Nuclear Instruments and Methods in Physics Research B6 (1985) 219-223 North-Holland. Amsterdam 219

AUTOMATIC CASSETTE TO CASSE’ITE RADIANT IMPULSE PROCESSOR

Ronald E. SHEETS

Tamarack Scientific Co. Inc., 1040 North Armando Street, Anaheim, CA 92806, USA

Single wafer rapid annealing using high temperature isothermal processing has become increasingly poputar in recent years. In addition to annealing, this process is also being investigated for silicide formation, passivation, glass reflow and alloying. Regardless of the application, there is a strong necessity to automate in order to maintain process control, repeatability, cleanliness and throughput. These requirements have been carefully addressed during the design and development of the Model 180 Radiant Impulse Processor which is a totally automatic cassette to cassette wafer processing system. Process control and repeatability are maintained by a closed loop optical pyrometer system which maintains the wafer at the programmed temperature-time conditions. Programmed recipes containing up to 10 steps may be easily entered on the computer keyboard or loaded in from a recipe library stored on a standard S/4” floppy disk. Cold wall heating chamber construction, controlled environment (N2, A, forming gas) and quartz wafer carriers prevent contamination of the wafer during high temperature processing. Throughputs of 1.50-240 wafers per hour are achieved by quickly heating the wafer to temperature (450-1400°C) in 3-6 s with a high intensity, uniform (? 1%) radiant flux of 100 W/cm’, parallel wafer handling system and a wafer cool down stage.

Rapid thermal annealing has been evaluated and re- ported on in a number of technical publications and conference proceedings [l-3]. At the present time, it is not being utilized extensively as a fabrication tool due to process control repeatability problems and in- adequate production throughout. Rapid annealing wa- fers one at a time requires accurate measurement and control of the temperature of each wafer as it is pro- cessed. In addition, the source used to heat the wafer must provide a uniform heat flux in order to maintain the wafer at a true isothermal condition. From a pro- duction consideration, it is necessary to bring the wafer to the desired temperature rapidly, hold the wafer at temperature for a minimum amount of time, cool the wafer quickly and automatically transfer the annealed wafer out of the chamber and a new one into the chamber.

This paper describes the engineering aspects and test results of a new production oriented machine which utilizes integrating optical coupling, closed loop temp- erature control and an automatic cassette to cassette handling system to rapidly anneal 3” to 6” diameter wa- fers with a production rate of 150 to 240 wafers per hour.

2. Heat source

One of the most important aspects of a rapid “isothermal” annealing system is the generation and delivery of radiant energy to the wafer. First of all, the radiation must be generated efficiently and by this we

0168-583X/85/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

mean the conversion of efectrical energy into thermal radiation. From an efficiency standpoint, the tungsten- halogen lamp used in the Model 180 system converts 86% of the electrical input power into radiation. This lamp has been used in numerous commercial lighting and heating applications since it was developed in the 1950s and is very reliable.

Tungsten-halogen lamps, sometimes referred to as quartz lamps, consist of a tungsten filament which is resistively heated inside a sealed quartz envelope con- taining a small quantity of one of the halogen (bromine or iodine) elements. This element is usually bromine which is solid at room temperature, although when the envelope has reached its required operating tempera- ture of 300-400°C, it forms a gas between the tungsten filament and the quartz envelope. When tungsten evaporates from the filament, it combines with a bromine atom to form tungsten bromine which is a gas. These molecules eventually strike the hot filament and decompose with the tungsten depositing back on the filament and the bromine atoms become available to form another cycle. This allows the tungsten filament to operate at a much higher temperature without en- velope darkening. Degradation of radiant output for the rated 2000 h of life is only 7-10% of new lamp output.

A major difficulty of the tungsten-halogen lamp is in coupling its radiation to the wafer in a uniform man- ner. The common practice has been to place a bank of linear lamps in an array very close to each side of the wafer [4,5] and heat the wafer from both sides. These systems work reasonably well when the dimensions of the lamp array are two to three times larger than the diameter of the wafer being annealed, however; as the

IV. IMPLANTER COMPONENTS / END STATIONS

220 R. E. Sheets I Automatic radiant impulse processor

wafer diameter approaches the lamp array size, the re- sulting end effects produce non-uniform heating. Large area tungsten halogen lamp arrays which heat the wafer from one side only have been reported by Lischner and Celler [6]. This system can maintain uniform tempera- tures over a 7.5” x 5” area as reported and utilizes 100 kW of input power.

3. optical coupling

The Model 180 radiant impulse processor utilizes an optical coupling system which takes into consideration wafer reflectivity, transmission and radiation in order to provide a uniform heat flux on the wafer. Radiant energy from the tungsten-halogen lamps is coupled to the wafer by the optical system shown in fig. 1. The water cooled walls of the cavity are optically polished, vacuum coated with aluminum or gold and overcoated (sealed) with a few hundred angstroms of SiOZ. Lamps are arranged in a double row at the top end of the cavity at a distance of 28 cm from the wafer top surface. Multiple reflections of light from the cavity walls inte- grate the energy into a uniform flux (+ 1%) at the wafer surface. Below the wafer is a reflecting cavity which reflects and integrates the radiation which misses the wafer, or is transmitted through or radiated by a hot wafer back to the bottom surface of the wafer. Energy reflected, transmitted or radiated by the wafer from the top surface or bottom surface is returned to the wafer plane with a uniform flux because of the numerous reflections encountered in traveling to either end of the cavity and back. With the high reflectivity coatings discussed, this radiant coupling method is very efficient and requires less than 50% input power in order to hold a wafer a 1200°C. The entire cavity is sealed and purged with NZ, A, Or or forming gas to

TUNGSTEN HALOGEN LAMP BANK

QUARTZ WINDOW

OPTICAL PYROMETER

Fig. 1. Schematic illustration of the optical coupling method used to provide a 7” diameter beam of uniform (k 1%) radiant flux at an intensity of 100 W/cm*.

control the wafer environment. Oxidation runs are used to verify wafer temperature uniformity and are discussed in a following section.

4. Handling system

The Model 180 handling system shown in fig. 2, is designed for high throughput and continuous opera- tion. The wafer handling system is set up with staging locations so that wafers can be simultaneously cycled into and out of the heating chamber. Wafers removed from the heating chamber are allowed to cool for S-10 s before being transferred back into a cassette. Dual elevator systems on each side of the machine provide non-stop operation since an operator can change cas- settes without interrupting machine operation.

Wafers are supported on quartz carriers which are mounted on the three station rotary turntable. These carriers are fabricated from a 2.5 mm diameter quartz rod formed into a circle with an i.d. equal to the wafer diameter plus 6 mm. Small 2 mm diameter rods are formed and fused to this rod in five places to center and support the wafer. The entire assembly is very low in cross section and does not perturb the uniform radiant flux field inside the chamber.

Throughput of the system is a function of the an- nealing cycle and can be calculated according to the following formula

3600 Throughput = (T, + Th + T, + Tc) wafersh,

where If, = transfer time to move wafers, T,, = time to heat to annealing temperature, T, = time to anneal at temperature, T, = time to cool to 700°C before the chamber is opened.

Example: T,,, = 3 s, T,, = 5 s (to heat to 1200°C), T, = 10 s, T, = 5 s:

RadIani He.tlna Cha,,,ber I

ol EL Unload StaHon

Fig. 2. Schematic illustration of the dual cassette handling system which simultaneously removes an annealed wafer and inserts a new wafer into the machine in 3 s.

R. E. Sheets I Automatic radiant impulse processor 221

3600 Throughput = (3 + 5 + 1O + 5) = 156 wafers/h.

5. Temperature control

Wafer temperature is controlled by a closed loop system consisting of a computer and an optical pyrome- ter. The optical pyrometer is filtered to h > 4.5 urn so that it can see only radiation from the wafer and not be influenced by radiation from the tungsten-halogen lamps. The pyrometer system is calibrated against a type K (chromel-alumel) thermocouple which is bonded into a silicon wafer. A calibration program steps the wafer in 50°C increments from 450 to 1350°C. The program holds the temperature at each point for 30 s to insure that the thermocouple has stabilized be- fore it is read. The resulting data is used by the compu- ter in order to set and control the temperature during automatic operation.

Programmed temperature-time recipes may be en- tered on the computer keyboard or loaded in from a recipe library stored on standard 5V4” floppy disks. Trapezoidal temperature-time recipes with tempera- ture programmable from 450°C to 1400°C and time programmable from 1 s (spike) to 999 s. Typical 12OO”C, 3 s recipe is shown in fig. 3. Complex recipes containing as many as ten temperature steps may be easily programmed into the computer.

6. Temperature uniformity tests

One of the major concerns in rapid annealing is ver- ifying that a uniform temperature profile has been achieved on the wafer during processing. Current and

MELTING

TIME (SECONDS)

Fig. 3. Temperature-time diagram for a 12OO”C, 3 s anneal cycle.

Yee [7] found that 40 keV B+ at a dose of 6 x lOI5 was very sensitive to temperature gradients when annealed in a double-sided tungsten-halogen rapid annealer. They also found that by increasing the time to 60 s they could improve the sheet resistance uniformity at the price of increased diffusion and an increase in total processing time. Their data points out that if the anneal time is increased, the effect of non-uniform wafer temperature is masked, or in effect sheet resistivity is not a good measure of wafer temperature uniformity. Lischner and Celler (61 have used oxidation runs in the 100 kW RTA apparatus in order to verify the wafer temperature uniformity. Here, the wafer is inserted into the rapid annealing system with a controlled at- mosphere of dry oxygen and heated to 1100°C for 30 min. At this temperature, the oxide will grow to a thickness of 850 A. A temperature variation of 10°C on the wafer will result in a 50 A change in the oxide thickness. By measuring the oxide thickness at a number of locations on the wafer, the effective temper- ature can be calculated.

On the Model 180 system, oxidation runs were made at temperatures from 1100°C to 1325°C for durations of 10-30 min. Data is presented for a 1325°C 10 min oxidation in fig. 4. Oxidation thickness was measured at seven points in the horizontal and vertical axis of the 100 mm diameter wafer. Wafer temperature as a func- tion of oxide thickness was calculated and is shown plotted in fig. 5. These data show that the wafer temp- erature is very uniform over the entire wafer. Temper- ature at the edge of the wafer was only 5°C lower than the highest point on the wafer indicating that the inte- grating optical cavity maintains a uniform circulating radiant flux which couples radiant energy into the edge of the wafer as well as the planar surfaces.

Fig. 4. Oxidation run on a 100 mm diameter silicon wafer. Measured oxidation thickness at the center of the wafer, 4 points at 16 mm radius, 4 points at 32 mm radius and 4 points at 47 mm radius.

IV. IMPLANTER COMPONENTS / END STATIONS

222 R. E. Sheets I Automatic radiant impulse processor

OXIMTION RUN IN DRY 02 llT 1325 DE6 C

FOR 10 IIIWTES

MY TEMPA327 C

-8 ““Iti TEMPA322 C*

-59 -25DIllt&R-MM 25

50

TEMPERllTURE FN OXIDE THICXNESS

Fig. 5. Wafer temperature profile based on the oxidation thickness presented in fig. 1.

7. Rapid annealing tests

Rapid annealing tests were performed with the Model 180 system on arsenic implanted 100 mm diame- ter wafers at medium dosage of 10” and high dosage of 1016. Sheet resistance values and standard deviation is given in table 1. Sheet resistance values for medium

Table 1 Sheet resistance of arsenic implanted 100 mm diameter wafers

Wafer Beam Dose Temp. Time Sheet Std. energy (cme2) (“C) (s) resistance dev.

(keV) (Q/O) (%)

#51 100 lOI 1000 1 95.17 #53 100 10’5 1000 95.46 x54 100 10’5 1000 1; 94.08

#23 40 10’6 1178 5 18.22 1.11 #22 40 10’6 1178 10 15.42 1.05 #03 40 10’6 1200 5 17.59 0.95

0.90 1.98 1.17

Fig. 6. Sheet resistivity map of a 100 mm diameter silicon wafer implanted with As+, 100 keV, 1015 and rapidly annealed at 1ooo”C for 1 s. Average sheet resistance = 95.17 Q/O, standard deviation = 0.90%.

dosage (10”) arsenic did not vary appreciably for an- neal times from 1 to 10 s at 1000°C. A map of the sheet resistance is shown in fig. 6. The high dosage arsenic (1016) was very sensitive to the temperature-time re- cipe. Basically, it was necessary to anneal at tempera- tures above 1150°C in order to activate the implanted ions. The best results were achieved with a rapid anneal cycle of 1200°C for 5 s, the sheet resistance map is shown in fig. 7.

Rapid annealing tests were also performed on 40 keV, 5 x 10” B implanted 100 mm diameter wafers. Annealing time was varied from 2 s to 60 s at 1150°C results are listed in table 2. Sheet resistance was mea- sured at 22.66 Q/O for the 2 s rapid anneal. Standard deviation for the 2 s anneal was 0.49%. Sheet resistance for the 10 s anneal cycle decreased 2.29% from the value at 2 s, with a standard deviation of 0.26%, see sheet resistance map in fig. 8.

Fig. 7. Sheet resistivity map of a 100 mm diameter silicon wafer implanted with AS+, 40 keV, lOI6 and rapidly annealed at 1200°C for 5 s. Average sheet resistance = 17.59 Q/U, standard deviation = 0.95%.

Table 2 Sheet resistance of boron implanted 100 mm diameter wafers

Wafer Beam Dose Temp. Time Sheet Std. energy (cmm2) (“C) (s) resistance dev.

(keV) (Q/O) (%)

#04 40 5 x 1o’j 1150 2 22.66 0.49 #05 40 5 x lOI 1150 10 22.14 0.26 #06 40 5 x 1ol5 1150 40 22.02 0.40 #07 40 5x10’5 1150 60 22.27 0.32

RX. Sheets i Automatic radian? impulse processor 223

Fig. 8. Sheet resistivity map of a 100 mm diameter silicon wafer implanted with B, 40 keV, 5 x 10” and rapidly annealed at 1150°C for 10 s. Average sheet resistance = 22.14 Sk/O, standard deviation = 0.26%.

8. Summary

The Model 180 system (fig. 9) provides rapid ther- mal annealing of ion implanted silicon wafers on a pro- duction basis. Uniform radiant heating of the wafer has been verified by using high temperature oxidation runs and measuring the resultant oxide thickness at multiple locations on the wafer. Closed loop temperature con- trol with programmable recipes provide the process control repeatability necessary to make rapid thermal annealing a viable production method.

References

[1] J.F. Gibbons, L.D. Hess and T.W. Sigmond, eds., Laser and electron beam-solid interactions, (North-Holland, Amsterdam, 1981).

Fig. 9. Photograph of the Model 180 Radiant Impulse Proces- sor system.

B.K. Appleton and G.K. Celler, eds., Laser and electron interactions with solids (North-Holland, Amsterdam, 1982). J. Narayan, W.L. Brown and R.A. Lemons, eds., Laser- solid interactions and transient thermal processing of materials (North-Holland, Amsterdam, 1983). K. Nishiyama, M. Arai and N. Watanabe, Jap. J. Appl. Phys. 19 (1980) L563. M. Kuzuhara, H. Kohzu and Y. Takayama, Appt. Phys. Lett. 41 (1982) 7.55. D.J. Lischner and G.K. Celler, in ref. [2], p. 759. M. Current and A. Yee, Sol. Stat. Technol. (October 19831.

IV. IMPLA~R COMPONENTS I END STATIONS