Thermal-wave measurements of ion implanted silicon

520

THERMAL-WAVE

Nuclear Instruments and Methods in Physics Research B21 (1987) 550-553 North-Holland, Amsterdam

MEASUREMENTS

OF ION IMPLANTED SILICON

B r a d f o r d J. K I R B Y , Lawrence A. L A R S O N a n d R u - Y u L I A N G National Semiconductor Corporation, M / S 4-105, P.O. Box 58090, Santa Clara, CA 9.5052-8090, USA

Results are presented of studies made to qualify photothermal-reflection spectroscopy for use as an ion implant process monitoring tool in a semiconductor production environment. The spectrometer used was the Therma-Probe 150 from Therma-Wave, Inc. Data is presented on the sensitivity of the system to the basic implant parameters: dose (fluency), energy, and species. Data is also presented on the sensitivity to ion channeling, dose rate, and time of measurement. The studies were limited to the low-to-medium-dose range, 1 X l0 n to 1 × 1014 ions/cm2, and to the energy range of 25 to 200 keV. Possible implications of the results on characteristics of implant damage are briefly discussed.

1. Introduction Photo-thermal-reflection spectroscopy has recently been introduced as a means of monitoring ion implanter dose accuracy and uniformity. As implemented in the Therma-Probe 150 from Therma-Wave, Inc., [1] this (" thermal-wave") method uses a modulated laser, beam to induce thermal oscillations in the silicon surface and a probe laser to measure resulting fluctuations in the surface reflectivity. It has been determined that the measured signal is sensitive to the level of damage present in the surface layers of the sample, and so also to an implanted dose in silicon. In this paper we present results of characterization studies of the method that are of particular interest to the implant process engineer. We also present some results on ion implant induced damage itself, since the method is a new tool for such studies.

2. Experimental conditions All implants were performed on Varian medium-current production implanters with standard Waytlow endstations masked for 100 or 125 mm wafers. Except in the case where it was a parameter, the beam current was chosen to give a nominal 10 s implant. The samples were 100 mm test-wafer-quality silicon wafers, typically 3 to 20 12 cm. For all but the channeling studies, the wafers were given a dechanneling thermal oxide coat of 25-30 nm as measured on a relative-reflectance spectrophotometer. Contour maps of the thermal wave signal were made of all wafers to eliminate implant uniformity as a contributor to the results. We present most results in the otherwise undefined "thermal-wave units" presented by the system. The measurements were taken over a period of 18 months, during which time a num-

0168-583X/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

ber of changes were made to the calibration of the system. We have confirmed the functionalities of the data presented, but the reader should not expect to reproduce the absolute values exactly.

3. Dependence on dose Of primary interest is the sensitivity of the thermalwave method to implanted dose. Fig. 1 shows the dependence of the reading on dose from 1 × l0 n to 1 x 1013 i o n s / c m 2 at 100 keV. Although there is a monotonic dependence, there is a plateau in the 1012-1013 i o n s / c m 2 range. The plateau results in a minimum in the measurement sensitivity defined as S = (dTW/TW)/(dN/N),

the percent change in measured signal ( T W ) caused by a unit percent change in implant dose (N). The sensitivity determined from the data of fig. 1 is plotted in fig. 2. It is our experience that important information can be masked for those cases where the value of S is below about 1/6, and, specifically, that uniformity maps become unrealistically featureless. In an attempt to determine whether the minimum in the sensitivity is a real damage phenomenon or a measurement artifact, we investigated the double-implant/ sheet-resistance method [2] for phosphorus over this dose range. This method also depends on implant damage. There was no obvious minimum above the noise level of the data, and any minimum there might have been is of much smaller magnitude than we see in the thermal-wave data. We conclude that the thermalwave method has a real sensitivity minimum in this damage range.

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The peak-to-peak noise levels we observe are 0.4% (thermal-wave units) point-to-point, 2% day-to-day. (These values were determined before the system change discussed in sect. 7, and may no longer be representative.)

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A strong dependence on ion energy is expected of a system that measures crystal damage. Fig. 3 shows this to be the case for arsenic and phosphorus. For boron, however, the thermal-wave signal levels off and then falls for higher energies. A possible explanation for the latter is that the probe is not sensitive to damage at the depths reached by the higher energy boron ions. To test this hypothesis, we used TRIM [3] to model the damage distributions, and assumed that the thermal-wave sensitivity is exponentially decreasing with depth [4]. The solid curve in fig. 3 is the integral of the function

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for boron, where x is depth below the silicon surface, V(x) is the TRIM function vacancies/angstrom/ion, and the fitting parameter b is 500 rim. (An overall

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Fig. 3. Thermal-wave readings as a function of implant energy at a dose of 1 x 1012 ions/era2 for three standard implant species. The solid curve is a model fit to the boron data. IX. MEASUREMENT TOOLS

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B.J. Kirby et aL / Thermal-wave measurements

normalization is included in V ( x ) 5 Although the agreement with the data is reasonable, this simple model does not fit all species simultaneously.

variation in damage due to channeling is much greater than the variation in depth.

6. D e p e n d e n c e on dose rate 5. D e p e n d e n c e on planar channeling

Planar channeling has been well studied using sheet resistance mapping techniques [5]. The channeling shows up as a relatively low sheet resistance stripe parallel to (110) planes on (100) wafers. The lower sheet resistance values are attributable to a deeper junction in those areas in which the ion beam strikes the wafer parallel to atomic planes. Similar maps are obtained with the thermal-wave technique, but now the lower signals are attributable to lower damage caused by channeled ions. Thermal-wave mapping appears to be more sensitive to channeling than is sheet-resistance mapping. It can detect channeling parallel to (400) planes in (100) wafers, which shows up as relatively low stripes rotated 45 ° from the (110) stripes. And thermal-wave maps at times have shown channeling patterns in arsenic implanted wafers that appeared perfectly uniform to sheet resistance mapping. We conclude that in these cases the

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Various papers [6,7 and references cited therein] have investigated the effect of dose rate on damage. These works have concentrated on the phenomenon of amorphization of the crystal lattice, and, in the dose ranges used, a major contributor to the residual damage level is the temperature history of the sample during the implant. (Dose level monitoring in such ranges-typically starting near or above 1 × 1014 i o n s / c m 2 - i s not amenable to the thermal-wave technique due to the presence of dynamic annealing as a complicating factor.) We have noticed a dose rate effect at rates not expected to have any effect on wafer temperature. Fig. 4 shows variations in readings for wafers implanted at 1 × 1014 i o n s / c m 2, 100 keV, over an order of magnitude in beam current. The readings are seen to increase with dose rate. No significant variation was observed for arsenic with these parameters, nor for phosphorus at 1 × 1012 i o n s / c m 2 over a range of 1.5 to 16 n A / c m 2. This data might be explained by the following hypothetical model. We assume that certain isolated defects created by an ion are unstable (in a time frame of seconds) at room temperature, but that aggregates (amorphous clusters?) of the same defects are stable. For light ions the defect/ion count is low and creation of a stable aggregate requires the defects from a number of ions. The residual damage will be independent of dose rate if the dose rate is very low (where there is little chance for cooperative effects) or very high (where cooperative effects dominate), but otherwise will be an increasing function of it. For heavy ions the defect/ion count is high and each ion can create stable aggregates. In this case the residual damage will be relatively independent of the dose rate. Further indication of stable and unstable damage forms is given in the next section.

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Other investigators have noticed a time dependence in thermal-wave readings [8]. Specifically, for most implants the thermal-wave reading will be observed to decrease over time, at a decreasing rate. The phenomenon has been referred to as a room-temperature anneal. Fig. 5 shows data for a 1 × 1014 ions/cm 2, 100 keV boron implant. There is noticeable decay for at least 4 months. The dependence does not appear to be simple exponential. For practical purposes, a vendor supplied

B.J. Kirby et a L / Thermal-wave measurements

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8. C o n c l u s i o n s

The thermal-wave technique shows a sensitivity to implant parameters that makes it a useful tool for monitoring machine performance. The dependencies are not simple, and often involve more parameters than one might naively assume. Comparison of results from different laboratories will be difficult for this reason. On the other hand, the technique will prove to be an interesting tool for the study of implant damage.

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References

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software modification minimizes this effect. However, we have found it to about double the day-to-day variation in readings of control wafers.

[1] W.L. Smith, A. Rosencwaig and D.L. Willenborg, Appl. Phys. Lett. 47 (1985) 584. [2] M.J. Markert, D.S. Perloff and E. Lee, ECS Extended Abstracts 83-1 (1983) 613. [3] J.F. Ziegler, J.P. Biersack and U. Littmark, The Stopping and Range of Ions in Solids (Pergamon Press, New York, 1985). [4] W.L. Smith, private communication. [5] M.I. Current, N.L. Turner, T.C. Smith and D. Crane, in: Ion Implantation Equipment & Techniques, eds., J.F. Ziegler and R.L Brown (North-Holland, Amsterdam, 1985) p. 336. [6] J.R. Dennis and E.B. Hale, Radiat. Eft. 30 (1976) 219. [7] J. Narayan and O.W. Holland, J. Electrochem. Soc.: S-SST 131 (1984) 2651. [8] J. Schuur, C. Waters, J. Maneval, A. Rosencwaig, W.L. Smith, L. Golding and J. Opsal, these Proceedings (Ion Implantation Technology, Berkeley, 1986) Nucl. Instr. and Meth. B21 (1987) 554.

IX. MEASUREMENT TOOLS