Within the European project FLASH [35], a new large area RTP machine was to be built for industrially relevant RTD of P emitter for silicon solar cells. Among the partners the question arose whether the P diffusion might be accelerated by additional illumination with UV light from excimer lamps. If so, this would allow to achieve the desired high-throughput without excessive increase of the diffusion temperature. For example, the widely used but temperature sensitive multicrystalline silicon materials like block-cast mc-Si could benefit from process temperatures below 900 C (see chapter 4.4). The scope of the experiments was extended by the veri cation the proposed accelerated diffusion of phosphorus in silicon by the irradiation of short wavelength photons from tungsten halogen lamps. In case of verification, the goal was to gain some knowledge about the conditions under which the enhancement can be observed, e.g. kind of P source.
Below, some of the possible causes for the photon-enhanced diffusion are discussed, along with the respective consequences for the design of the experiments to be carried out in order to clarify the origin of the acceleration:
Photon-enhanced chemical processes within the deposited P dopant source (e.g. densification of the spin-on film [123, 124]). This could eventually provide higher concentrations of mobile P or an increase in the mobility of P within the spin-on layer, and thus lead to an increased concentration of P at the dopant-silicon interface. In this case, one might expect a dependency of this effect on the chemical composition of the P source applied. The diffusion of P within the bulk silicon would not be enhanced. The use of several diffusion sources (e.g. various spin-on sources, APCVD SiO :P and POCl pre-diffused) may allow us to identify this phenomenon.
Higher P concentrations at the very Si surface and consequently deeper P profiles due to lowering of the entrance energy barrier for P or due to the increase of entering sites.
The reason for this could be the creation of charged vacancies due to a photon-induced shift in the Fermi-Level as has been suggested to explain nonthermal effects observed for diffusion on Si surfaces [24]. Alternatively, it was suggested that pairs consisting of a P atom and a Si self-interstitial (denoted as PI pair) are injected into the Si substrate by the presence of a highly doped spin-on source. This mechanism might be enhanced by extra UV illumination [108]. If so, CFP pre-diffused wafers where the PSG was removed prior to an RTD drive-in would not exhibit any photon-induced enhancement.
Increased diffusivity of P within the bulk of the silicon wafer [166]. In this case the profile depth would be increased even if the surface concentration is not affected by the photon-effect. For example, superfluous charged vacancies created by a photon-induced shift in the Fermi-level [108] could be responsible for the enhancement as P is assumed to diffuse as (PV2-) and (2PV-) pair in the extrinsic regime [94, 67]. In this case, the enhancement should be visible for CFP pre-diffused wafers where the PSG was etched before RTD.
To assess whether SiP precipitates play a role in the mechanism, additional pre-diffused wafers that contain no more inactive excess P after diffusion are of interest.
Another explanation for the enhancement is that it is simply caused by thermal effects rather than by photon or other athermal effects. In this case, the apparent enhancement would be the result of an erroneous temperature determination in the RTP units. This is why we wanted to avoid all problems associated with temperature-controlled diffusion processes to clearly distinguish (UV) photon-induced from temperature effects. As a consequence, open-loop processing was applied. In addition, some of the wafers were coated with the dopants on both surfaces, but only one was exposed to the photon irradiation. As the temperature difference between the two surfaces during the diffusion is negligible [115] a difference in the sheet resistance or the P profile of the surfaces would unambiguously prove photon-enhanced diffusion.
Silicon wafers
For the experiments two types of boron doped, (100)-oriented, 4 inch Cz silicon wafers were used:
A) Single-sided polished, 525 m thick, specific resistivity of 7-21 cm.
B) Double-sided polished, 350 m thick, specific resistivity of 0.5-2.0 cm.
Both types were of prime quality. Sheet resistance as well as profile measurements can be performed on the polished sides. Wafers of type A were coated single-sided with different P sources. The coated side faced the UV lamps and the uncoated backside faced the THLs.
However, wafers of type B offer the possibility to apply the P source to both surfaces. Wafers coated with the dopant on both surfaces can be used to easily verify the claimed observation that the sheet resistance is lower when the wafer surface with the dopant faces the halogen lamps than when it is directed away from them. Please note that above temperatures of 700 to 800 C Si becomes opaque, and all visible and UV photons are absorbed in the first few micrometers of the wafer. They do not reach the opposite surface. In contrast to the experiments presented in other publications (see [109] and [124]), where the dopant was deposited only single-sided and thus two diffusion runs with different wafer arrangement had be carried out, we need just one run. The advantage of our method is that, as long as both surfaces are at the same temperature, knowledge of the absolute temperature is not necessary to evaluate the effect of the photon irradiation. No comparison with a second run has to be made which would require accurate temperature measurements to ensure the comparability of the two runs with respect to diffusion temperature.
P sources
Phosphorous sources with different chemical composition were used. They were deposited after rinsing of the wafers in deionized water. The native oxide remained on the wafers. Composition, deposition and characteristics of the phosphorous sources were as follows:
1. The Filmtronics spin-on dopants P507, exhibiting a medium P concentration and P508, exhibiting a high P concentration were used. 800 l of the respective SOD was spun-on at 3000 rpm and baked in an oven at approximately 200 C for 10 to 15 min. In the case of double-side coated wafers, the opposite surface was coated after baking of the firstly deposited film and the wafer was baked again. As the film structure changes during baking, the two sides are not perfectly identical.
2. SiO :P was deposited by atmospheric pressure chemical vapor deposition (APCVD). The P weight percentage was either 14 % or 20 %. In the case of double-sided deposition the wafers had to be sent through the APCVD machine twice. In order to protect the hygroscopic PSG from humidity an additional thin undoped layer of APCVD-SiO2 was deposited on top of the PSG.
3. Wafers were pre-diffused double-sided in a conventional quartz tube furnace1 using 250 mg of POCl3 per minute. The diffusion was carried out in air atmosphere at approximately 900 C for 10 min (excluding 3 min for loading and 3 min for unloading of the quartz boat). After the initial diffusion, the wafers were divided into 3 groups which were treated differently prior to the RTD drive-in:
i) The PSG was left on the wafer to serve as an external P source in the subsequent RTD step in addition to the internal source of SiP precipitates.
ii) The PSG was removed in HF. Only the internal surface near SiP precipitates serve as P source during RTD. Here, the photons from the THL and the excimer lamps impinge directly onto the Si surface during RTD.
iii) The PSG was removed in HF and a second CFP diffusion was carried out at 900 C for 15 min to dissolve the SiP precipitates. Like in the case ii), the photons directly hit the Si surface.
Experimental procedure
The spin-on films were prepared just before RTD was carried out whereas the APCVD deposit-ion and the CFP pre-diffusdeposit-ion were carried out some days in advance. RTD was performed using the standard open-loop recipe presented in Fig. 2.2. The plateau power of the THLs was varied between 66 and 77 %. Prior to the actual diffusion a test run was carried out to establish the initial conditions. Then, one sample was diffused without UV illumination and subsequently an identically prepared one with UV light from the front. This was done for all the different dopant sources which were sometimes deposited on both surfaces. It has to be noted that the front of a wafer was defined as the surface facing the excimer UV lamps regardless of whether they were on or off. The wafer backside rested on the pins of the wafer holder and thus always faced the THL bank.
Characterization
After RTD the wafers were etched in HF and rinsed in deionized water. The sheet resistance was determined with the conventional four point probe method. The systematic error of the system was below 5 %. The mean and its standard deviation were calculated from 25 points located in a 3 cm diameter circle in the wafer centre. Values at the wafer edges were not taken into account. The profiles of the atomic P concentration were measured by secondary ion mass spectroscopy (SIMS) on samples cut out of the middle of the wafers. The measurements were carried out at the CEA located in Grenoble and, in addition, a few samples were sent to RTG Mikroanalyse GmbH in Berlin for comparison. Both institutions used a Cameca system and a Cs beam for sputtering.
1It might be of interest that during the CFP pre-diffusion the later front side was pointing towards the inlet of POCl3at the backend of the quartz tube. This might explain minor differences between the sheet resistance of the front and the back of these wafers. In general, the front received slightly more doping than the back.