Improved gauge capability for ion implant monitors using temperature compensation for resistivity measurements

Section V Process control and yield

Nuclear Instruments and Methods in Physics Research B74 (1993) 229-233 North-Holland

NOMB

Beam interactions with Materials&Atoms

Improved gauge capability for ion implant monitors using temperature compensation for resistivity measurements Walter H. Johnson, Chester L. Mallory and W. Andrew Keenan Prometrti Corp., 3255 Scott Blvd., Santa Clara, CA 95054, USA

Dennis Karnenitsa Eaton Corp., 2433 Rutland Drive, Austin, 7x 78758, USA

Recent gauge ~pabil~~ studies for ion implant monitors have shown that the ambient tempera~re variation in the fab is a large contributor to the lack of reproducibility in the sheet resistance measurement. The temperature variation in many fabs can be as large as 4-8°C. For films with a temperature coefficient of resistance (TCR) of O.S%/“C this causes a 3.2 to 6.4% variation in sheet resistance. TCR values are well documented for National Institute of Science and Technology (NIST) Standard Reference Material (SRM) and accurate temperature measurement allows for easy correction of measured sheet resistance to the nominal temperature of 23”C, at which the standards are certified. Unfort~ately, the TCR values of most implant films have not been documented so it has not been possible to correct sheet resistance readings for variations in measurement temperature. An automated apparatus has been developed to measure the TCR of implanted layers in the temperature range of 15 to 35°C. The variation of sheet resistance with temperature can be considered linear over this range and these TCRs can be used to correct subsequent R, measurements to a nominal reference temperature (usually 23°C). With ever tightening process capability and gauge capability requirements the use of temperature correction will often mean the difference between a process or gauge that meets requirements and one that does not. The TCRs are also important in understanding how temperature variations will effect device performance where resistance is important. Examples of sheet resistance versus temperature and TCRs are presented for some typical ion implants. Results of a gauge study repeated on the same set of wafers reported on at IIT’90, using this apparatus are also reported.

1. Introduction

Many process engineers tend to b&eve their faciiities manager when they are told that the total temperature variation in the fab is only a few degrees Celsius. Not only does the temperature vary 4-8°C from location to location in some fabs but the temperature variation at a single tester may be as much as 4-8°C. We have recently learned of a world class fab where the pedometer was located in the ha11 adjacent to the process area. This did not cause a problem until construction was started on a new line across the hall which resulted in the exterior doors being constantly opened and closed. Even in ideal conditions the fab thermometers are seldom located in areas where hightemperature intensive process equipment like fm3aces, reactors and crystal pullers are located. There are exceptions such as SEMATFXH where the temperature is controlled to within fractions of a degree of the set point for that zone. This is a good start but needs 0168-583X/93/$06.00

to be expanded to having a common set point for the entire fab then a common reference temperature for all fabs. Until this can be done temperature variations need to be tracked and compensated for, The specifications on cross wafer variation (or uniformity) and the wafer to wafer variation are becoming more critical parameters to both users and vendors of process equipment. Because the temperature coefficient of resistivity (TCR) generally increases with increasing resistivity it can consume a considerable amount of the wafer specification, for low dose implants if not accounted for properly.

2. Test method

In order to accurately measure the sheet resistance uniformity of a wafer one should use the dual configuration technique developed by Dr. David Perloff [l].

0 1993 - Elsevier Science Publishers B.V. All rights reserved

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WH. Johnson et al. / Improved gauge capability for monitors

230

Silicon Wafer

I

w

stage

ResMivitV(ohmcmk

I

This technique involves making four point probe measnrements for each ~n~~rat~on and calculating the geometric correction factor from the ratio of the two readings. This factor is then used to correct for varia-

to the variations in sheet resistance me~urements across a wafer. Fig. 1 [31 plots the TCR values for nand p-type bulk single crystal silicon. This table shows the TCR for a 10 Q cm wafer (for example) is O.g%/‘C which would introduce a 3.2 to 6.4% error in sheet resistance or resist&&y with a 4-8°C temperature variation. MST standards are often used to establish the accuracy of the resistivity tester. The most difficult part of this method has been accurately measuring and correcting for the wafer temperature. Once accurate sheet resistance and temperature measurements have been made the appropriate TCR can be used to calculate the sheet resistance at 23°C. These corrections can be done for bulk silicon but unfortunately the TCR may not be known when dealing with ion implanted layers. An attempt has been made to develop a relatively easy to use system for rapidly determining TCR values. The system developed uses a heated stage to ramp the temperature of the wafer. Sheet resistance CR,) measurements are made at a series of increasing temperatures. The temperature (T) is measured to O.l”C precision. The system automatically plots the variation of

tions in probe spacing and any offset due to current crowding as the probe deviates from the center of the wafer. Dr. James Ehrstein of NIST has demonstrated that the dual ~n~~ration (or ~n~~ration s~tc~n~ technique provides more accurate readings than the single configuration measurement, which have been corrected using the ASTM F84 geometric correction factor tables [2,3]. As semiconductor device manufacturers attempt to effectively use more of the available wafer real estate they are specifying process unifo~i~ closer to the wafer edge. In is imperative then to use dual configuration to monitor the sheet resistance of wafers and processes in order not to introduce errors near the wafer edge. The use of a 25 mil spacing probe is also recommended [4]. It is also impo~ant to have good ohmic contacts. This is commonly verified by a probe qualification program, where a standard deviation of less than 0.2% is often used as the qualification limit [5]. This qualification indicates the contribution of probe contact noise

27

27.5

t SENSOR

Fi. 2. The method of temperature measurement is a thin film resistor which is in an opposing position to the four point probe measurement. A resistive heater surrounds the sensor for the development of the TCR curve.

Fig. 1. The temperature coefficient of resistivity (TCR) varies with resistivity and carrier type in single crystal silicon.

26.5

I

28

28.5

29 DEGREES

28.5

Jo

30.5

31

31.5

32

CELCIUS

Fig. 3. Sheet resistance versus temperature readings for a 120 keV phosphorus implanted wafer with 5 X 1012 ions/cm*, the described apparatus, which are in turn used to calculate the TCR value.

taken on

PKH. Johnson et al. / Improved gauge capability for monitors

231

Table 1 The results of a gauge capability study performed on the RS5S/tc system using 120 keV phosphorns implanted wafer with 5 x 1012 ions/cm’ (TCR = 0.0041) Operator 1 a

u-2 u-3 u-4 w-5 U-6 u-7 u-9 Average

Operator 2 a

Operator 3 a

1

2

3

Diff.

1

2

3

Diff.

1

2

3

Diff.

3102 3244 3793 3739 3207 3494 3555 ~3440

3107 3241 3799 3747 3217 3495 3557

3107 3238 3799 3741 3219 3492 3551

5 6 6 8 12 3 6

3105 3235 3797 3735 3221 3491 3554

3104 3231 3794 3745 3215 3492 3557

3105 3246 3803 3735 3210 3497 3556

3108 3237 3798 3736 3209 3491 3550

3105 3241 3800 3745 3215 3499. 3542

3099 3231 3798 3738 3223 3486 3546

9 10 2 9 14 13 8

3446

3444

8

3443

3443

3445

1 15 9 10 11 6 3 ----8

3443

3446

3442

9

Grand average 1 = 3443

Grand average 2 = 3444

Grand average 3 = 3444

Grand average range = 8 Xbar diff. = 1 Target = 3400 TT=398

%E.V. = 6.16% %A.V. = 1.22% %R&R = 6.31%

E.V = 24.65 A.V. = 4.848 R&R = 25.13

a Trial numbers 1.2 and 3.

sheet resistance culates

as a function

of temperature

and cal-

the TCR.

TCR = sIope,‘R,@23”C, TCF = 1 - (TCR( T, - 23)), where R&J23 is the sheet resistance at 23°C and T, is the measured temperature of the wafer. The temperature and R, data values are stored along with the TCR. The TCR, from the appropriate process, and the measured wafer temperature (T,) are then used to calculate a temperature correction factd (TCF) that can be automatically applied to tlie wafer. This allows all data to be referenced to a standard temperature. The system can accommodate two 25

25.5 deg C

23.1 deg C 3382.4 Ohms&q

3435.3 ohmsiq

4.13 % S.d.

4.1% QiaS.d.

Fig. 4. Repeated measurements made on the same wafer at different temperatures, showing the difference in mean but no change in the wafer uniformity.

3,100

0

0

2

3

f

I

3,ooo f

1

Opefa~of

Fig. 5. Repeated measurements

4

1

5

5

Operator 2

7

8

s

10

11

Operator 3

by different operators on a set of 8 wafers implanted with 5 X 101’ ions/cm2 120 keV.

of phosphorus at

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KH. Johnson et al. / Improvedgauge capabilityfor monitors

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.6

0.9

1

PfT Ratio

Fig. 6. Process tolerance ratios for different testing procedures, includ~g hand loading the wafer, using a robotic loader, meas~ing wafer temperature with a hand held thermocouple and automaticafly measuring temperature on the R&55,%.

wafer boats. Trend charts and SQC charts are immediately available, with any out of specs or out of control wafer indicated. Many wafer vendors have exotic home made systems for measuring the sheet resistance and the wafer temperature. But these systems are slow and cumbersome with little or no automation. They are used to correct for the TCR at the measurement temperature but because of the inherent in~nve~ence they are seldom used to check the TCR of the wafer under test. Fig. 2 shows a block diagram of the system used in this study. Temperature can be raised in a fraction of a degree increments to aid in the development of the TCR curve. The temperature is measured, using an embedded thin film sensor, along with the sheet resistance. The accuracy of the temperature monitoring system is 0.2% between 15 and 35°C.

reported that after reducing placement and probe contact errors, temperature vacations were the greatest contributor to sheet resistance variations. This wafer set was remeasured using the above described temperature measurement and compensation scheme and the results are presented in table 1. Fig. 4 shows the sheet resistance contour maps for one wafer taken at different temperatures, illustrating that the difference in the wafer mean is not caused by spurious measurements but a consistent mechanism acting on the entire wafer. Fig. 5 is a graphic presentation of the data listed in the more traditional table form (table 1). It can be clearly seen that the variation between repeated measurements utilizing temperat~e compensation is very small relative to the variation in the process. Repeated measurements of wafers U-2 and U-4 have very Iittle variation but there is a large variation between the two wafer means.

3. Results Fig. 3 shows the sheet resistance versus temperature curve for a wafer, with a 5 X 101’ phosphorus implant at 120 keV, measured with this system. The TCR in this temperature range is 0.0041/Y!. This TCR value was then applied to repeated measurements made on a set of implant wafers in a duplication of an earlier gage study [55]. By el~inating the variation of resist&y caused by temperature, the gauge capability of the system is significantly improved. By improving the gauge capability of the monitoring system the process capability (Cpk) is improved by eliminating a significant source of variation. In a paper given at the last Ion Implant Technology Conference (IITW), data was reported on gauge studies done on several implants, with a 5 x 1012, 120 keV phosphorus implant proving the most difficult. It was

4. Summary The P/T or the precision to tolerance ratio calculated in table 1 is shown in fig. 6 along with the P/T ratios determined in the original study for various test methods. The P/T ratio for this system the RS55/tc, improved on the results obtained by hand measuring the temperature with an external thermocouple meter and correcting for the variations. The TCR value used in the original study I.51was 0.0044 based on thermocouple readings which showed much greater noise then the me~~ements reported here. The addition of an accurate automatic wafer temperature sensor permits the application of temperature compensation during unattended operation.

NH. Johnson et al. / Improved gauge capability for monitors 5.

Conclusion

It has been shown that utilizing temperature correction techniques for monitoring ion implants with a four point probe can significantly reduce measurement error. This will in turn improve gauge capability results and allow less influence from the measurement tool on the process capability (Cpk) values. The standardization of temperature compensated sheet resistance measurements will allow for more consistent process or information transfers between manufacturing facilities or laboratories. Much work still needs to be conducted to determine the TCR values for the broad range of implant parameters used in the semiconductor industry.

233

References 111D.S. Perloff, J.N. Gan and F.E. Whal, Solid State Techn. (1981) 112. 121SEMATECH Workshop on Silicon for Mega-IC Applications, report (1991). F-84-73, Annual Book of ASTM Standards, vol. 14.02. [41Prometrix Resistivity Applications note No. RS-16. [51W.H. Johnson, W.A. Keenan and T. Wetterroth, Nucl. Instr. and Meth. B55 (1991) 148. [31Designation

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