Techniques for dose matching between ion implanters

Nuclear

Instruments

and Methods

in Physics

Research

243

B55 (1991) 243-249

North-Holland

Techniques P. Lundquist

for dose matching

between

ion implanters

and S. Mehta

Varian Ion Implant Systems, Gloucester, MA 01930, USA

T. Black and D. Jackson Intel/Fab

6 5000, W. Chandler Blvd., Chandler, AZ 85226, USA

Dose matching between systems installed in a device manufacturer’s fab is a critical issue in ion implantation, especially when the process is to be transferred from one system to another. Ion implantation systems, like other types of semiconductor capital equipment, are constantly evolving to keep pace with the ever-increasing demands of device manufacturers. Some hardware changes may be retrofitted to existing equipment, while major changes often result in a new system design. As a result older, retrofitted systems as well as the latest generation equipment may be found operating side by side in the wafer fab. While dose uniformity and repeatability for a given implanter have always been specified, dose matching between implanters, allowing the transfer of process from one system to another, is addressed in this paper. Dose matching characteristics for several Varian implanters will be presented and techniques for matching dose between systems will be discussed. In addition, the key hardware and process issues that may cause mismatching in dose will be addressed.

1. Introduction Every implant used in a semiconductor fab is described by a recipe. The recipe specifies: species, dose, energy, beam current, wafer tilt angle, and several other parameters for implant setup. Frequently, when a process is developed in a new fab, the implant recipes are determined by qualification exercises done on the new ion implanters. In contrast, when an existing fab upgrades or purchases new equipment, the goal is for the new or retrofitted equipment to run the established recipes, within the tight control limits of the process, already developed on existing equipment. The goal is driven by the need to bring the equipment on line quickly with the minimum cost, and to avoid getting approvals for recipe changes. The absolute dose has not been as important as dose matching to the existing process. N.B.S. standards are not available for an implanted dose. Implant round robins done throughout the industry for a given recipe. indicate there is approximately a 12% variation of sheet resistance due to the implanters when the wafers are all processed together [l-3]. Theoretical models must consider a large number of variables to be able to predict for all recipes, what the sheet resistance should be. Some of the difficult variables are wafer charging, beam neutralization, and onset of wafer amorphization coupled with its change on channeling effects. Hence, one should be cautious about using theoretical models for a target sheet resistance value [3]. A reference value to consider is the baseline implant done during the instal0168-583X/91/$03.50

0 1991 - Elsevier Science Publishers

lation of an implanter. The ion implanter manufacturer can provide these recipes which have been tested on many ion implanters. Typically, an ion implanter has control capability that allows the user to increase or decrease all of the doses by the same amount for dose matching to the existing process recipes of another ion implanter. On Varian serial ion implanters control of the implant area setting provides this control capability. The implant area setting is a number entered into the dose calculation electronics. The implant area is the effective area (square centimeters) at the wafer plane, which is scanned by the ion beam. For a given wafer size, increasing the area setting will increase all the implanted doses. This paper discusses considerations to be made before adjusting the area setting. The steps for selecting the area setting for matching the dose on two ion implanters are presented along with the data from two production implanters.

2. Variables affecting dose In dose matching two ion implanters, the average value of the dose (ions/cm’) implanted into the wafer, the uniformity of the dose across the wafer, and the depth profile must match, for the measurement of wafer dose to match. Dose matching must be checked for each recipe that is used in production. Product device data is the ultimate judge that the dose matching is successful

B.V. (North-Holland)

III. THROUGHPUT

& YIELD

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when ‘botl~ implanted procbts are within the process control limits. Fig. la is 8 fishbone (Istik~~~~) diagram [4] of seria1 implanter variables which can cause the “measurement of wafer dose“ to change. Six main categories are lisied. The br2m&es off of the “implant recipe” are the recipe items, any one of which, if changed, could cause a, change in the measurement of wafer dose (average value, uniformity, or depth profile). Many of the fishbone items are not called out in a recipe. For exarrnpk there is a branch off of the “dcshetry system” cafegnfy d&d “curred &eg~~d’~ It I~asbranches comxcting to it. One branch is “passes calibration check” for ‘“dc currents” and “pulsed currents”, which, if out of calibration, will cause the measurement of wafer dose to change. If there is only one current range that is bad, then dose matc~ng between two implantem might be successful except for hxplants on &at cnrrent range. Thus, the calibration of modules which ccrdd affect dose must be verified to be correct before startLug dose matching. Newer implanters use software routines to

n~.Ator power sqqdks and check rrxzny calibrations, to warn of implanter problems. A different type of dose matching problem is not due to an implanter failure but to implanter design. The “pfaten fl&xzss’” is ty@za& F&d ark an implanter. If a fab has two implantem, one with a flat platen, and the othex with a domed cooling platen then the “ beasn incident angle to tha wafer crystal” structure will be different between the two implanters. This according to the fishbone diagram could cause ‘“channeling effects OS implant depth profiles”, which could show up oa four-point probe maps as an unusual sheet resistance value with a poorer uniformity map [5]. This wonld show up less or not at all for high dose, high energy As’ implants that amorphize the crystal lattice and have minimal channeling. This small difference in implanter ~nfig~atio~ could cause the closes on the two ~~~a~te~ to match for some recipes h*t not for all. For a particular i,m@nter design, Qne must look at the fishbone diagram and determine the variables that apply and their corresponding effects. If variables are

246

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added, make sure that they are well defined and measurable. A dose matching checklist can then be made from the customized fishbone diagram. If a variable is

not controlled, and dose matching is performed, when that variable changes (which one may not be aware of) the implanter may no longer be dose-matched to the

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P. Lundquist

247

et al. / Dose matching between ion implanters

own control chart like fig. 3. Once the implanters are matched, the monitor can be periodically implanted as a quick partial implanter check. Typical mo~to~g process is to use an anneal, etch, and 4-point probe, or to use a thermal-wave measurement.

other implanter or the process. Fig. 2 is the layout of a 300 XP serial implanter for reference in locating implanter main assemblies mentioned in the fishbone. Fig. lb is a fishbone diagram of process variables which can cause the “measurement of wafer dose” to change. Because of all the variables in processing the wafers, it is necessary to split a “lot” of wafers, which meet the wafer requirements through both implanters. Combine the wafers after implant. Randomize the wafers for the anneal, in order to separate the anneal effects from the implanter effects 161.Measure the wafer dose on the same system. A checklist should also be made for each processing step. In addition to the recipes that must be matched, it is valuable to develop at least one dose matching monitor. The monitor can be the same as one of the recipes. The monitor should have the following qualities. It should have a quick turn around time of 4 hours or less, compared to a few weeks for device wafers. It must be sensitive to the dose so that it can pinpoint small dose changes. Its measured value must be stable over time, so that the time after implant that the measurement is made does not affect the result [7]. It should have its

3. Dose matching steps and results The steps for dose matching implanters will now be presented along with the data from two production implanters. Step I. Consider the implanter variables due to calibration, recipe requirements, and implanter config~ation. Go through the checklist and fix any problems. Dose matching was done between a new 300XP serial implanter, and a 350D serial implanter retrofitted with a new Faraday system [8], similar to the one used on the 300XP. The retrofitted 350D is called a 350DE. Before the retrofit was done the 350D and 300XP had been dose-matched. The variables for the 350DE and 300XP implanters were checked. The significant dif-

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dose matching monitor wafers on both implanters. On the implanter already process qualified, implant the monitors with the production area setting. On the other implanter use three different areas, the calculated area provided by the implanter manufacturer, an area between 2 and 4% greater than, and an area 2 to 4% less than the calculated area. Implant 3 monitor wafers for each area setting. After this test is successfully done, it can be repeated with other monitor implants if desired. Twelve dose matching monitors were implanted, and measured on a thermal wave system. The 300XP used its production qualified area of 212.0 cm2. Three areas were implanted with on the 350DE: 200.5, 204.5, and 207.5 cm’. Fig. 4 is a graph of 350DE area setting vs. thermal wave signal. From it one concludes that an area of 207.5 cm’ initially matches the 350DE to the 300XP. Step 3. Implant a split lot of product wafers on both implanters. On the implanter already process qualified, use the production area setting. On the other implanter use three different areas, the dose matching area found in step 2, an area between 2 and 4% greater than, and an area 2 to 4% less than the dose matching area found in step 2. Implant 3 product wafers for each area setting. This test was performed with microprocessor product wafers. E-test results in fig. 5 show that device threshold voltage measurements demonstrate that the area of 207.5 cm2 matches the 350DE to the 300XP. E-test results in fig. 6 show that a sensitive source to drain depletion current measurement demonstrates that the correct dose matching area is 207.5 cm2. Step 4. Repeat step 3 with a larger quantity of wafers to gain more confidence in the measurement [6]. Referring back to fig. 3, it shows the stability of the threshold voltage measurement over time for the 350DE, the 3OOXP, and another implanter on site, the 350D. Step 5. Repeat step 3 and step 4 for each recipe. This can be done over the course of a few weeks, so that as time passes more and more recipes are production-qualified on the implanter. The implant area of 207.5 cm2 was used on the 350DE to match it to the 300XP for the other device implants. All the implants matched the 350DE with an area of 207.5 cm2 to the 300XP with an area of 212.0 cm2.

249

Since the thermal wave measurement agreed with the product wafer device measurements that the area of 207.5 cm2 is correct, the thermal wave measurement is used as a daily monitor.

4. Conclusions Dose matching between two implanters must be carefully done taking into account implanter variables, wafer processing, and dose measurement. After the implanter is configured as needed, and passes its checklist, then on serial implanters use a sensitive, stable monitor and we suggest to adjust the implant area to dose-match the implanters. All recipes used must be checked before turning over to full production. In the event that two different model implanters do not match for all doses, then we would suggest to match them for the one with the tightest control specs. If some of the other recipes fall out of spec, then that recipe dose will have to be changed unless the implanter configuration can be changed to bring that recipe back into spec.

Acknowledgements The authors acknowledge done on the serial implanters for work on the figures.

A. Dranchak for work at Intel, and P. Mansfield

References [l] H. Glawischnig

[2] [3]

[4] [5]

[6] [7] [8]

and G. Lang, Proc. 7th Int. Conf. on Ion Implantation Technology, Nucl. Instr. and Meth. B37/38 (1989) 624. L. Larson, Proc. 7th Int. Conf. on Ion Implantation Technology, Nucl. I&r. and Meth. B37/38 (1989) 628. L.A. Larson and G.L. Kennedy, Proc. 6th Int. Conf. on Ion Implantation Technology, Nucl. Instr. and Meth. B21 (1987) 421. K. lshikawa, Guide to Quality Control (Asian Productivity Organization, Japan, 1988) p. 18. M.I. Current, N.L. Turner, T.C. Smith and D. Crane, Proc. 5th Int. Conf. on Ion Implantation Equipment and Techniques, Nucl. Instr. and Meth. B6 (1985) 336. A.R. Alvarez, D.J. Welter, and M. Johnson, Solid State Technol. 26(7) (1983) 127. J.T.C. Chen, Proc. 6th Int. Conf. on Ion Implantation Technology, Nucl. Instr. and Meth. B21 (1987) 526. P. Lundquist, C. McKenna, R. Brick and P. Corey, Proc. 6th Int. Conf. on Ion Implantation Technology, Nucl. Instr. and Meth. B21 (1987) 414.

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