Anal. Chem. 1994,66, 1015-1020
Infrared Reflection Absorption Spectroscopy of Photoresist Films on Silicon Wafers: Measuring Film Thickness and Removing Interference Fringes Chrlstopher J. Gamsky, Glenn R. Howes, and James W. Taylor' Department of Chemistry, Universiry of Wisconsin, Madison. Wisconsin 53 706
Infrared reflection absorption spectroscopy (IRRAS)of thin films produces IR spectra in sampling situations where transmission spectroscopy would be impossible, such as in situ spectroscopy of fdms on thick substrates, and it is also more sensitive because the path length is more than doubled. Unfortunately, interpretation of IRRAS data can be complicated by the existence of interference fringes which prevent the accuratedetermination of peak heights and areas. A method has been devised to use the informationprovided by these fringes to obtain the thickness and complex refractive index of the film in regions of little or no absorption. This information potentially could be used to calculate the fringefree optical constant spectra by use of a Kramers-Kr6nig transformation. We also confirm that the interferenceeffects can be eliminated to a large extent by incorporating a mirror behind a highresistivity double-polished silicon wafer during IRRAS data collection. Infrared reflection absorption spectroscopy (IRRAS) is a useful technique for studying the chemistry of thin films. IRRAS has been used to examine electrode surfaces,' Langmuir-Blodgett films,2 and other systems including some in the semiconductor i n d ~ s t r y .It~ is ~ ~especially useful for monitoring the chemical changes that take place in photoresist films during lithographic processing due to the nondestructive nature of the technique and its ability to acquire data in real time. Unfortunately, the spectra of thin films, especiallythose cast on partially reflective substrates, such as single- and double-side polished silicon wafers, contain interference fringes. These fringes can be several times larger than the absorption peaks of interest and can make accurate quantitation of chemical changes difficult. Various approaches have been applied to the problem of eliminating these fringes, including calculation of the film optical constants using Kramers-Krbnig a n a l y s e ~ , ~ calculation -l~ of effective film thickness,13 and modification of the optical system.14J5 (1) McIntyre, J. D. E. In Optical Techniques in Elecfrochemistry;R. H. Muller, Ed.; Jon Wiley & Sons: New York, 1973; Vol. 9, pp 61-166. (2) Brinkhuis, R.H. G.; Schouten, A. J. Macromolecules 1991.24, 1496-1504. ( 3 ) Tcschner, U.; HBbner, K. Physica B 1990, 159, 917-926. (4) Li, J.; Pons, S. Elecfroanal. Chem. 1987, 233, 1-18. ( 5 ) Graf, R. T.; Kocnig, J. L.; Ishida, H. Appl. Specfrosc. 1985, 39, 405-408. (6) Rocssler, D. M. Br. J . Appl. Phys. 1965, 16, 1359-1366. (7) Rocssler, D. M. Br. J . Appl. Phys. 1%5, 26, 1119-1123. (8) Rocsslcr, D. M. Br. J . Appl. Phys. 1966,17, 1313-1317. (9) Tickanen, L. D.; Tejedor-Tejedor, I. M.; Anderson, M. A. Appl. Specfrosc. 1992, 46, 1848-1858. (10) Dignam, M. J.; Mamiche-Afara, S. Spectrochim. Acfa 1988,441435-1442. (11) Bardwell, J. A.; Dignam, M. J. Anal. Chim. Acta 1986, 181, 253-258. (12) Bardwell, J. A.; Dignam, M. J. Anal. Chim. Acra 1985, 172, 101-110. (13) Brendel, R. J . Appl. Phys. 1991, 69,7395-7399.
0003-2700/94/0366-1015$04.50/0 @ 1994 American Chemical Society
In this paper, we describe two approaches to dealing with these interference fringes in IRRAS spectra of photoresist films on silicon substrates. The first technique uses the fringe spacing and height to calculate the thickness and the complex refractive index of the photoresist film. This is accomplished by iteratively changing the input parameters of a three-phase reflectivity calculation in order to obtain the best match between thecalculated and observed IRRAS spectra. In order to match correctly the observed reflectivity, one must know the optical constants of the silicon wafer substrate. Because these constants vary significantly from wafer to wafer, they are evaluated for each wafer using a Kramers-Krbnig transformation from reflectivity data. From two IRRAS spectra, one of the bare silicon wafer and one of the same wafer coated with photoresist, one can determine the film thickness and complex refractive index of the film. This provides a quick and effective way to obtain refractive index values in the IR, which are useful for Kramer-KrBnig determinations of optical constants in regions of high absorption. This method can also provide measurements of film thicknesses down to about -0.05 pm. These thicknesses can be used either as independent measurements or as checks for other thickness measurement techniques. The second method discussed here provides a way to minimize the interference fringe height in IRRAS spectra of films on moderately reflective substrates with low absorption coefficients, such as double-polished silicon wafers. This is accomplished by placing a mirror behind the substrate in the IRRAS experiment as suggested by Edgar and others.13-ls The fact that the resist film and the silicon wafer have low absorption coefficients throughout most of the IR region, combined with the fact that the condition necessary for destructive interference of the reflected beam produces constructive interference in the transmitted beam, means that the reflectivity of this mirror-backed system should be nearly equal to 1 at any frequency. One can then use IRRAS instead of transmission FT-IR to study chemical changes in the resist during lithographic processing. The advantages of IRRAS include the longer path length compared to transmission and the ability to perform experiments in situ during exposure or processing of the resist. For example, the postexposure baking step can be monitored while the wafer is on a vacuum hot plate. This provides one of the major objectives for this study because of the application to the control of chemicallyamplified (14) Edgar, R. F.; Stay, B. J. SPIE 1985, 590, 316-320. (15) Farrington,P. J.; Hill, D. J. T.;O'Donncll, J. H.;Pomery, P. J. Appl. Spectrosc. 1990,44,901-903.
Analytical Chemistry, Voi. 66, No. 7, April 1, 1994
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resists where monitoring is desirable during baking on a vacuum hot plate. EXPERIMENTAL SECTION Materials. Three resists were used: a positive tone photoresist AZ-PF 514 (Hoechst Celanese), a negative tone resist SAL-605 (Shipley Co., Marlboro, MA), and 950 000 MW PMMA (KTI Chemicals). Both single-side polished (Tygh Silicon, Pleasanton, CA) and double-side polished (4 in., type p, 100 orientation, >5 O/cm resistivity, P2S surface and 17-20-mil thickness) (Polishing Corp. of America, Santa Clara, CA) wafers were used. Film Preparation. The photoresist films were spin coated onto silicon wafers using a Soltech spinner. For example, a 1-hm AZ-PF film was prepared by spinning at 4500 rpm for 1 min. The films were than baked on a vacuum hot plate to evaporate the remaining solvent and to relieve the stresses induced during spinning. Typical baking conditions were 1 10 "C for 1 min. Thickness Measurements. The thickness of the resist films was measured using an AutoEL I1 ellipsometer (Rudolf Instruments) and a NanoSpec/AFT Model 210 (Nanometrics). The NanoSpec/AFT collects a reflection spectrum in the visible region of the spectrum and then calculates the film thickness on the basis of the resulting interference fringes. IRRAS. IRRAS measurements were performed using a Nicolet 5 10M FT-IR spectrometer (Nicolet Instruments, Madison, WI) equipped with DTGS and MCT detectors and a KBr beam splitter. A variable-angle external reflection accessory (Pike Technologies, Madison, WI) with a useful range of angles from 30' to 7 5 O off normal and a 16-mm spot size was used to accquire spectra at various angles of incidence. In order to simplify reflectivity simulations, all spectra were obtained with radiation polarized perpendicular to the plane of incidence (s-polarized) using a wire grid polarizer (Harrick Scientific, Ossining, NY). Reflection spectra were the result of taking the ratio of 64 sample scans to 64 reference scans, where the reference was an aluminum or gold mirror. The moving mirror speed was 0.6329 cm/s for the DTGS detector and 1.266 cm/s with the MCT detector, and the spectral resolution was 4 cm-I. The spectrometer was interfaced to a Macintosh IIfx computer, and all data acquisition and manipulation were performed on that system using NicIR software (Nicolet Instruments) and the Igor data analysis software package (WaveMetrics, Lake Oswego, OR). A translator program, written in Think C 5.0 for the Macintosh and available upon request, was used to convert Nicolet data files either to text files or to a format specific to the Igor software. Calculation of IRRAS Baseline. In order to calculate the IRRAS baseline, the resist film on the silicon substrate was modeled as a three-phase system: a semiinfinite air phase, a resist film of finite thickness, and a semiinfinite silicon substrate. It was assumed that the film was an isotropic layer with parallel boundaries at the air and silicon interfaces and that the refractive index of the resist varied linearly as a function of frequency. The reflectivity could then be calculated using the equations formulated by Heavens16 and applied by (16) Heavens, 0. S.Optical Properties of Thin Solid Films; Dover Publications, Inc.: New York, 1965; p 261.
1016 Analytical Chemistry, Vol. 66, No. 7, April 1, 1994
3.6 3.4
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Flgure 1. Apparent refractive index for two typical slllcon wafers (angle of incidence 35') calculatedby use of Roessler's method. Curve B is representative of a moderately doped wafer while curve A is typical for a wafer containing a low dopant level-high resistivity.
several workers including Allara et al.14 and Greenler.17 In order to calculate this reflectivity correctly, the complex refractive index of both the film and the silicon substrate, the thickness of the resist film, the angle of incidence, and the polarization of the incident radiation must be known. The complex refractive index of a silicon wafer can be calculated using the method mentioned below. Determination of Apparent Optical Constants of Silicon Wafers. The apparent optical constants of the silicon wafer were determined using a method reported previ~usly.~ The Kramers-Kronig transformation was used to determine the phase change upon reflection from a silicon wafer. The phase was corrected, using the method of Roessler,6 by assuming that the phase change was zero (Le., there is no absorption) at 850 and 4500 cm-l. The apparent optical constants could then be determined from the corrected phase using eqs 1-4. (1 - R,) a=
1
COS
,$
+ R, - 2 cos $R,'i2
(1)
-2 cos 4 sin $ R , ~ / ~ b=
1
+ R, - 2 cos $R,'I2
(2)
In these equations ,$ is the angle of incidence, $ is the phase change upon reflection, and R, is the perpendicular component of the reflectivity. The terms a and b can then be used to determine n and k from the following simultaneous equations: a2 - b2 = n2 - k2 - sin2 ,$
(3)
ab = nk (4) Figure 1 shows the apparent refractive index for two silicon wafers. These optical constants were then used in a twophase calculation of the reflectivity of the silicon wafer. The measured and calculated IRRAS spectra of two typical silicon wafers are shown in Figure 2. Curve A is characteristic of a wafer with a high resistivity while curve B arises from a doped wafer. Mirror-Baked IRRAS with Double Polished Wafers. Mirror-backed IRRAS (MBIRRAS) experiments were performed by placing an aluminum or gold front-surface mirror behind (1 7 ) Greenler, R. G. J . Chem. Phys. 1966, 44, 3 1 C-3 15
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Figure2. Calculatedreflectivity(dark curves)andmeasuredreflectivity (light curves) of typlcal silicon wafers for s-polarized light at 35' off normal incidence. Curve A Is for a hlgh resistivity wafer, curve B is for a doped wafer, and curve C Is the reflectivity calculated by we of common values for the optical constants of silicon (n was obtained by fitting a curve to literature values1*between 400 and 8000 cm-l, while k was taken to be zero).
-
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U"
B
.-0
0.2
= $
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(cm")
Flgure 3. Comparlson of measured reflectivity (dark curves) and calculatedreflectivity(Ilght curves)of single-sklepolishedsilicon wafers coated with 1.60 (A) and 1.39 pm (B) of photoresist. The angle of IncMence was 40' and radiation was s-polarized.
a double polished silicon wafer separated from the wafer by a gap of approximately 250 pm. The ratios of sample spectra against a reference spectrum collected with the aluminum or gold mirror in place of the sample were then taken. The back of the wafer was cleaned with an acetone-dampened cloth to remove any residue deposited during processing and handling of the wafer.
RESULTS AND DISCUSSION Reflectivity Calculations. The basic problem of analyzing a photoresist film in reflectance can be illustrated in Figure 3. In this spectrum of SAL-605, it is known that the peak at 3300 cm-1 can be utilized for monitoring the products formed during the postexposure bake step following exposure to X-rays. As demonstrated in Figure 3, however, constructing a baseline to observe quantitatively the changes in the peak area or peak height at 3300 cm-l will require calculational techniques to remove effects of interference. In much of the work involving calculations of reflectivity for systems where silicon is the substrate, the authors devote little attention to the optical constants of the silicon. However, the optical constants of silicon wafers can vary substantially, depending on doping. Figure 2 shows the reflectivity of two silicon wafers, light curves A and B, obtained under identical conditions.
Both wafers are of the sort typically used in the semiconductor industry. Figure 2 also shows the calculated reflectivity, curve C, using literature values18 for the refractive index and zero for the extinction coefficient of the silicon wafer. It is clear that these values for the optical constants are inadequate for calculating the reflectivity of a system where either of these two typical wafers are used as the substrate. In order to predict the reflectivity of such a system more accurately, the optical constants weremeasured for each wafer described in this paper. This was done in a four-step process: (1) collection of IRRAS data, (2) transformation from reflectivity to phase change, (3) correction of phase for limited frequency range, and (4) calculation of complex refractive index (n - ik) from phase information. Because the above calculation is valid for a bulk material only, the constants obtained in this manner do not necessarily correspond to the bulk optical constants of silicon. This is due to the finite thickness of the wafer (as opposed to a semiinfinitely thick bulk material), the layer of native silicon dioxide on the wafer surfaces (-20 A on each surface), and the fact that the back of the wafer may be either polished or rough. A consequence of using the apparent instead of the actual constants is that a new set must be obtained for each angle of incidence. The refractive indices of two typical silicon wafers obtained in this manner are shown in Figure 1. It was found that the extinction coefficient was sufficiently low for all wafers tested that a value of zero was used in all calculations. These constants were then used in a two-phase reflectivity calculation as a check of their accuracy. Figure 2 demonstrates that even though the constants are not strictly correct, they can be used successfully tocalculate the reflectivity of the wafer. Excellent agreement is found between the measured and calculated reflectivities (light and darkcurves) over most of the frequency range of interest. These constants should then allow calculations to be performed accurately for the reflectivity of the resist-coated wafer. The agreement between measured and calculated reflectivity for the wafers is obtained by careful choice of the anchor wavelengths used in the phase-correction algorithm. For all wafers in this paper, the anchor points were 850 and 4500 cm-l. Choosing a point at lower frequency (e.g., 750 cm-l) improved the agreement at low frequency, but worsened the fit at higher frequency. The low-frequency part of the spectrum is where most organic resists have characteristic absorption peaks. Therefore, theoptical constants of the resist are changing rapidly in this region, making it impossible to fit the IRRAS baseline by use of a simple model for the dispersion and a constant value of the extinction coefficient. For this reason, the anchor points were chosen in order to provide a good match between the calculated and measured reflectivity in the range between 7000 and 2500 cm-I where resist absorption is low. This region also includes the peak at 3300 cm-l in the photoresist spectrum which is useful for following the postexposure bake step in resist processing. Once the optical constants of the wafer have been determined, the reflectivity of the resist film can be calculated. This reflectivity depends on the complex refractive index as well as the thickness of the resist film and the angle of incidence. These parameters can be modified to produce the best (18) Edwards, D. F. In Handbook of Optical Constants of Solids; E. D. Palik, Ed.; Academic Press, Inc.: New York, 1985; pp 547-569.
Analytical Chemistry, Vol. 66, No. 7, April 1, 1994
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0*35
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{
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er
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Figure 4. Comparison of the measured reflectivity (dark curve) and calculated reflectkity (light curve)of a singleside polishedsilicon wafer coated with 0.19 pm of photoresist. The angle of incidence was 40' and radiation was s-polarized.
agreement between the measured and calculated reflectivities, thus fixing the values of these parameters. The iterative approach can be summarized as follows: (1) modify the film thickness to get the correct number of fringes over the frequency range of the measured reflectivity; (2) change the refractive index to fit the height of the fringe minumum a t highest frequency; (3) modify the extinction coefficient to give the best agreement between the measured and calculated reflectivity maximum at highest frequency; (4) change the film thickness to fit the measured reflectivity on the highfrequency side of the highest frequency fringe; and (5) modify the slope of the refractive index curve to produce the best agreement between the measured and calculated fringes over the entire frequency range, neglecting the areas of strong absorption by the resist. If good agreement exists at this point, the process is terminated and the constants are recorded. Otherwise, the process is continued until good agreement is obtained. Figure 3 demonstrates the accuracy with which the measured reflectivity can be matched for two different thicknesses of resist. Once the complex refractive index of a particular resist has been determined, the thickness can be calculated for thinner films that may have one, or even less than one fringe, in the entire frequency range of the IRRAS measurement. Figure 4 shows the calculated and measured reflectivities for a 0.19-pm resist film on a silicon wafer where there is less than half of an interference fringe present. Approximating Reflection Spectra Baselines. The calculated reflectivity for a resist film is the reflectivity that would be measured in an actual film if there were no absorption. In that case, it is analogous to the baseline in an IRRAS measurement and could be subtracted in order to produce an interference-free spectrum. Figure 5 shows the results of such a baseline subtraction. Curve A is the absorbance spectrum of SAL-605, a negative tone chemically amplified X-ray photoresist, before baseline correction, while curve B is the same spectrum after subtracting the calculated absorbance spectrum (simulated baseline). Curve B is free of interference effects, but the baseline near the absorption peaks is incorrect. This is because the calculated reflectivity is incorrect in regions of anomalous dispersion, causing some of the absorbance peaks to be negative. The agreement between the measured and calculated reflectivity in Figure 3, and other similar cases, is excellent l Q l 8 Analytical Chemistry, Vol. 66,No. 7, April 1, 1994
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Figure 5. Comparison of the absorbance of a 1.13-pm SAL-605 film before (A) and after (B) subtraction of the calculated baseline. lhe~ angle of incidence was 40' and redlation was s-polarired. Note the absence of interference fringes between 6500 and 3500 cm-l. Table 1. Comparkon of Redrt Film T M c k f " M " d Udng an Elllp8omoter and a NanospecIAFT wlth Those Calculated from IRRAS Spectra thickness ( u m P
sample AZ-PF 514 1
Nanospec
1.604 f 0.022 1.397 f 0.059 1.145 f 0.029 1.490 f 0.018 1.128 k 0.008 0.9541 f 0.0012 1.72f 0.11 1.931 f 0.062 2.204 f 0.068 silicon dioxide 0.8750k 0.0045
AZPF 514 2 AZ-PF 514 3 S A L 605 1 SAL 605 2 SAL 605 3 PMMA 1 PMMA 2 PMMA 3
ellipsometer
IRRAS
1.597 f 0.011 1.391 f 0.039 1.158 f 0.024 1.448 f 0.015 1.1316 f 0.0037 0.9590f 0.0018 1.703 f 0.059 1.915 0.060 2.157 f 0.044 0.8730 f 0.0024
1.603 f 0.0391 1.387 i 0.045 1.155 f 0.026 1.420f 0.002 1.136 f 0.045 0.9583 f 0.0036 1.721 f 0.036 1.923 f 0.063 2.213 f 0.072 0.8728f 0.0030
a Mean values obtained from meanurements at three locationson the wafer surface. Error is three times the standard deviation.
at frequencies between 7000 and -3800 cm-l but begins to deteriorate below -3500 cm-'. One reason for this is that the complex refractive index used for the film is only valid in the area where normal dispersion is occurring. As soon as the resist begins to absorb strongly (Le., anomalous dispersion) the fit begins to worsen. Another reason for the lack of agreement is that thecalculated opticalconstants for the wafers chosen for this illustration cannot be used accurately to predict the reflection spectra of the silicon wafers below -850 cm-I (see Figure 2). Therefore, the three-phase reflectivity calculation, which relies on these constants, could not be expected to produce accurate results in the region below 850 cm-1. However, the agreement is satisfactory throughout a large region of the IR, suggesting that the complex refractive index and thickness calculated in this manner are representative of the resist film. Determining Film Thickness and Refractive Index from Reflectivity Calculations. In order to assess the accuracy of this method for measuring film thickness and refractive index, the method was performed on various films of commercially available photoresists and silicon dioxide. The calculated film thicknesses appear in Table 1. IRRAS data were collected at three locations on the wafer surface, and the resulting film thicknesses were averaged. Also shown in Table 1 are results obtained from two commercial instruments for measuring film thickness: an ellipsometer and a Nanospec/AFT. Since it is difficult to determine the absolute accuracy of either of the
I
vu
{;\
eemax Accessory
IR out
I I
IR in 0
not drawn to scale
Flgure 6. Experimentalsetup for MBIRRAS experiments. The angle of incidence, 8, is measured from the surface normal. The spacer is a piece of a silicon wafer.
commercial instruments, uncertainties for all measurements represent the film thickness variations across the wafer surface. Another source of differences among the methods is the size of the area sampled. The spot size in the IRRAS method is 16 mm, much larger than the spot in either ellipsometry (2 mm) or the Nanospec (0.03 mm). These uncertainties are probably larger than the instrumental uncertainty, although there may be systematic errors in either of the two sets of data. In any case, the thicknesses measured using these methods agree to within the experimentaluncertainty in almost all cases. These data suggest that for films ranging in thickness from several tenths of a micrometer to several micrometers (thickness often associated with resist films in semiconductor manufacturing) the IRRAS method is comparable to either a Nanospec or an ellipsometer for measuring film thickness. One advantage of the IRRAS method of film thickness determination over ellipsometryis that no prior knowledge of the substrate or the film is required. All the necessary information is obtained from two IRRAS spectra, one of the bare wafer and one of the resist-coated wafer. In fact, the optical constants of wafers from a single lot are usually similar enough that the IRRAS spectrum of only one wafer from the lot needs to be taken, and the calculated optical constants used for the rest of the wafers in that lot. For ellipsometry, one must determine the optical constants of the film at the wavelengths used by the instrument before the thickness can be calculated. The measurement of these constants can be performed using the ellipsometer, but it requires the preparation of additional samples and an experienced operator. Another benefit of the IRRAS measurement technique is that it can be performed on any FT-IR that has a polarizer and some kind of reflection accessory. Such equipment is fairly standard in most industrial and academic laboratories, so film thickness measurementscan be performed without substantial investments in new specialized equipment. Mirror-Backed IRRAS. A more desirable approach than removing the interference effects through a simulation is to change the optical system in such a way that the fringes disapper. The MBIRRAS experiment, shown in Figure 6 , accomplishes this. Figure 7 shows the IRRAS spectra of three films of AZ-PF 5 14,a positive tone chemically amplified X-ray photoresist, on double polished silicon wafers with and without a mirror behind the sample. Obviously, it is much easier to extract quantativeinformation from the mirror-backed spectra. The reproducibility of MBIRRAS is also quite good. Figure
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Figure 7. Comparison of spectra for three thicknesses of AZ-PF 5 14 on double polished silicon wafers with (A) and without (6) a mirror behind the sample. The angle of incidence was 40' and the gap between the mirror and the wafer was -2250 pm.
f
T
0.25 aJ 0.20;C
e v)
o Q
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Figure 8. Five MBIRRAS spectra (space between wafer and mirror -250 pm) of the same wafer coated with AZ-PF 514. The angle of incidence was 40' and radiation was s-polarized. The wafer was removed from the sampling accessory between collection of spectra. The error bars are three times the standard deviation of the peak height at that location.
8 shows five MBIRRAS spectra of the same resist coated wafer. The wafer was removed from the sampling accessory between collection of spectra. These data suggest that the difference in peak height associated with two measurements of these films (perhaps before and after the postexposure bake step in photoresist processing) is less than 0.01 absorbance unit. The change in absorbance associated with a thickness variation of 5% (a reasonable value for these particular samples) would be on the order of 0.005 absorbance unit for a moderately high extinction coefficient (0.2) at 1000 cm-l. This means that most of the variation observed in Figure 8 is probably due to film thickness variation and is not inherent to the MBIRRAS technique. The reason the interferencefringes disappear when a mirror is placed behind the wafer in a reflection experiment is because the mirror recombines the light which was transmitted through the wafer with the light that was reflected at the resist and wafer surfaces. Figure 9 illustrates this concept. If the film thickness were such that the path difference between beams 1 and 2 in Figure 9 introduced a 0' phase shift between the two beams, then they would interfere constructively (Le., beam 1 is shifted 180' upon external reflection from the air-resist interface while beam 2 undergoes a 180' shift for each pass through the film and a 180' shift upon external reflection Analytical Chemistv, Vol. 66, No. 7, April 1, 1994
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Resist (-1pm)
Wafer (-250pm)
\A/
4000
Air (0 - 250wm)
Flgurr S. Diagram of the light paths In the MBIRRAS experiment. The air gap between the wafer and the mirror was varied from 0 to 250 Mm.
from the resist-wafer interface. This produces a net phase difference between the two of OO). However, this same film thickness would produce destructive interference between beams 3 and 4 (Le, beam 3 is shifted 180' when it passes through the film while beam 4 is shifted 180' for each pass through the film and 180' upon external reflection a t the resist-wafer interface. This produces a net phase difference between beams 3 and 4 of 180'). So when beams 3 and 4 are recombined with beams 1 and 2, by inserting the mirror, the total reflected light will be equal to the incident light minus any absorption by the system. This phenomenon will occur for any thickness film since any condition which produces constructive or destructive interference between beams 1 and 2 causes destructive or constructive interference, respectively, between beams 3 and 4. For high-resistivity, double-side polished wafers and typical resist materials there is little absorption throughout most of the IR. Therefore, the total reflectivity of these systems is nearly one at all wavelengths, except those where the resist has its characteristic infrared absorption bands. The gap between the back of the wafer and the mirror in the MBIRRAS experiment must either be zero or else greater than -60 pm at 4-cm-I resolution in order to completely remove the effects of interference. This is because fringes arise from interference between the beams reflected from the back surface of the wafer (not shown in Figure 9) and the mirror. Figure 10 shows the results of experimentally varying the gap in the MBIRRAS experiment. If a metal is coated directly onto the back surface of the wafer, then there is no gap and hence no fringes (curve A in Figure 10). If there is a gap between the mirror and the back of the wafer, then fringes whose periods depend on the thickness of the gap will be observed. Curves B-E in Figure 10 correspond to gaps of 15,30,60, and 250 pm, respectively. It is clear that one must either directly coat the back surface of the wafer with metal or have a gap of greater than 60 Mm in order to remove the interference fringes a t 40° off normal incidence and 4-cm-I resolution. It should also be noted that interference can occur between the beams reflected from the front and back surfaces of the wafer. These fringes have a very short period, compared to fringes due to the resist film and gap and are not observed at 4-cm-I resolution for a wafer that is 250 pm thick. However, 1020 AnalytlcalChemistry, Vol. 66,No. 7, April 1, 1994
3500
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-
Flgure 10. MBIRRAS spectra of a SAL-605 (thickness 1.0 pm) film on a silicon wafer with varlous gaps between the wafer and the mirror. Gaps: (A) no gap and (8) 15, (C)30, (D)60, and (E) 250 pm. The angle of Incidence was 40' and the radiation was epolarlzed.
they have been observed for a 250-pm-thick wafer at 2-cm-l resolution. This means that there is a minimum useful thicknessfor wafers at a given angle of incidenceand resolution.
CONCLUSION In situations where mirror-backed IRRAS is possible, this technique appears to be the most successful a t removing interference fringes. It has also been shown that coating the back surface of the wafer with metal or separating the mirror from the back of the wafer by a sufficiently large air gap improves the quality of the resulting MBIRRAS spectrum. This technique can provide reproducible, fringe-free reflection spectra for thin films on transparent substrates. If one must perform IRRAS on films coated on strongly absorbing substrates, single-side polished wafers, or metals, then qualitative information can be obtained by fitting a baseline to the IRRAS spectrum using the method discussed above. In addition, a method has been devised to determine quickly the film thickness and complex refractive index of thin films if a substantial number of fringes (free of absorption peaks) appear in the IRRAS spectrum. The method provides a means of determining a reasonably accurate absolute film thickness without investing in a dedicated instrument. Furthermore, the IRRAS film thickness measurement method appears to have comparable accuracy, within experimental uncertainty, to some commercially available instruments for the determination of film thickness, yet requires only two IRRAS spectra for each sample-one of the bare substrate and one of the film on the substrate. ACKNOWLEDGMENT This work was supported by the SEMATECH Center of Excellence Program of the Semiconductor Research Corp. under Contract 92-MC-507, and the Office of Naval Research (ARPA) supplied funding for facilities support through Grant NOOO14-91-5-1876. The Synchrotron Radiation Center is operated with support of the National Science Foundation under Grant DMR 9212658. Received for review December 3, 1993. Accepted January 25, 1994." Abstract published in Adoance ACS Abstrocts. March I , 1994.
