Difficulties associated with temperature measurements in RTP

In an RTP unit, the wafer temperature can be measured by a TC mounted onto the wafer and by a calibrated pyrometer. The pyrometer calibration procedure also requires mounting of a TC onto a test wafer with optical properties identical to those of the wafers to be processed.

Both methods can yield erroneous temperature measurements if not applied correctly. As a basic requirement for TC measurements, the TC temperature has to coincide with the wafer

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∆ T=100°C

TC facing...

UV lamps, UV off UV lamps, UV on THL, UV off THL, UV on

Process time [s]

TC m ea su re m en t [ °C ]

Fig. 2.3: Temperature of a TC embedded in the middle of a Si wafer upon subjection to the standard open-loop process with a THL power plateau of 73 %. The TC either faced the UV lamps or the THL bank. The UV lamps were switched on optionally.

temperature. Though trivial, this requirement is often not fulfilled as a TC actually measures its own temperature. The basic requirement for correct pyrometer measurements is that the pyrometer signal is a monotonous function of the radiation emitted by the wafer. Again, this prerequisite is often not fulfilled. In the following, two examples are presented where TC and pyrometer measurements fail in the determination of the absolute wafer temperature.

TC measurement

A type K TC was embedded with ceramic glue in the middle of a virgin Si wafer featuring optically identical mirror-polished surfaces. The wafer was subjected to the standard open-loop process with a 120 s plateau of 73 % of the THL power. In one case the TC faced the UV lamps above and in the other case the wafer was turned over, now with the TC facing the THL bank. For both arrangements two runs were performed, first with the UV lamps switched off, and subsequently with the UV lamps switched on to 100 %.

According to Fig. 2.3 the influence of the UV lamps is very small. This confirms the results shown in Tab. 2.1. The slightly higher temperature is due rather to heating of the surrounding chamber during the first run than to the UV irradiation. However, when the TC faces the THL bank and is directly subjected to its radiation, a much higher temperature is measured. The difference with the non-irradiated case is as much as 100 C. The actual wafer temperature could not possibly increase in the same way because the two wafer surfaces were optically

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Process time [s]

uncoated Spin-on P507 Spin-on P508 APCVD 20% P

P yr om et er o ut pu t [ a. u. ]

Fig. 2.4: Pyrometer measurement of the emission of Si surfaces coated with different P dopant films upon heating with the same open-loop process.

identical apart from the tiny spot where the TC was mounted. In addition, it was shown earlier that the run-to-run repeatability of the open-loop process is better than 3 C.

The result clearly demonstrates that a TC measures its own temperature which, in non-equilibrium conditions such as the ones present in a field of irradiation, does not have to coincide with the wafer temperature, because their emissivities are different. The experiment also shows that the measurement of the absolute temperature of a wafer with a TC in an RTP unit requires much effort and is hardly achieved. Naive measurements with a TC exposed to the irradiation definitely leads to erroneous results. Unfortunately, at least two publications (see [165] and [109]) claiming the enhanced P diffusion, are based on evidently wrong temperature measure-ments such as the ones described above.

Pyrometer measurement

The basic requirement for a correct pyrometer measurement is that the pyrometer output is only a function of the radiation emitted by the wafer. If the surface of a wafer is coated with a film, the film can change its optical properties during processing although the temperature stays constant. This can, for example, take place by a continuous densification of the film.

Unfortunately, in the UV-RTP set-up (see Fig. 2.1), the pyrometer is located on the same side as the excimer UV lamps. Hence, the experiments on the UV-enhanced diffusion would require a pyrometer measurement of the wafer temperature via the dopant film covered surface. In order to check the behavior of different P dopant s during RTD, films were deposited on the polished

side of single-sided polished wafers. The wafers were subjected to the standard open-loop process (73 % THL power plateau for 4 min) with the uncoated surface facing the THL bank.

The emission of the coated side was measured with the pyrometer. Since the pyrometer output signal was calibrated to the emissivity of a blank polished wafer it should not be interpreted as absolute temperature in the case of the coated wafers.

Fig. 2.4 shows the pyrometer output as a function of the process time. For the uncoated wafer, the increase during the plateau is similar to that observed earlier with an embedded TC.

The optical properties of the uncoated surface do not change during the plateau. In contrast, some of the coated surfaces exhibit strong changes in their optical properties: For the APCVD SiO2:P and the SOD P508 the wafer emission decreases even though the THL power incident on the uncoated back stays constant. It can hardly be said to which proportion this decrease is caused by a real decrease in the wafer temperature or by a change in the emissivity at the working wavelength of the pyrometer. Interestingly, the small hump in the emission-time profile of the P508 occurred each time. Most likely, it arises from a transition in the composition of the SOD. In conclusion, most of the dopant films show ambiguous dependence of the pyrometer output on the emitted radiation. Thus it is impossible to calibrate the pyrometer and hence impossible to utilize it for absolute temperature measurements of the coated wafers.

Conclusions

These two examples on TC and pyrometer measurements show how measurements of the absolute process temperature can fail and how a naive belief in the measurements can lead to a misinterpretation of experimental results and erroneously proposed physical effects. These examples shall motivate the use of open-loop processing for the experiments on the photon-enhanced diffusion. One drawback is the impossibility of measuring the absolute process temperature and hence no temperature dependent kinetic constants like the diffusion coefficient can be deduced.