`
`60
`TIME (SECONDSJ
`SINGLE PIXEL
`9 PIXEL AM.
`
`FIG. 7
`
`170-
`
`POL YSIL ICON HOL E PEG ION
`
`li)U-\
`
`130-
`
`§
`•^
`
`110-
`
`I
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`k
`TP
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`i
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`1
`r
`
`' f / V^
`
`BARE POLY
`
`REGION-^
`
`i
`40
`
`I
`80
`TIME (SECONDS)
`
`FIG. 8
`
`/I r I
`
`i
`120
`
`i
`160
`
`^\
`
`s.
`
`s
`
`LAM Exh 1004-pg 6
`
`

`
`U.S. Patent
`
`Sep. 12, 1995
`
`Sheet 5 of 13
`
`5,450,205
`
`1500
`
`1000-
`
`s
`
`500-
`
`^
`
`BARE POLY:2960 A/HIH
`
`0.00
`
`0.02
`
`0.06
`0.04
`DIHEHSIOHLESS FREQUEHCY
`
`0.08
`
`FIG. 9
`
`20
`
`80
`60
`40
`HASHETIC FIELD STREHGTH (GAUSS)
`
`FIG. 13
`
`100
`
`120
`
`LAM Exh 1004-pg 7
`
`

`
`U.S. Patent
`
`Sep. 12, 1995
`
`Sheet 6 of 13
`
`5,450,205
`
`2995
`
`0.0
`
`?$
`
`FIG. 10 A
`
`1.00
`
`0.00
`0.00
`
`0.67
`0.33
`POSITION ALONG 01 E
`
`1.00
`(cm)-X
`
`FIG. 10B
`
`LAM Exh 1004-pg 8
`
`

`
`U.S. Patent
`
`Sep. 12, 1995
`
`Sheet 7 of 13
`
`5,450,205
`
`>
`
`*
`
`FIG. 11
`
`LAM Exh 1004-pg 9
`
`

`
`U.S. Patent
`
`Sep. 12, 1995
`
`Sheet 8 of 13
`
`5,450,205
`
`*-
`
`\
`
`FIG. 12
`
`LAM Exh 1004-pg 10
`
`

`
`U.S. Patent
`
`Sep. 12,1995
`
`sheet 9 of 13
`
`5,450,205
`
`-378
`
`-340
`
`71
`
`380 s
`
`336
`
`1 382-
`
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`342
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`344
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`354-
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`350
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`346
`
`V
`316
`
`-302
`
`FIG. 14
`
`LAM Exh 1004-pg 11
`
`

`
`U.S. Patent
`
`Sep. 12, 1995
`
`Sheet 10 of 13
`
`5,450,205
`
`INITIALIZE VARIABLES
`
`•1504
`
`DETER HI HE SENSOR PARAMETERS
`
`•1506
`
`i
`I
`
`WAIT FOR'"BEGIN" SIGNAL
`
`-1508
`
`1510-
`
`.NO
`
`IS
`MEMORY
`FULL
`YES
`
`X 1512
`
`SYNCHRONIZE AN
`IMAGE
`
`XL 1514
`TRANSMIT IMAGE SIGNAL
`FROM SENSOR TO
`COMPUTER MEMORY
`
`SAVE SIGNAL TO FILE
`
`JH5I6
`
`s-'517
`
`\CHANGE TO NEXTA\
`
`ACQUISITION I
`I
`i.
`I
`
`1520 2
`
`CLOSE FILES
`
`4
`
`YESJ
`
`-1518
`
`NO
`
`EXIT
`COMMAND
`RECEIVED? >
`
`1522-^
`
`CFrnJ
`FIG. 15
`
`LAM Exh 1004-pg 12
`
`

`
`U.S. Patent
`
`Sep. 12,1995
`
`sheet 11 of 13
`
`5,450,205
`
`•1604
`
`-1605
`
`INITIALIZE VARIABLES
`
`I
`DISPLAY IMAGE I
`
`SELECT LOCATIONS Effl ANALYSIS
`
`-1606
`
`DETERMINE DATA VALUE VS. TIME AT EACH ANALYSIS
`
`-1608
`
`LOCATION I
`I
`
`DETERMINE TEMPORAL LIMITS OF ANALYSIS
`
`NORMALIZE FOR LINEAR RISE WITH TIME
`
`-1610
`
`-1612
`
`•1614
`
`COMPUTE FFT AT EACH ANAL YSIS LOCATION
`
`I
`
`DETERMINE DOMINANT FREQUENCY FROM EACH FFT
`
`-1616
`n
`
`1619-
`V
`
`"1
`DETERMINE REFLECTIVITY
`i
`VS. X FROM RAD. INTENSITY 1
`1
`
`1621-^
`1
`j
`\
`L
`
`1
`FIFREFLETTWITY 7 F]
`THEORETICAL VALUES TO
`I
`DETERMINE FILM
`THICKNESS
`
`.J
`
`r"
`
`1
`
`H 1618
`CALCULATE ETCHING RATE
`
`DISPLAY RESULTS
`
`i a 1620
`I r. 1622
`I
`
`SAVE RESULTS
`
`r~l626
`TEMP. END
`POINT
`REACHED?
`YES
`C^if~Y~ 1624
`
`NO
`
`FIG. 16
`
`LAM Exh 1004-pg 13
`
`

`
`U.S. Patent
`
`Sep. 12, 1995
`
`Sheet 12 of 13
`
`5,450,205
`
`1754
`
`5mV HeHe LASER
`
`L-1770
`
`FIG. 17
`
`LAM Exh 1004-pg 14
`
`

`
`U.S. Patent
`
`Sep. 12, 1995
`
`Sheet 13 of 13
`
`5,450,205
`
`Csfm~\^im
`
`INITIALIZE VARIABLES
`
`-1804
`
`SELECT LOCATIONS FOR AHALYSIS
`
`-1806
`
`-1808
`DETERMINE DATA VALUE VS. TIME AT EACH ANALYSIS
`LOCATION
`
`I
`1
`I
`
`-1810
`
`YES ' AUTONLATIC)"0
`
`1814 2L
`COMPARE SLOPE
`AFTER EXTREMA
`
`L 1812
`
`DISPLAY RESULTS
`
`J^I8I0
`
`NO
`/ENDPOINTh
`
`-1814
`
`TAKEACTION
`
`1824 FIG. 18
`
`LAM Exh 1004-pg 15
`
`

`
`5,450,205
`
`APPARATUS AND METHOD FOR REAL-TIME
`MEASUREMENT OF THIN FILM LAYER
`THICKNESS AND CHANGES THEREOF
`
`The present invention relates generally to the field of
`thin film analysis. The invention relates more specifi(cid:173)
`cally to the field measuring the thickness of a thin film.
`The invention has applications in the semiconductor
`fields.
`fabrication field as well as other thin film
`
`5
`
`10
`
`FIG. 1 depicts a thin film 100 of thickness d and
`refractive index nj on a wafer substrate 120 with refrac(cid:173)
`tive index 113. A pedestal 122 supports the wafer sub(cid:173)
`strate 120. (In some cases, more than one thin film layers
`will make up the thin film layer 100. In such a case, the
`light is reflected at each interface of different indices of
`refraction. Further, the upper most layer may have a
`patterned photoresist layer coating it.) Light rays 102 at
`a wavelength of X travel through a first medium 130
`with a refractive index ni and strike the interface be(cid:173)
`tween the medium 130 and the thin film 100 at an angle
`61 relative to the surface normal. Part 104 of the light
`rays 102 are refracted inside the film to an angle 62 to
`the surface normal. Another part 106 of the incident
`light ray is reflected from the interface between the first
`medium 130 and the film 100. Following the develop(cid:173)
`ment of Hecht & Zajac (Eugene Hecht & Alfred Zajac,
`OPTICS
`(Addison Wesley, Reading Mass., 1979),
`p.295-297), the optical path length difference, Alq,?,
`between light 106 reflected from the interface between
`the first medium 130 and the film 100 on the one hand,
`and light 108 that has traveled through the film 100 and
`been reflected from the lower interface between the
`film 100 and the wafer substrate 120 on the other hand
`can be written as:
`
`Inid
`A/,
`"I" - 005(62)
`
`-
`
`2nirftan(e2)sin(ei).
`
`(1)
`
`Light rays 110, which are the result of multiple re(cid:173)
`flections within the film, are usually sufficiently low in
`intensity that they can be neglected in the analysis dis(cid:173)
`cussed below. Using Snell's Law, equation (1) can be
`written in terms of © alone, and simplified to yield:
`
`&.lap,=2n2 d cos (62).
`
`(2)
`
`A maximum in the intensity of the net reflected light
`resulting from interference between the two reflected
`light rays 106 and 108 will occur when the optical path
`difference Al0pris an integral multiple of the wavelength
`X of the light. If ni is assumed to be 1, then Ad/?/m, the
`change in film thickness that has occurred between the
`occurrence of adjacent (in time) maxima or minima
`(known as "extrema"), is given by:
`
`Ad/i/m
`
`2n2COS(02)
`
`(3)
`
`F or normal incidence,
`duces to simply
`
`(i.e., 61=62=0) Eq. (3) re-
`
`Arfyz/m =
`
`X
`2n->
`
`(4)
`
`The term cos (62) is a correction for non-normal
`incidence, which can be rewritten in terms of measur(cid:173)
`able parameters (i.e., 61 and 112) as:
`
`005(62) = 0-[^J)-
`
`(5)
`
`A practical known apparatus for laser interferometry
`is shown in FIG. 2. An incident beam " I" from HeNe
`laser 202 strikes the interface between the wafer 208 and
`
`15
`
`20
`
`25
`
`30
`
`40
`
`45
`
`BACKGROUND
`Uniformity of film etching rate and uniformity of film
`thickness has always been an important issue in plasma
`etching. Wafer-to-wafer uniformity within a batch is no
`longer a major concern because batch reactors that
`treat numerous small wafers are being replaced with
`large, single-wafer etchers. A dominant concern that
`remains is etching process uniformity across a single
`wafer, especially as silicon wafer diameters are increas(cid:173)
`ing. See, J. W. Cobum, "Summary Abstract: Diagnos(cid:173)
`tics in Plasma Processing," J. Vac. ScL Technol. A 4(3),
`1830 (1986). State-of-the-art wafer processing is done
`on 2(X)-mm diameter wafers. The next generation of
`wafers will probably be 250 and/or 300-mm diameter.
`Process parameters include the uniformity of etching
`rate, and the absolute thickness of specific structures.
`Presently, rate uniformity is determined ex situ, and
`after the etching step, by optically measuring film thick(cid:173)
`ness (for example, with a Nanospec) before and after a
`partial etch of known duration. See generally, A. G.
`Nagy, "Radial Etch Rate Nonuniformity in Reactive
`Ion Etching," /. Electrochem. Soc. 131(8), 1871 (1984)
`and Alan S. Kao & Harvey G. Stenger, Jr., "Analysis of ,5
`Nonuniformities in the Plasma Etching of Silicon," J.
`Electrochem. Soc. 137(3), 954 (1990). This process is not
`suitable as a diagnostic for real-time process control,
`i.e., control of the etching process as it is being con(cid:173)
`ducted.
`A number of techniques exist to measure etching rate
`and/or film thickness in situ. Among these are laser
`interferometry,
`optical
`emission
`interferometry
`("OEI") and ellipsometry. See generally Paul J. Mar-
`coux & Pang Dow Foo, "Methods of End Point Detec(cid:173)
`tion for Plasma Etching," Solid State Technology, 24(4),
`115 (1981)(laser interferometry) and Stemheim, W. van
`Gelder & A. W. Hartman, "A Laser Interferometer
`System to Monitor Dry Etching of Patterned Silicon,"
`/. Electrochem. Soc. 130(3), 655 (1983); F. Heinrich,
`H.-P. Still, and H.-C. Scheer, "New and Simple Optical
`Method to in situ etch rate determination and endpoint
`detection," AppL Phys. Lett. 55(14), 1474 (1989)(OEI);
`and David Angell & Gottlied S. Oehrlein, "Grazing
`Angle Optical Emission Interferometry for End-Point
`Detection," Appl. Phys. Lett. 58(3), 240 (1991); and Ste(cid:173)
`ven A. Henck, "In situ Real-Time Ellipsometry for
`Film Thickness Measurement and Control," J. Vac. ScL
`Technol. A 10 (4), 934 (1992).
`Laser interferometry and optical emission interferom(cid:173)
`etry both analyze the interference of light reflected
`from a thin film, but they use different light sources.
`Laser interferometry uses a laser beam (typically a 632.8
`nm Helium-Neon (HeNe), while optical emission inter(cid:173)
`ferometry uses etch reactor plasma optical emission as
`the light source. Ellipsometry measures the change in
`polarization of light upon reflection of the light from a
`surface.
`
`50
`
`55
`
`60
`
`65
`
`LAM Exh 1004-pg 16
`
`

`
`5,450,205
`
`the chamber environment 230 such as a plasma of the
`reactor chamber 214. The reflected beam "R" is di(cid:173)
`rected through a bandpass filter 210 to a photodiode
`212, where an interferometry signal is recorded as a
`function of time. The bandpass filter 210 prevents 5
`plasma emission (i.e. unwanted light) from entering the
`photodiode 212, while allowing the reflected laser beam
`R to strike the photodiode 212.
`Similarly, a known optical emission interferometry
`apparatus is shown in FIG. 3. Light C is collected from 10
`the chamber environment 230 within chamber 214 with
`a lens 218, and passed through a bandpass filter 220 and
`into a photodiode 212. Here, the bandpass filter 220
`defines the wavelength of light being used for interfer(cid:173)
`ence and blocks light at unwanted wavelengths to pre- 15
`vent the plasma background from reaching the photodi(cid:173)
`ode 212. The etching rate is calculated as
`
`Arffffa
`
`ER =
`
`(6) 20
`
`40
`
`where 8rm is the measured time between received adja(cid:173)
`cent (in time) maxima or adjacent minima in the inter(cid:173)
`ferometry signal.
`Using the known apparatus, the spatial relationship
`between the region of the film 208 being analyzed, the
`lens 218 and the photodiode 212 is such that the light
`from a specific spot on the film is not focused on the
`photodiode. The photodiode is located roughly at the 30
`focal length of the lens. Thus, all of the light from over
`a relatively large region of the film 208 is delivered to
`the photodiode, which registers essentially an average
`effect over the entire area. Even if light from only a
`relatively small region of the film is focused onto the 35
`photodiode, the photodiode signal does not provide any
`information about the location of the region on the film.
`Thus, another method must be used to determine this
`location, such as a laser pointing device, or a line of
`sight approximation.
`Both optical emission and laser interferometry have
`been used to monitor etch rate at different locations in
`situ. Davies et al. monitored photoresist thickness using
`a white light excitation source and a photodiode spec(cid:173)
`trometer array. Etching rate could have been deter- 45
`mined at up to five regions on a wafer; however, this
`was not done (the method was mentioned, but etching
`rate measurements were made only over a single re(cid:173)
`gion). See John T. Davies, Thomas Metz, Richard N.
`Savage & Horace Simmons, "Real-time, in situ Mea- 50
`surement of File Thickness and Uniformity During
`Plasma Ashing of Photoresist," Proc. SPIE, 1392, 551
`(1990), using white light and a view window as large as
`the analysis region.
`Economou et al. measured etch rate in situ at five 55
`regions across a wafer by multichannel laser interferom(cid:173)
`etry. However, their technique is not readily applicable
`to industrial plasma etching tools. It requires a-priori
`selection of sites for analysis and the splitting and align(cid:173)
`ment of multiple laser beams. Without some extrinsic 60
`evidence, such as correlation with an image taken by a
`standard image capture device, it is difficult to know
`from the light signal collected what location on the
`wafer is being analyzed. It is similar to viewing the
`night sky through a telescope, without being able to 65
`take stock of a larger field of view to identify the rela(cid:173)
`tionship between the scene in the telescope and the
`known constellations.
`
`Also, the multichannel laser interferometry technique
`requires an optical viewport the same diameter as the
`wafer being measured to obtain a full wafer view. See
`D. Economou, E. Aydil & G. Bama, "In situ Monitor(cid:173)
`ing of Etching Uniformity in Plasma Reactors," Solid
`State Technology, 34(4), 107 (1991). Wafers are now
`normally 200 mm across and will soon be 250-300.
`Standard viewing windows that are available in reac(cid:173)
`tors are on the order of 50 mm (two inches) across. It is
`difficult to change the size of a window in a standard
`reactor. Further, changing the size of such a window
`may change conditions inside the reactor. In addition, it
`is advantageous for the etching process to keep the
`window as small as possible. It is commonly necessary
`to replace windows that are affected by the etching
`process. Smaller windows are less expensive to replace.
`Further, the etchant removes window constituents and
`those constituents become part of the plasma environ(cid:173)
`ment, acting as contaminants. It is beneficial to mini(cid:173)
`mize this contaminating effect.
`The use of in situ ellipsometry to measure etching
`rates is fairly new and has not been used to measure rate
`uniformity.
`It would also be beneficial to be able to determine the
`uniformity of the rate of film removal in processes other
`than plasma wafer etching.
`In addition to the removal of thin film material, it is
`useful to be able to know the rate of change of film
`thickness in processes where additions are made to the
`thickness of regions of thin films, over the entire surface
`area of the film. Such processes include: sputter deposi(cid:173)
`tion; chemical vapor deposition ("CVD"); plasma en(cid:173)
`hanced CVD; physical vapor transport; evaporation,
`thermal processing and others.
`It is also useful to be able to determine the absolute
`thickness of a thin film, either as it is being etched away
`from or added to a preexisting substrate. For instance,
`in etching, it is useful to know when all of the film at a
`certain location has been etched away from a substrate
`(the so-called process "end-point") so that the etching
`process can be stopped. End-point is typically identified
`by observing a characteristic, and then noting when the
`characteristic undergoes a gross change. The change
`typically signifies that the endpoint has been reached,
`and that the process has changed. For instance, as
`shown in FIG. 1, as the thin film 100 is being etched
`away, the plasma environment will include a certain
`chemical composition. After the thin film is all re(cid:173)
`moved, the substrate wafer 120 begins to slowly etch
`away, and the chemical composition of the plasma envi(cid:173)
`ronment changes. This change in environment can be
`noted, and used to identify the occurrence of the end-
`point.
`This method of endpoint determination has draw(cid:173)
`backs. First, the wafer reaches endpoint at different
`times at different locations around the wafer. However,
`the known methods typically give only information
`about the state of a single point or the average state of
`the entire wafer, for instance when endpoint has been
`reached at enough locations so that the chemical com(cid:173)
`position of the reactor environment has changed to a
`certain degree. Thus, known techniques are not very
`sensitive. Further, such techniques require that the pro(cid:173)
`cess actually proceed beyond the minimal endpoint.
`This is undesirable, for processes which can not tolerate
`much, if any, overshoot. Further, such techniques are
`not suitable for monitoring operations where film is
`being added to a layer, rather than being removed,
`
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`

`
`5,450,205
`
`because there is typically no physical change in the
`environment when such an accumulative endpoint has
`been reached.
`There are also other instances in which it is beneficial
`to know the absolute thickness of a film, for instance to 5
`evaluate the uniformity of film thickness at the outset or
`end of a process.
`Thus, a significant need exists to be able to monitor
`and measure the uniformity across a wafer of the rate
`that material is being etched away from or deposited 10
`upon a substrate. It is desirable to be able to monitor
`etching rate in real-time, as the film material is being
`etched away, so that process parameters can be changed
`or observed to bring the etching rate into uniformity or
`as desired. There is also a similar need for apparatus that ^
`can monitor and measure the rate of addition of material
`to a substrate. It is desirable to monitor the process over
`the entire surface of a workpiece, such as a wafer, for
`instance on the order of at least 200 to 300 mm in diame-
`ter, while requiring a viewing port of no more than a
`much smaller diameter, e.g. on the order of 50 mm, as is
`common with a standard plasma reactor. It is also an
`object of the invention to determine the absolute thick(cid:173)
`ness of films that are being either etched away from or
`added to a body of film, without needing to know the
`original thickness, or the history of etching rate. Fur(cid:173)
`ther, it is desirable to be able to measure the absolute
`thickness over the entire surface area of a thin film,
`through a viewing port that is significantly smaller than 30
`the subject film. Another object of the invention is to
`readily identify the locations on the thin film that are
`being analyzed, without need to resort to an additional
`image capture or laser direction device.
`
`25
`
`35
`
`SUMMARY
`A preferred embodiment of the invention is an appa(cid:173)
`ratus for measuring change in the thickness of a thin
`film on a substrate. The apparatus comprises a support
`for the thin film and substrate and, spaced away from 40
`the support, an array of means for sensing electromag(cid:173)
`netic radiation (such as light) reflected from the thin
`film, the array providing an individually recoverable
`signal for each sensing means of the array, each signal
`corresponding to a region of the film. A filter transmits 45
`electromagnetic radiation of at least one selected range
`of wavelengths to the array. A lens for focusing electro(cid:173)
`magnetic radiation reflected from the film is arranged
`between the thin film and the array such that an image
`of a region of the film is focused onto said array. A 50
`storage is provided for storing a time sequence of data
`signals corresponding to the signals generated by the
`array over a period of time in response to radiation
`transmitted through said filter means. The storage is
`connected to data processing means for comparing the 55
`data signals for at least one sensing means of the array
`over a portion of the time period, and from the compari(cid:173)
`son, determining the change of the thickness of the film
`over the portion of the time period at the region of film
`focused upon array. The apparatus may also include a 60
`timer to measure the time over which the signals have
`been collected, from which the data processing means
`can determine the rate of change of the film thickness.
`The data processor can also compare such changes in
`thickness or such rates of change of thickness from one 65
`region of the wafer to another, thereby providing a
`measure of the uniformity of such parameters over the
`wafer.
`
`According to a preferred embodiment, the array of
`sensors is a CCD. The apparatus may include a display,
`for displaying an image of the wafer generated from the
`signals. The image may be modified from time to time to
`indicate the change in thickness over time of the various
`regions of the wafer at different times, as well as the rate
`of change of thickness. The apparatus may be used in
`conjunction with a walled environment, such as a
`plasma etching apparatus, having a window for passage
`of the light. The window may be smaller than the analy(cid:173)
`sis region of the wafer, due to the focusing action of the
`lens.
`the
`According to another preferred embodiment,
`filter is capable of sequentially transmitting radiation of
`at least two different ranges of wavelengths. The inten(cid:173)
`sity of the light received indicates the reflectivity of the
`wafer. The data processing equipment may include
`means for correlating the measured reflectivities for
`each wavelength, to known reflectivities for wave(cid:173)
`lengths for certain film thicknesses, thereby enabling
`determination of the absolute film thickness.
`Another preferred embodiment of the invention is an
`apparatus for collecting information regarding
`the
`change in the thickness of a thin film being reduced in
`thickness on a substrate. For instance, the operator can
`determine whether an etching endpoint has been
`reached. The apparatus comprises a support for the thin
`film and substrate. Spaced away from the support is an
`array of means for sensing electromagnetic radiation
`(such as light) reflected from the thin film, the array
`providing an individually recoverable signal for each
`sensing means of the array, each signal corresponding
`to a region of the film. A filter transmits electromag(cid:173)
`netic radiation of at least one selected range of wave(cid:173)
`lengths to the array. A lens for focusing electromag(cid:173)
`netic radiation reflected from the film is arranged be(cid:173)
`tween the thin film and the array, such that an image of
`a region of the film is focused onto the array. Connected
`to the array are storage means for storing a time se(cid:173)
`quence of data signals corresponding to the signals
`generated by the array of sensing means over a period
`of time in response to radiation transmitted through the
`filter. The operator can use the data stored to determine
`if individual components of the time sequence of data
`conform or not to a predetermined relationship among
`the data, such as rates of change of amplitude, thus
`determining whether certain process points have been
`reached.
`Another preferred embodiment of the invention is a
`method for determining when a layer of a thin film has
`been completely removed by a thickness reduction pro(cid:173)
`cess at a plurality of locations over the area of said thin
`film. The method comprises the steps of reflecting elec(cid:173)
`tromagnetic radiation from the thin film during the time
`the film is being reduced in thickness. At least one se(cid:173)
`lected range of wavelengths of electromagnetic radia(cid:173)
`tion reflected from the film is focused onto an array of
`means for sensing electromagnetic radiation. The array
`provides an individually recoverable signal for each
`sensing means of the array, each signal corresponding
`to a region of the film. A time sequence of data signals
`is generated corresponding to the signals generated by
`the array of sensing means over a period of time in
`response to radiation focused on the array. The ampli(cid:173)
`tude of the data signals are correlated with the time
`during which the signals were generated. A periodic
`pattern in the rate of change of the amplitude of the data
`signals is identified and at selected time intervals, the
`
`LAM Exh 1004-pg 18
`
`

`
`5
`
`rate of change of the amplitude of the data signals is
`compared with the periodic pattern. A determination is
`made that the film has been removed if the rate of
`change of the amplitude of the signals departs from the
`periodic pattern.
`Another preferred embodiment of the invention is a
`method for determining the rate of change of the thick(cid:173)
`ness of a thin film at a plurality of locations over the
`area of the thin film, the method comprising the steps of
`reflecting electromagnetic radiation from the thin film 10
`during the time the film is being changed in thickness
`and focusing at least one selected range of wavelengths
`of the electromagnetic radiation reflected from the film
`onto an array of means for sensing electromagnetic
`radiation which provides an individually recoverable 15
`signal for each sensing means of the array, each signal
`corresponding to a region of the film. A time sequence
`of data signals is generated, corresponding to the signals
`generated by the array of sensing means over a period
`of time in response to radiation focused on said array. 20
`The amplitude of the data signals is correlated with the
`time during which the signals were generated and a
`periodicity in the rate of change of the amplitude of said
`data signals is identified. The rate of change of the
`thickness of the film is determined by relating said peri(cid:173)
`odicity to said selected range of wavelengths. The array
`may be a CCD according to another preferred embodi(cid:173)
`ment.
`Yet another embodiment of the invention is a method
`for determining the thickness of a thin film at a plurality
`of locations over the area of the thin film. The method
`comprising the steps of reflecting electromagnetic radi(cid:173)
`ation from the thin film during the time the film is being
`changed in thickness and focusing at least two selected 35
`ranges of wavelengths of the electromagnetic radiation
`reflected from the film onto an array of means for sens(cid:173)
`ing electromagnetic radiation which provides an indi(cid:173)
`vidually recoverable signal for each sensing means of
`the array, each signal corresponding to a region of the 43
`film. A set of data signals are generated corresponding
`to the signals generated by the array of sensing means in
`response to radiation of the selected at least two ranges
`of wavelengths focused on the array. The amplitudes of
`the data signals are correlated with the wavelength of 45
`the electromagnetic radiation from which the signals
`were generated and compared to a predetermined cor(cid:173)
`relation between amplitudes and film thickness for each
`wavelength. From this comparison, the thickness of the
`film is determined.
`
`50
`
`30
`
`BRIEF DESCRIPTION OF THE FIGURES
`These and other features, aspects, and advantages of
`the present invention will become better understood
`with regard to the following description, appended 55
`c