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Thermal wave implant dosimetry for process control on product wafers
Nuclear Instruments and Methods in Physics Research B21 (1987) 559-562 North-Holland, Amsterdam
559
T H E R M A L WAVE IMPLANT DOSIMETRY FOR PROCESS CONTROL O N PRODUCT WAFERS M a r k A. W E N D M A N * Intel Corporation, 250 N. Mines Rd., Livermore, CA 94550, USA
W . Lee S M I T H Therrna-Wave, Inc., 47734 Westinghouse Dr., Fremont, CA 94539, USA
The ability of thermal wave techniques to nondestructively measure ion implant doses on NMOS product wafers is demonstrated. The correlation of thermal wave measurement signal to electrical parameters and as-dialed dose is examined. Results are presented for thermal wave measurements over an implant dose range of 4.0-8.0Ell ions/cm2 for a 50 keV boron enhancement implant and 1.5-2.5E12 ions/cm 2 for a 25 keV arsenic depletion implant.
1. Introduction Control of ion implant dose is critical to good device performance and wafer yield in silicon integrated circuit manufacturing. In particular, in MOS VLSI manufacturing, nonparasitic threshold voltage control is paramount to a reliable manufacturing line. In NMOS technology, two critical electrical parameters are the depletion transistor drain current, Idd, and the enhancement transistor threshold voltage, Vie. Variations of + 15% of the depletion drain current and +5% of the enhancement transistor threshold voltage can dramatically affect wafer yield and device speed. In a stable VLSI manufacturing line, threshold adjust ion implants are one of the few process steps which at present cannot be adequately monitored until the end of the manufacturing cycle, typically 2 - 4 weeks after implantation. Techniques [1-4] for monitoring low dose implants at present all require the use of special test wafers to determine implant dose, and therefore are not of use in verifying that particular product wafers have received the desired implant dose. In particular, no means have been available to measure low dose implants ( 1 E l l - l E 1 3 ) on product wafers immediately after implantation to permit real time implant process monitoting. The ability to measure the implanted dose directly on product wafers in a nondestructive fashion is invaluable in a manufacturing environment, particularly when product or mask checkout lots are being run. This is where the thermal wave implant dose measurement technique [5] has a distinct advantage over existing dose measurement methods. *Presently at Cray Research, Inc., Chippewa Falls, WI 54729, USA. 0168-583X/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
The recently introduced commercial thermal wave measurement system [6] has been applied to measurement of ion implant doses [7-10], but as yet there have been no reports in the literature of direct correlation of thermal wave signal, measured just after implantation, to electrical test measurement results obtained on the finished product circuits. Since the ultimate task of the implant process engineer is to ensure good control of the end-of-line transistor characteristics, a true evaluation of this implant dose measurement technique must include a cross-correlation of thermal wave signal to dose and implanted transistor parameters. In this article we present data correlating thermal wave measurements to implant dose and electrical test parameters to verify that, indeed, this real time implant dose monitoring technique is truly useful as a tool for VLSI manufacturing cost reduction.
2. Product wafer sample preparation The objective of this experiment was to determine the feasibility of using thermal wave measurements to control critical transistor characteristics. The process vehicle used for the experiment was an NMOS technology using four ion implants. Because there is at present no other means to measure low doses on product wafers, we decided to characterize the two critical low dose implants: a 25 keV arsenic depletion implant and a 50 keV boron enhancement implant. Cross-sectional views of these regions of the device structure are depicted schematically in fig. 1. The ion implants were performed on Varian medium-current implanters with a tilt angle to dechannel the implanted ions. The implanted dose values are listed in table 1. IX. MEASUREMENT TOOLS
560
a ~...Es, J j
M.A. Wendman, W.L. Smith / Thermal wave imp~ant dosimetr)'
F,E o ox,o
"b ~ -
1.8
-
I
1.8
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Fig. 1. Cross-sectional view of MOS transistor structures on product wafers showing the gate regions used for depletion (a) and enhancement (b) implant inspection measurements with the Therma-Probe system.
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oh 1.2
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Table 1 Implant doses
1.0
~
Boron dose values (ions/cm 2)
Arsenic dose values (ions/cm2)
4.10Ell 4.70Ell 5.30Ell 5.90Ell 6.50Ell 7.10Ell 7.70Ell
1.55E12 1.70E12 L85E12 2.00E12 2.15E12 2.30E12 2.45E12
Twenty-four wafers were procured from a standard product manufacturing lot before arsenic depletion implant. After both depletion and enhancement implants were performed according to the standard manufacturing process flow, thermal wave measurements were made on a Therma-Probe 150 system. To comply with manuscript length constraints, we must refer the reader to other published works [5,10-12] for description of thermal wave measurement techniques used for ion implant dose inspection. For the depletion implant, five measurements were made per wafer at top, center, bottom, left and right locations. For the enhancement implant, three measurements per wafer were made at top, center and bottom locations. The wafers were then sent on to complete the standard manufacturing process flow which included electrical parameter testing of each of the 24 wafers on the same transistors as measured earlier with the ThermaProbe system. Five measurement sites on the wafer (top, center, bottom, left and right) were used in collecting the parametric data. No anomalies relating to thermal wave measurements were found. This measurement technique has proven to be entirely nondestructive.
3. R e s u l t s
Graphs of electrical parameters versus implant dose or thermal wave signal, and of thermal wave signal versus implant dose are shown in figs. 2-7. The graphs clearly show a strong correlation between implant doses and the corresponding thermal wave and electrical parameters. Linear regression calculations were made to
0.9 380.0
° 380.0
400.0
420.0
440.0
T h e r m a l Wave Signal ( TW units )
Fig. 2. Graph of enhancement threshold voltages measured on completed product wafers versus thermal wave signal measured after the enhancement low-dose implant.
extract the least squares best fit parameters of slope, intercept and correlation coefficient. These are summarized in table 2. The correlation coefficients for the depletion implant samples are quite good, with all except one greater than 0.95. The exception is the correlation coefficient of the depletion drain current as a function of thermal wave signal [Idd (TW)]. Between the Idd (TW) and the Vie(TW) data in table 2, there is a rather large discrepancy of approximately 0.10 in the correlation coefficient, with the enhancement implant having a correlation coefficient of no less than 0.98. Interestingly enough, such a discrepancy is not observed when comparing the respective data of implanted dose versus thermal wave signal [TW(DOSEe) and TW(DOSEd) in table 2], where both the enhancement and depletion implant data sets have best-fit correlation coefficients greater than 0.96. The fact that the TW versus dose data for the two implants have equally good correlation coefficients indicates that both implants have similar dose control. We expect that the difference in the correlation coefficients for the data on electrical parameter versus TW signal for the two implants arises from one main factor that is implant-process related. Whereas the enhancement implant is made through a silicon dioxide film (fig. 1) which greatly reduces ion channeling, the depletion implant is made into bare silicon. Despite the tilt angle employed in the ion implanter, the wafers may still suffer channeling related to the "twist" angle of the wafer. Since this twist angle was not controlled, the amount of channeling and thus the effective depth of the implant are affected in a random manner from wafer to wafer. This effect is enhanced by the low ion energy (25 keV) which increases the acceptance angle for channeling. Since the arsenic has a lower diffusion rate than boron, the diffusion caused by subsequent
M.A. Wendman, W.L. Smith
/
561
Thermal wave implant dosimet o, 460.0
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T h e r m a l W a v e S i g n a l ( TW u n i t s )
7
6
8
I m p l a n t D o s e ( x l 0 1 1 ions / cm 2 )
Fig. 3. Graph of depletion drain current measured on completed product wafers versus thermal wave signal measured after the depletion low-dose implant.
1.6
A
|
1.5
Fig. 6. Graph of thermal wave signal measured after the enhancement low-dose implant versus the as-dialed implant dose.
610,0
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600,0
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~
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560.0
i
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4.0
5.0
6.0
7,0
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Implant D o s e ( x l O 11 ions / cm 2 )
15
1.7
1.9
2.1
2.3
2.5
I m p l a n t D o s e ( x l 0 TM i o n s / c m 2 )
Fig. 4. Graph of enhancement threshold voltage measured on completed product wafers versus as-dialed enhancement implant dose.
Fig. 7. Graph of thermal wave signal measured after the depletion low-dose implant versus as-dialed implant dose.
280.0
240.0
200.0
160.0 ©
0 120.0
80.0 1.5
1.7
1.9
2.1
2.3
2.5
Implant D o s e ( x l O TM ions / cm 2 )
Fig. 5. Graph of depletion drain current measured on completed product wafers versus as-dialed depletion implant dose.
processing will have little effect in masking the effects of channeling in the depletion implant. The end result of the deeper arsenic depth is an increase in the depletion transistor drain current, Idd. On the other hand, ion channeling causes a reduction in TW signal since less lattice damage is produced by a channeled ion. These opposing effects reduce the correlation between Idd and TW signal Finally, this same scatter in the Idd measurements should also adversely affect the Idd versus enhancement dose data. But because the as-dialed dose values are not correlated with channeling, the correlation coefficient between Idd and depletion dose can be expected (and is observed) to exceed that for Idd versus TW signal. IX. MEASUREMENT TOOLS
562
M.A. Wendman, W.L. Smith / Thermal wave implant dosimetr).
Table 2 Summary of least-squares-fit data Graph Y(X)
Fig.
Slope d Y/d X
Intercept Y( X = 0)
Correlation coefficient
Vt¢(TW) Idd(TW) Vtc(DOSEe) Idd(DOSF-AI) TW(DOSEe) TW(DOSEd)
2 3 4 5 6 7
0.007447 V/TW 2.984/~A/TW 0.1772 V/1Ell 160.9/~A/1E12 23.31 TW/1Ell 52.70 TW/1E12
- 1.808 V - 1560.0/~A 0.2287 V - 149.7/tA 276.6 TW 474.8 TW
0.9814 0.8843 0.9932 0.9532 0.9681 0.9597
4. Conclusions
References
To summarize the product wafer results described here, the thermal wave dose measurement technique has proven to be a very powerful diagnostic for real time, low dose measurements. A strong correlation of thermal wave signal to implanted dose and transistor electrical characteristics has been demonstrated. Indeed, it can be foreseen that thermal wave dose measurements will be introduced as a standard technique in a VLSI manufacturing line to tightly control the often troublesome low dose, threshold adjust implants, thus making these steps significantly easier to optimize and maintain.
[1] G.A. Gruber, Solid State Technol. 26 (1983) 159. [2] J.C. Cheng and G.R. Tripp, Solid State Technol. 26 (1983) 143. [3] H. Glawischnig, K. Hoerschelmann, W. Holtschmidt and W. Wenzig, Nucl. Instr. and Meth. 189 (1981) 265. [4] R.O. Deming and W.A. Keenan, Solid State Technol. 28 (1985) 163. [5] W.L. Smith, A. Rosencwaig and D. Willenborg, Appl. Phys. Lett. 47 (1985) 584. [6] Therma-Probe Systems, product of Therma-Wave, Inc., Fremont, CA. [7] W.L. Smith, R.A. Powell and J.D. Woodhouse, Proc. SPIE 530 (1985) 188. [8] W.L. Smith, M.W. Taylor and J. Schuur, Proc. SPIE 530 (1985) 201. [9] W.L. Smith, R.H. Reuss, W. Clark and D. Rensch, Materials Research Society Syrup. Proc. 45 (1985) 247. [10] W.L, Smith, A. Rosencwaig, D. Willenborg, J. Opsal and M.W. Taylor, Solid State Technol. 29 (1986) 85. [11] A. Rosencwaig, J, Opsal, W.L. Smith and D.L. Willenborg, Appl. Phys. Lett. 46 (1985) 1013. [12] J. Opsal and A. Rosencwaig, Appl. Phys. Lett. 47 (1985) 498.
We would like to thank L. Brooks of Intel for her help in coordinating the processing, M. Rocke of Intel for his help and advice over the length of the project, and M. Taylor of Therma-Wave for his help with the implant inspection measurements.
559
T H E R M A L WAVE IMPLANT DOSIMETRY FOR PROCESS CONTROL O N PRODUCT WAFERS M a r k A. W E N D M A N * Intel Corporation, 250 N. Mines Rd., Livermore, CA 94550, USA
W . Lee S M I T H Therrna-Wave, Inc., 47734 Westinghouse Dr., Fremont, CA 94539, USA
The ability of thermal wave techniques to nondestructively measure ion implant doses on NMOS product wafers is demonstrated. The correlation of thermal wave measurement signal to electrical parameters and as-dialed dose is examined. Results are presented for thermal wave measurements over an implant dose range of 4.0-8.0Ell ions/cm2 for a 50 keV boron enhancement implant and 1.5-2.5E12 ions/cm 2 for a 25 keV arsenic depletion implant.
1. Introduction Control of ion implant dose is critical to good device performance and wafer yield in silicon integrated circuit manufacturing. In particular, in MOS VLSI manufacturing, nonparasitic threshold voltage control is paramount to a reliable manufacturing line. In NMOS technology, two critical electrical parameters are the depletion transistor drain current, Idd, and the enhancement transistor threshold voltage, Vie. Variations of + 15% of the depletion drain current and +5% of the enhancement transistor threshold voltage can dramatically affect wafer yield and device speed. In a stable VLSI manufacturing line, threshold adjust ion implants are one of the few process steps which at present cannot be adequately monitored until the end of the manufacturing cycle, typically 2 - 4 weeks after implantation. Techniques [1-4] for monitoring low dose implants at present all require the use of special test wafers to determine implant dose, and therefore are not of use in verifying that particular product wafers have received the desired implant dose. In particular, no means have been available to measure low dose implants ( 1 E l l - l E 1 3 ) on product wafers immediately after implantation to permit real time implant process monitoting. The ability to measure the implanted dose directly on product wafers in a nondestructive fashion is invaluable in a manufacturing environment, particularly when product or mask checkout lots are being run. This is where the thermal wave implant dose measurement technique [5] has a distinct advantage over existing dose measurement methods. *Presently at Cray Research, Inc., Chippewa Falls, WI 54729, USA. 0168-583X/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
The recently introduced commercial thermal wave measurement system [6] has been applied to measurement of ion implant doses [7-10], but as yet there have been no reports in the literature of direct correlation of thermal wave signal, measured just after implantation, to electrical test measurement results obtained on the finished product circuits. Since the ultimate task of the implant process engineer is to ensure good control of the end-of-line transistor characteristics, a true evaluation of this implant dose measurement technique must include a cross-correlation of thermal wave signal to dose and implanted transistor parameters. In this article we present data correlating thermal wave measurements to implant dose and electrical test parameters to verify that, indeed, this real time implant dose monitoring technique is truly useful as a tool for VLSI manufacturing cost reduction.
2. Product wafer sample preparation The objective of this experiment was to determine the feasibility of using thermal wave measurements to control critical transistor characteristics. The process vehicle used for the experiment was an NMOS technology using four ion implants. Because there is at present no other means to measure low doses on product wafers, we decided to characterize the two critical low dose implants: a 25 keV arsenic depletion implant and a 50 keV boron enhancement implant. Cross-sectional views of these regions of the device structure are depicted schematically in fig. 1. The ion implants were performed on Varian medium-current implanters with a tilt angle to dechannel the implanted ions. The implanted dose values are listed in table 1. IX. MEASUREMENT TOOLS
560
a ~...Es, J j
M.A. Wendman, W.L. Smith / Thermal wave imp~ant dosimetr)'
F,E o ox,o
"b ~ -
1.8
-
I
1.8
1.4
Fig. 1. Cross-sectional view of MOS transistor structures on product wafers showing the gate regions used for depletion (a) and enhancement (b) implant inspection measurements with the Therma-Probe system.
I
[
1.3
oh 1.2
~Ot#o
1.1
Table 1 Implant doses
1.0
~
Boron dose values (ions/cm 2)
Arsenic dose values (ions/cm2)
4.10Ell 4.70Ell 5.30Ell 5.90Ell 6.50Ell 7.10Ell 7.70Ell
1.55E12 1.70E12 L85E12 2.00E12 2.15E12 2.30E12 2.45E12
Twenty-four wafers were procured from a standard product manufacturing lot before arsenic depletion implant. After both depletion and enhancement implants were performed according to the standard manufacturing process flow, thermal wave measurements were made on a Therma-Probe 150 system. To comply with manuscript length constraints, we must refer the reader to other published works [5,10-12] for description of thermal wave measurement techniques used for ion implant dose inspection. For the depletion implant, five measurements were made per wafer at top, center, bottom, left and right locations. For the enhancement implant, three measurements per wafer were made at top, center and bottom locations. The wafers were then sent on to complete the standard manufacturing process flow which included electrical parameter testing of each of the 24 wafers on the same transistors as measured earlier with the ThermaProbe system. Five measurement sites on the wafer (top, center, bottom, left and right) were used in collecting the parametric data. No anomalies relating to thermal wave measurements were found. This measurement technique has proven to be entirely nondestructive.
3. R e s u l t s
Graphs of electrical parameters versus implant dose or thermal wave signal, and of thermal wave signal versus implant dose are shown in figs. 2-7. The graphs clearly show a strong correlation between implant doses and the corresponding thermal wave and electrical parameters. Linear regression calculations were made to
0.9 380.0
° 380.0
400.0
420.0
440.0
T h e r m a l Wave Signal ( TW units )
Fig. 2. Graph of enhancement threshold voltages measured on completed product wafers versus thermal wave signal measured after the enhancement low-dose implant.
extract the least squares best fit parameters of slope, intercept and correlation coefficient. These are summarized in table 2. The correlation coefficients for the depletion implant samples are quite good, with all except one greater than 0.95. The exception is the correlation coefficient of the depletion drain current as a function of thermal wave signal [Idd (TW)]. Between the Idd (TW) and the Vie(TW) data in table 2, there is a rather large discrepancy of approximately 0.10 in the correlation coefficient, with the enhancement implant having a correlation coefficient of no less than 0.98. Interestingly enough, such a discrepancy is not observed when comparing the respective data of implanted dose versus thermal wave signal [TW(DOSEe) and TW(DOSEd) in table 2], where both the enhancement and depletion implant data sets have best-fit correlation coefficients greater than 0.96. The fact that the TW versus dose data for the two implants have equally good correlation coefficients indicates that both implants have similar dose control. We expect that the difference in the correlation coefficients for the data on electrical parameter versus TW signal for the two implants arises from one main factor that is implant-process related. Whereas the enhancement implant is made through a silicon dioxide film (fig. 1) which greatly reduces ion channeling, the depletion implant is made into bare silicon. Despite the tilt angle employed in the ion implanter, the wafers may still suffer channeling related to the "twist" angle of the wafer. Since this twist angle was not controlled, the amount of channeling and thus the effective depth of the implant are affected in a random manner from wafer to wafer. This effect is enhanced by the low ion energy (25 keV) which increases the acceptance angle for channeling. Since the arsenic has a lower diffusion rate than boron, the diffusion caused by subsequent
M.A. Wendman, W.L. Smith
/
561
Thermal wave implant dosimet o, 460.0
Up3 A
I
c~ t~ 240
'
O
O
220
°~°
d
200
c .~
180
c
160
u
u
A
° ~ _~
440.0
~
~o
!
420,0
0
--
0
m
i5 400.0
E Pp-
380.0
nfl ~
140 o
--~
¢
D ~
120
80
H
360.0 550
570
590
610
5
T h e r m a l W a v e S i g n a l ( TW u n i t s )
7
6
8
I m p l a n t D o s e ( x l 0 1 1 ions / cm 2 )
Fig. 3. Graph of depletion drain current measured on completed product wafers versus thermal wave signal measured after the depletion low-dose implant.
1.6
A
|
1.5
Fig. 6. Graph of thermal wave signal measured after the enhancement low-dose implant versus the as-dialed implant dose.
610,0
I 8o
600,0
g 1.4
590.0
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~
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~
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560.0
i
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550,0
4.0
5.0
6.0
7,0
8.0
Implant D o s e ( x l O 11 ions / cm 2 )
15
1.7
1.9
2.1
2.3
2.5
I m p l a n t D o s e ( x l 0 TM i o n s / c m 2 )
Fig. 4. Graph of enhancement threshold voltage measured on completed product wafers versus as-dialed enhancement implant dose.
Fig. 7. Graph of thermal wave signal measured after the depletion low-dose implant versus as-dialed implant dose.
280.0
240.0
200.0
160.0 ©
0 120.0
80.0 1.5
1.7
1.9
2.1
2.3
2.5
Implant D o s e ( x l O TM ions / cm 2 )
Fig. 5. Graph of depletion drain current measured on completed product wafers versus as-dialed depletion implant dose.
processing will have little effect in masking the effects of channeling in the depletion implant. The end result of the deeper arsenic depth is an increase in the depletion transistor drain current, Idd. On the other hand, ion channeling causes a reduction in TW signal since less lattice damage is produced by a channeled ion. These opposing effects reduce the correlation between Idd and TW signal Finally, this same scatter in the Idd measurements should also adversely affect the Idd versus enhancement dose data. But because the as-dialed dose values are not correlated with channeling, the correlation coefficient between Idd and depletion dose can be expected (and is observed) to exceed that for Idd versus TW signal. IX. MEASUREMENT TOOLS
562
M.A. Wendman, W.L. Smith / Thermal wave implant dosimetr).
Table 2 Summary of least-squares-fit data Graph Y(X)
Fig.
Slope d Y/d X
Intercept Y( X = 0)
Correlation coefficient
Vt¢(TW) Idd(TW) Vtc(DOSEe) Idd(DOSF-AI) TW(DOSEe) TW(DOSEd)
2 3 4 5 6 7
0.007447 V/TW 2.984/~A/TW 0.1772 V/1Ell 160.9/~A/1E12 23.31 TW/1Ell 52.70 TW/1E12
- 1.808 V - 1560.0/~A 0.2287 V - 149.7/tA 276.6 TW 474.8 TW
0.9814 0.8843 0.9932 0.9532 0.9681 0.9597
4. Conclusions
References
To summarize the product wafer results described here, the thermal wave dose measurement technique has proven to be a very powerful diagnostic for real time, low dose measurements. A strong correlation of thermal wave signal to implanted dose and transistor electrical characteristics has been demonstrated. Indeed, it can be foreseen that thermal wave dose measurements will be introduced as a standard technique in a VLSI manufacturing line to tightly control the often troublesome low dose, threshold adjust implants, thus making these steps significantly easier to optimize and maintain.
[1] G.A. Gruber, Solid State Technol. 26 (1983) 159. [2] J.C. Cheng and G.R. Tripp, Solid State Technol. 26 (1983) 143. [3] H. Glawischnig, K. Hoerschelmann, W. Holtschmidt and W. Wenzig, Nucl. Instr. and Meth. 189 (1981) 265. [4] R.O. Deming and W.A. Keenan, Solid State Technol. 28 (1985) 163. [5] W.L. Smith, A. Rosencwaig and D. Willenborg, Appl. Phys. Lett. 47 (1985) 584. [6] Therma-Probe Systems, product of Therma-Wave, Inc., Fremont, CA. [7] W.L. Smith, R.A. Powell and J.D. Woodhouse, Proc. SPIE 530 (1985) 188. [8] W.L. Smith, M.W. Taylor and J. Schuur, Proc. SPIE 530 (1985) 201. [9] W.L. Smith, R.H. Reuss, W. Clark and D. Rensch, Materials Research Society Syrup. Proc. 45 (1985) 247. [10] W.L, Smith, A. Rosencwaig, D. Willenborg, J. Opsal and M.W. Taylor, Solid State Technol. 29 (1986) 85. [11] A. Rosencwaig, J, Opsal, W.L. Smith and D.L. Willenborg, Appl. Phys. Lett. 46 (1985) 1013. [12] J. Opsal and A. Rosencwaig, Appl. Phys. Lett. 47 (1985) 498.
We would like to thank L. Brooks of Intel for her help in coordinating the processing, M. Rocke of Intel for his help and advice over the length of the project, and M. Taylor of Therma-Wave for his help with the implant inspection measurements.