Nuclear Instruments and Methods in Physics Research B.55 (1991) 725-729 North-Holland 725 Thermal-wave measurements of high-dose ion implantation Michael Taylor, Kurt Hurley, King Lee, Mark LeMere and Jon Opsal Therma-Wave, 47320 Mjssion EW.s Court, Fremont, CA 94539, USA Tim O’Brien Micron Technology Inc., 2805 East Columbia Road Boise, ID 83706, USA Thermal-wave measurements of high-dose ion implantation into silicon have been characterized. In order to evaluate this technique for use in production as an ion-implant monitor, correlations were performed between thermal-wave and four-point-probe sheet-resistance measurements on test wafers. On device wafers, thermal-wave measurements were correlated to contact resistance on resistor test structures. 1. ~n~oducti~n Thermal-wave measurements are commonly used to characterize and monitor ion implantation at low and medium doses [l-3]. This technique has advantages over other methods because it is nondest~ctive and may be used directly on product wafers. Until recently, thermal-wave measurements had a practical upper ap- plication limit of about 1 X 10” ions/cm’. This is be- cause implants at higher doses from a subsurface amorphous layer that causes a nonmonotonic behavior of the thermal-wave signal. A recently developed capa- bility for thermal-wave me~~ements now extends the measurable dose range to 2 x 7O1”ions/cm’. To evaluate the effectiveness of this technology for production monitoring of high-dose ion implantation, we performed a series of studies to determine the repea- tability of the technique and to correlate thermal-wave measurements of dose with other established tech- niques. We also present data correlating dose measure- ments with electrical test data on actual device wafers. 2. Technology The thermal-wave measurement technique employs two low-power laser beams focused to a 1 pm spot on the sample surface. A simplified schematic of the sys- tem is shown in fig. 1. Absorption of light from an intensity-modulated Ar-ion “pump” laser generates thermal and plasma waves within the surface region of the wafer. These waves are detected by the HeNe laser Helium Neon Probe Incident Probe c Fig. 1. Thermal and plasma waves are generated and detected by two low-power nondestructive laser beams focused to a 1 pm diameter spot on the sample surface. Absorption of light from an acousto-optically modulated (I MHz) argon-ion pump laser generates thermal and plasma waves within the surface region of the sample. These waves are detected by the helium neon probe laser through the pump-induced modulation of the sample reflectivity or other material characteristics. 0168-583X/91/$03.50 0 1991 - Elsetier Science Publishers B.V. (North-Holland) VI. MATERIALS SCIENCE
Transcript
Nuclear Instruments and Methods in Physics Research B.55 (1991) 725-729 North-Holland
725
Thermal-wave measurements of high-dose ion implantation
Michael Taylor, Kurt Hurley, King Lee, Mark LeMere and Jon Opsal Therma-Wave, 47320 Mjssion EW.s Court, Fremont, CA 94539, USA
Tim O’Brien Micron Technology Inc., 2805 East Columbia Road Boise, ID 83706, USA
Thermal-wave measurements of high-dose ion implantation into silicon have been characterized. In order to evaluate this technique for use in production as an ion-implant monitor, correlations were performed between thermal-wave and four-point-probe sheet-resistance measurements on test wafers. On device wafers, thermal-wave measurements were correlated to contact resistance on resistor test structures.
1. ~n~oducti~n
Thermal-wave measurements are commonly used to characterize and monitor ion implantation at low and
medium doses [l-3]. This technique has advantages
over other methods because it is nondest~ctive and
may be used directly on product wafers. Until recently, thermal-wave measurements had a practical upper ap- plication limit of about 1 X 10” ions/cm’. This is be- cause implants at higher doses from a subsurface amorphous layer that causes a nonmonotonic behavior of the thermal-wave signal. A recently developed capa- bility for thermal-wave me~~ements now extends the measurable dose range to 2 x 7O1” ions/cm’.
To evaluate the effectiveness of this technology for production monitoring of high-dose ion implantation,
we performed a series of studies to determine the repea- tability of the technique and to correlate thermal-wave measurements of dose with other established tech- niques. We also present data correlating dose measure- ments with electrical test data on actual device wafers.
2. Technology
The thermal-wave measurement technique employs two low-power laser beams focused to a 1 pm spot on the sample surface. A simplified schematic of the sys- tem is shown in fig. 1. Absorption of light from an intensity-modulated Ar-ion “pump” laser generates thermal and plasma waves within the surface region of the wafer. These waves are detected by the HeNe laser
Helium Neon Probe
Incident Probe
c
Fig. 1. Thermal and plasma waves are generated and detected by two low-power nondestructive laser beams focused to a 1 pm diameter spot on the sample surface. Absorption of light from an acousto-optically modulated (I MHz) argon-ion pump laser generates thermal and plasma waves within the surface region of the sample. These waves are detected by the helium neon probe laser
through the pump-induced modulation of the sample reflectivity or other material characteristics.
126 M. Taylor et al. / Thermal-wave measurements of high-dose ion implantation
“probe” through the modulation of the sample reflectiv- ity. The modulation AR/R is defined as the thermal- wave (TW) signal. The effects of the thermal and plasma waves on the silicon reflectivity is very sensitive to the presence of crystalline disorder created in the surface region of the wafer by the ion-implantation process [4]. Therefore, the thermal-wave signal can be directly cor- related to the ion implantation dose in crystalline sili- con.
3. High-dose ion implantation
A layer of amorphous silicon will be produced when crystalline silicon is implanted at a high dose with sufficient energy. For example, arsenic implants will cause amorphization at doses greater than about 1 x 1Or4 ions/cm’ for energies above 10 keV. The thickness of the amorphous layer increases with dose for a fixed energy and conversely, increases with energy for a fixed dose.
Calculations of dc laser reflectivity and modulated reflectance (TW signal) from an amorphous layer on a crystalline silicon substrate have been performed previ- ously [5,6]. As seen there, the modulated reflectance and dc reflectivity show an interference behavior with in- creasing amorphous thickness. Because of the non- monotonicity of the modulated reflectance signal, a simple correlation to dose is not possible (fig. 2). There- fore, in order to extract dose from modulated reflec- tance and dc reflectivity measurements, a model calcu- lation must first be performed to determine the thick- ness of the amorphous layer. The amorphous-layer thickness is then a monotonically increasing function of
60000
40000 B El
c 30000
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0 -I 0 1000 2000 3000 4000 5000
Amorphous Si thickness (A)
Fig. 2. High-dose ion implantation results in the formation of a subsurface amorphous silicon layer in crystalline silicon. The modulated reflectance signal (thermal-wave signal) demon- strates an interference behavior as function of increasing
amorphous-silicon thickness.
dose, and a simple calibration can be made between the two.
4. Experimental results and discussion
We examined the capabilities of the high-dose ther- mal-wave measurement technique utilizing a commer- cially available Therma-Probe 300 system (Therma- Wave). Experiments were performed to determine the repeatability of the measurement technique and correla- tion to existing monitoring techniques. Additional stud- ies were conducted to determine the sensitivity of the thermal-wave tectique to electrical-device parameters on product wafers.
Fig. 3. Two wafers were measured repeatedly over a period of approximately three weeks. The + la dose repeatability is better than 1% in both cases.
M. Taylor et al. / Thermal-wave measurements of high-dose ion implantation 721
In order to measure the repeatability of the method, a calibration was performed for an arsenic implant at 40 keV. Two wafers (2 X 101’ and 5 X 1O’5 ions/cm2) were measured repeatedly over a period of approxi- mately three weeks. Fig. 3 shows the result of dose measurements. In both cases the measurements demon- strated better than 1% (lo) dose repeatability over the measurement period. This is significantly smaller than the lo-20% dose limit specification for many high-dose implant processes.
A correlation was performed between the thermal- wave measurement of dose and four-point-probe sheet- resistance measurements. Each day an implant monitor wafer was implanted under the same conditions. A thermal-wave measurement was performed before the wafer was furnace-annealed and measured on the four- point probe. This comparison was performed over a period of about one month. The results of the measure- ments are shown in fig. 4. As expected, the sheet-resis- tance and thermal-wave dose measurements show an anticorrelation, since the sheet resistance drops as the dose rises.
Fig. 5 shows the thermal wave measurement of dose plotted against sheet resistance. Even though the total variation in the dose is relatively small, a correlation to sheet resistance can be discerned with a correlation coefficient of 0.77. Comparisons between thermal-wave measurements and four-point-probe measurements of sheet resistance are instructive, but must be analyzed with caution. Thermal-wave measurements are made directly after ion implantation with no intervening pro- cess steps. Sheet-resistance measurements are made after the implanted species is activated through a high-tem- perature anneal. Therefore, the sheet-resistance uni- formity may be affected by the uniformity of activation during the anneal. Also, some diffusion (redistribution)
21
Correlation coefficient = 0.77
7.50e+l5 8.50e+l5 TW Measured Dose
-I 9.50e+l5
Fig. 5. Comparison of thermal-wave measurements with sheet- resistance maps for an implanter stability monitor demon- strates good correlation. Some variation between the two mea- surement techniques is expected since the sheet-resistance maps involve an extra anneal step that introduces another parameter
into the experiment.
of the implanted species may take place during anneal. Finally the spatial resolution of the two techniques are quite different. Thermal-wave measurements are made with a spatial resolution on the order of 1 pm, whereas four-point-probe sheet-resistance measurements average the data over an area of about 1 cm.
Fig. 6 shows a comparison of thermal-wave dose maps and four-point-probe measurements taken on the same test wafer. In this case the standard deviations of the two maps match quite closely. Again, as with the trend data, the contours reverse since a higher dose corresponds to a lower sheet-resistance value.
Since the thermal-wave measurement is used to pre- dict and control electrical parameters on actual devices, it is important to characterize the correlation of dose
1 5e+16
13ec16 i;i
E x I !z
1 let16 g 0
i F e
mt15
7 Oe+15
Fig. 4. Process-stability trend data was collected by thermal-wave measurements of dose and simultaneously by a four-point probe. One wafer per day was implanted and measured on both systems. The trends appear as a mirror image since the sheet resistance
decreases as dose increases.
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728 M. Taylor et al. / Thermal-wave measurements of high-dose ion implantation
Fig. 6. Comparison of the actual maps measured on the same implant monitor wafer: 30 keV arsenic; (a) 9.94~ 10n ions/cm2, 1.61% standard deviation, 1.0% contour interval; (b) 19.1 a/O, 1.56% standard deviation, 1.0% contour interval. The contours reverse since
a higher dose corresponds to a lower sheet-resistance value.
measurements to electrical test parameters. This was done by implanting a series of device wafers at different doses, performing a thermal-wave measurement on the wafers directly after implantation, and comparing these measurements to electrical parameters on the completed devices.
The device wafers were implanted at five different doses: target dose, +7%, -7%, + 15% and - 15%. The implant was arsenic with a nominal dose of 9 X 1015 ions/cm2 with a nominal beam current of 5 mA per- formed on a Varian 120-10 implanter. No screen oxide was used. Measurements were performed on the wafers at three locations, top, center and bottom.
After completing the measurements, the wafers were sent on to complete the manufacturing process. Electri- cal measurements were made on simple resistor test structures to determine the implanted dose. The results
IO - I
El El
q
Ei
31 m I r I n I ’ I m I m I
7 8 9 10 11 12 13
Measured Dose (x E15) Fig. 7. Correlation of contact resistance to thermal-wave-mea-
sured dose on device structures on product wafers.
are plotted in fig. 7. A clear dependence can be seen of contact resistance on thermal-wave measured dose. The significant scatter of the data in contact-resistance read- ings for each dose grouping may be attributable to critical-dimension (CD) variations in the resistor test structures.
There is a group measurements at approximately 1.1 x 1016 that do not correspond to a particular im- plant target dose. These points do lie in a smooth line with the other data points with respect to contact resis- tance. This indicates that these measurements actually represent areas of the wafer that received a substantially different dose from the implant target.
5. Conclusions
We have demonstrated the capabilities of the ther- mal-wave measurement technique to monitor high-dose implants on test and device wafers. The results have been correlated to sheet-resistance measurements for test wafers. On device wafers, we have correlated the thermal-wave measurements to electrical-device param- eters. The repeatability of the measurement technique was demonstrated to be superior to the typical dose ranges of most high-dose implant processes.
Acknowledgement
The authors wish to thank Mike O’Connor for help- ing provide the experimental results on four-point-probe and electrical-test correlations.
M. Taylor et al. / Thermal-wave measurements of high-dose ion implantation 129
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[Z] R. Martini, C. Whichard, W.L. Smith and M.W. Taylor, Solid State Technol. 30 (5) (7987) 89.
[3] W.L. Smith, A. Rosencwaig and D. Willenborg, Appl. Phys. Lett. 47 (1985) 584.
[5] S. Wurm, P. Alpem, D. Savignac and R. Kakoschke, Appl. Phys. A47 (1988) 147.
[6] J. Opsal, in: Review of Progress in Quantitative Nonde- structive Evaluation, vol. SB, eds. D.O. Thompson and DE. Chimente (Plenum, 1987) p. 1241.
