Multichannel process monitor for real-time film thickness and rate measurements in dry etching and deposition

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1 Pergamon PII: S X(98) Vacuum/volume 51/number 4/pages 497 to 502/1998 ã 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain X/98 Sl - see front matter Multichannel process monitor for real-time film thickness and rate measurements in dry etching and deposition F Heinrich, a D Heinze, a T Kowalski, a P Hoffmann b and P Kopperschmidt c, a PAS Plasma Analytics Systems GmbH, Fraunhofer Str.3, Itzehoe, Germany, b Fraunhofer-Institut fu«r Siliziumtechnologie ISIT, Fraunhofer Str. 1, Itzehoe, Germany, and c Max-Planck-Institut fu«r Mikrostrukturphysik, Weinberg 2, 06120, Halle, Germany The versatile multichannel process monitor system, MPM-X, designed for the online control of plasma and ion beam etching and deposition is introduced. It provides a number of in situ process data like etch rate and selectivity, deposition rate, film thickness, uniformity and endpoint. This is achieved by an integration of optical emission spectroscopy (OES) and optical interferometry, where no external light source is required. By using the plasma itself as a light source for the wafer illumination, a large number of wavelengths, ranging from the deep ultra violet to the near infrared, is available for the interferometric process control. The process control capabilities of the MPM-X system are demonstrated in this paper for various etch and deposition applications. ã 1998 Elsevier Science Ltd. All rights reserved Introduction In plasma etching and deposition, process development and process optimization is often an extremely cumbersome and time consuming work since a variety of performance data must be controlled, including the etch and deposition rates of di erent layers, the corresponding selectivity and the uniformity, as well as the quality of the lms and the quality of the pattern transfer. A confusingly large number of di erent tools for plasma processing are currently commercially available. They range from the good old parallel plate reactors and triode systems to di erent kinds of radio frequency and microwave driven high density plasma (HDP) tools, which are based on inductively coupled plasma (ICP) excitation and electron cyclotron resonance (ECR) excitation. 1±3 Unfortunately, understanding of the plasma and processing behavior, in particular of the advanced HDP systems, is still poor. Thus, the use of adequate real-time process diagnostics appears to be indispensable to obtain fast process development and subsequent reliable production control. The MPM-X multichannel process monitor was designed for real-time optical process control in etching and deposition, where plasma and ion beams are used. It is a versatile diagnostic tool that favourably combines the capabilities of OES with those of interferometry. Beyond the conventional OES applications like endpoint detection and spectroscopic plasma analysis, the MPM-X is able to monitor the growth and etching of thin lms in situ. As described elsewhere, 4±11 the underlying method does not require any external light source but takes advantage of the plasma and ion beam emission itself. The bene ts of this methodðapplied to single layers, multilayer stacks as well as to patterned wafersðhave been demonstrated for several applications, like ordinary reactive ion etching RIE 5, 9 and magnetically enhanced RIE, 6 ICP at panel display processing, 3 ECR 7 and reactive ion beam etching. 4, 8 In this paper, we give representative examples for the MPM-X process control capabilities in di erent etch and deposition applications of commercial plasma tools. Experimental Figure 1 shows a photograph of the MPM-16 and the MPM- 2S plasma and surface monitoring systems both integrated in a190 frame. The MPM-16 hardware set up is essentially the same as described in Ref. 3. In brief, it includes a selected photo multiplier tube (PMT) with 16 parallel channels connected to a sixteen channel ampli er/noise lter and an assembly of optical band pass lters for wavelength selection, mounted in an exchangeable lter holder in front of the PMT cathodes. As distinct from the system described in Ref. 3 Ð where one single wavelength bandpass lter was used for all 16 channels to measure the etch rate uniformity across the wafer surfaceðwe have employed a lter holder where each PMT cathode is equipped with an individual bandpass lter of 10 nm band width. 497

2 Figure 1. MPM-16 multichannel process monitor and MPM-2S two channel spectroscopic monitoring system both integrated in a 190 rack. The MPM-16 is equipped with fixed wavelength interference filters. Thus, up to 16 different wavelengths can be monitored simultaneously by using a split fiber arrangement as shown here. The sixteen channels can be also used to get locally resolved process data. The MPM-2S provides free tunable wavelengths on two independent channels allowing a spectroscopic plasma analysis in addition to the film thickness monitoring. The MPM-2S hardware essentially consist of a dual scanning spectrometer with a focal length of 250 mm and an integrated detection unit including two single photo multiplier tubes, ampli ers and noise lters. It allows for free wavelength tuning in a spectral range of 200 to 900 nm at a maximum spectral resolution of 0.2 nm. Data acquisition, monitoring and evaluation of the MPM-X signals are performed via a multichannel AD-board with 16 Bit resolution and software. The optical coupling of the MPM-X system to the plasma system is performed by quartz bers. The multichannel process monitor system can be employed in a very exible way. Depending on the lter arrangement and on the optical ber coupling and its respective viewing position, it can work in di erent modes of operation such as: (i) rate and lm thickness control by single and multiple wavelength interferometry, (ii) process uniformity control, (iii) control of seasoning and chamber cleaning processes and (iv) optical emission spectroscopy. For the cases (i) and (ii), the bers must have a view to the wafer surface, where the viewing angles can be varied in a wide range between perpendicular and near grazing incidence, i.e., 08 to about 758 with respect to the surface normal. For cases (iii) and (iv), there is no need to view the wafer surface. Only the plasma bulk has to be in the visual eld of the optical bers. Due to the high sensitivity of the MPM-X detection units, the size of the viewports can be quite small. Viewport diameters <1 mm may be su cient in many applications. Thus, the adaption to a number of process plasma tools is possible without disturbing plasma and process. Because of the small viewport sizes, the coating of the window, which is known to be a severe problem, particularly in deposition applications, can be avoided. Unless otherwise mentioned, the measurements shown in the following chapter were performed by using the MPM-16 system. The MPM-2S was used in combination with the MPM- 16 in Figure 7(a±d). Results and discussion The experiments were carried out by applying the MPM-X system to di erent process applications. The measurements shown in the following sections were obtained for three di erent commercial plasma reactors: an Anelva ECR etcher [see Figures 2±4, Figure 7(a±d)], an Applied Materials RIE system with magnetic con nement (Figures 5 and 6) and an Anelva ECR deposition system (Figures 8 and 9). Poly silicon etching The schematics of the ECR etch system and the geometry of the MPM-X optical ber coupling is shown in Figure 2. Please note that no e orts have been made here to give a detailed drawing of the technical details of the etch tool. In brief, the microwave is coupled by a waveguide into the plasma via a quartz plate on top of the etch chamber. 498

3 Figure 2. Schematics of MPM-X optical fiber adaption to an Anelva ECR etching system. The optical coupling is similar to that described in Ref. 7. However, instead of one single optical ber, in the present work we employed two ber bundles both split into several sub bundles on the MPM-X detector side, allowing multiwavelength detection. Both ber bundles were equipped with simple optics for parallel light detection. One of the ber bundles (top view) looks through a small viewport in the microwave guide through the plasma onto Figure 4. MPM-X signals during quarter micron gate patterning. thickness of gate oxide <5 nm, poly silicon 150 nm. Curve (1) side view l = 530 nm, (2) normalized top view. the center of a 60 wafer at zero degree observation angle with respect to the surface normal. The other ber bundle (side view) looks from a side window into the plasma bulk only. Thus, the top view signals detect the wafer status and the side view signal yields information essentially on the plasma status. The measurements shown in Figure 3 were performed during the etching of a poly silicon wafer with about 450 nm thickness over 100 nm gate oxide on a silicon substrate under chlorine/bromine chemistry. The process has been monitored at two di erent emission wavelengths, 530 nm and 375 nm. The full etching process, including the breakthrough of the native oxide and the main etch, highly selective to the oxide, is monitored in Figure 3. Curves 1 and 2 show the intensity variation of the plasma detected by the side view ber. It demonstrates that the plasma emission needs some time to reach a stable state after the main etch is started. On the other hand, the emission intensities change on approaching the poly sili- Figure 3. MPM-X signals during poly silicon etching. (1) side view l = 375 nm, (2) side view l = 530 nm, (3) top view l = 530 nm, (4) normalized top view l = 530 nm, poly±si thickness 450 nm, gate oxide 100 nm. Figure 5. Schematics of the MPM-X optical fiber adaption to an Applied Materials RIE system with rotating magnetic field. 499

4 Figure 6. MPM-X signals without (upper curve) and with software filter at l = 530 nm in poly silicon gate patterning. The magnetic eldðdirection perpendicular to the surface normal of the waferðrotates around the normal of the wafer surface. Since the plasma density is increased in areas where the magnetic eld is active, the plasma light emission uctuates according to the rotation frequency. These regular `high frequency oscillations' are superimposed on the interference signal resulting from the poly silicon gate etch process. One way to get rid of these magnetic eld induced disturbances is to use a normalization procedure similar to that shown in Figures 3 and 4. As demonstrated in Figure 5, however, a clear monitoring of the interference signal can also be gained by employing an integrated software lter. The average etch rate derived from the interference signal was 264 nm/min. The etch process was stopped by the online computer evaluation at a poly silicon rest lm thickness of 30 nm. con/oxide interface due to a change in the plasma chemistry. Although no attempts for an identi cation of the underlying chemical species has been made here, we believe that the 530 nm emission mainly originates from the feed gas, whereas the 375 nm emission is due to a poly±si etch product. This is suggested by earlier experiments 9 and the di ering intensity behaviour of the 530 nm and 375 nm side view emission when approaching the oxide lm. In addition to the endpoint behavior, indicated by curves 1 and 2, curve 3 shows the evolution of the poly silicon lm thickness variation by interference of the plasma emission re ected at the wafer surface and detected by the top ber. In order to monitor the etching rate and lm thickness, respectively, it can be advantageous to normalize the top view emission to the side view emission. This is demonstrated in curve 4 which depicts the 530 nm top view emission normalized to the 530 nm side view emission. In this case, changes due to the changing plasma chemistry are ruled out and the remaining intensity behaviour is in a good approximation due to surface e ects only. The etch rate obtained by an on-line computer evaluation of curve 3 and 4 provided a value of 150 nm per minute. In view of the quarter micron gate patterning for VLSI applications, the gate oxides becomes very thin which is a challenge for process control and respective diagnostics. The potential of the MPM-X system for these applications may be demonstrated in Figure 4 where a sub quarter micron poly silicon gate patterning process with a <5 nm thick gate oxide was monitored at an open area >50%. The thickness of the poly silicon lm was about 150 nm. The curves show the normalized 530 nm top view emission together with the 530 nm side view emissions (cf. Figure 2). Although the degree of modulation of the interference signal is considerably weaker than for the thicker gate oxide in Figure 3, the instantaneous thickness of the poly silicon layer can be clearly monitored due to the high signal-to-noise ratio of the MPM-X detection unit. This allows us to stop the main etch safely before the oxide is reached and to continue with a highly selective overetch in order to avoid unwanted e ects like notching or trenching. 12 The adaption of the MPM-X system to an RIE etch tool with magnetic con nement and one example of poly silicon etch monitoring in this system is shown in Figures 5 and 6. Oxide etching One of the bene ts of the MPM-X system is its capability to use very short wavelengths. This is demonstrated in Figure 7(a±d) for oxide etching using a carbon uorine feed gas mixture. The experimental setup was the same as in Figure 2. In this case, a ber bundle with three sub bundles on the MPM-X detector side was used, allowing for the simultaneous monitoring of three di erent wavelengths. The plasma parameters were the same for Figure 7(a±d). Figure 7(a) shows MPM-X signals recorded at wavelengths of l = 530 nm and l = 330 nm during the etching of an unpatterned oxide lm with a thickness of 300 nm. Both signals provide a clear interference pattern from which the etch rates can be simultaneously evaluated. The advantages in using shorter wavelengths is that the status of the lm thickness is obtained at better time resolution and higher accuracy, according to the increased number of oscillations. A second advantage in using short wavelengths is demonstrated in Figure 7(b±d), which shows the MPM-X signal behaviour during the etching of quarter micron lines and spaces in a 300 nm thick oxide lm. An i-line photoresist mask was used with 50% open area. The etching conditions were similar to that in Figure 7(a). Compared to the etching of unpatterned wafers, the signal behaviour becomes more complicated when the etching of both the resist mask and the oxide lm contribute to the interference signal. This is demonstrated in Figure 7(b,c) for wavelengths of 530 and 330 nm. Up to the estimated endpoint, indicated as arrows in the gures, we see an overlap of resist and oxide modulation. The signals become simple only after the oxide is etched through. The single modulation frequency actually re ects a resist etch rate of 136 nm/min. In order to have clear monitoring of the oxide lm thickness evolution, we used the shortest available emission wavelength by tuning the MPM-2S system to l = 200 nm. Since the photo resist shows strong absorption at this wavelength, 9 the interference e ects resulting from the resist etching are now e ectively suppressed. The remaining modulation is solely due to the oxide etching. Hence, a simple intensity modulation is obtained, as in the case of Figure 7(a), as well as a distinct endpoint. The oxide etch rate obtained from Figure 7(d) was 209 nm/min, the selectivity of the oxide:photoresist mask was 2.4, as derived from Figure 7(b±d). 500

5 Figure 7. MPM-X signals at different wavelengths during the etching of a 300 nm thick oxide film, (a) no pattern, (b)±(d) with i-line photoresist `lines and spaces' pattern, structure widths down to 0.25 mm; experimental setup as Figure 2. The oxide etch rate and the oxide endpoint could be clearly detected by the 200 nm signal of the MPM-X system. Silicon nitride deposition In several etch and deposition systems, there is no viewport available that would allow for a perpendicular view to the wafer surface. In this case, we can take advantage of the fact that large observation angles can be used by the MPM-X. This was studied on an ECR downstream deposition tool, as shown in Figure 8. A view of the wafer surface was possible only via a side ange. In this case, the optical ber, equipped with simple optics for parallel light detection, looked o -axis through the ange onto a silicon area of about 1 cm 2. The observation angle with respect to the surface normal was 558. The interference signal resulting from a silicon nitride deposition process in SiH 4 /N 2 atmosphere is shown in Figure 9. The signal was used to stop the process at a lm thickness of 170 nm. Other examples demonstrating the MPM-X's capability to monitor the etch rates and the lm thickness at non-zero degree observation angles are given in Ref. 13 for several applications, including etching and deposition. Figure 8. MPM-X optical fiber adaption to an Anelva ECR deposition system. Figure 9. MPM-X signal at 336 nm during deposition of a 170 nm thick silicon nitride film on silicon. 501

6 Conclusion In this paper, the MPM-X multichannel plasma and surface monitoring system was introduced as a real-time optical detection system for the interferometric process and lm thickness control in etching and deposition, where the plasma itself was used as a light source. Its control capabilities were investigated for several etching and deposition applications on commercial plasma tools including: (i) poly silicon patterning with gate oxides of thicknesses down to <5 nm, (ii) oxide patterning and (iii) silicon nitride deposition. For all applications, etch or deposition rates and lm thickness evolution could be clearly derived from the emission signals. Several problems in plasma processing could be overcome: (i) The fairly low degree of modulation of interferometric signal from poly silicon etching, when the underlying gate oxide is very thin. This was matched by the usage of high sensitivity photomultiplier tubes with high signal-to-noise ratios. (ii) In oxide patterning, the disturbing superposition of the modulation resulting from the etching of the resist mask could be avoided by selecting wavelengths as low as 200 nm, where only the oxide remains transparent. (iii) The applicability of the MPM-X system for lm thickness monitoring at non zero degree observation angle with respect to the surface normal was demonstrated in silicon nitride deposition on an ECR deposition system. From the results presented in this paper, we may conclude that the MPM-X system can be considered as a powerful tool for real-time rate determination and lm thickness monitoring in etching and deposition. In particular, we expect quite a high reliability in process control applications due to the high degree of redundancy of the information which can be gained by the monitoring of plasma and surface state using up to 16 parallel channels simultaneously. References 1. Moisan, M. and Pelletier, J. (eds.), Plasma Technology, 4, Microwave Excited Plasmas. Elsevier, Amsterdam, London, New York, Tokyo, 1992, ISBN Hopwood, J., Plasma Sources Sci. Technol., 1992, 1, Heinrich, F., BaÈ nziger, U., Jentzsch, A., Neumann, G. and Huth, C., Journal of Vacuum Science & Technology B, 1996, 14, Heinrich, F., Stoll, H.-P. and Scheer, H.-C., Appl. Phys. Lett., 1989, 55(14), Heinrich, F., Stoll, H.-P., Scheer, H.-C. and Ho mann, P., in Proceedings of SPIE-Symp. on Microelectronic Integrated Processing, Santa Clara, CA., U.S.A., Oct. 8± , SPIE, Vol. 1188, pp. 185± Heinrich, F. and Ho mann, P., J. Appl. Phys., 1992, 71(4), Ho mann, P.J., Heinrich, F., Renner, S. and Deutschmann, L., in 187th Meeting of The Electroch. Soc., Reno, Nevada, May 21± , Extended Abstracts, Vol. 95-1, pp. 222± Heinrich, F. and Ho mann, P., in Proceedings of `Technology and Applications of Ion Beams', Loughborough, U.K., April 7± , (Vacuum, 1993, 4(3/4), 275). 9. Heinrich, F. and Neumann, G., in Proceedings of European Tegal Plasma Seminar, Geneva, April 10th 1994, pp. 1± Heinrich, F. and Kopperschmidt, P., in Proceedings of 10th International Colloquium on Plasma Processes, Antibes, France, June 11±15, 1995, pp. 268± Heinrich, F., US Patent Nr , Final technical report, JESSI E 96 Project, funded by the European Community: Multiprocessing Modules Based Upon Advanced Plasma Sources (ECR), Scheer, H.-C., Zink, J., Ningel, K.P. and Soll, C., These proceedings. 502