Deposition concept and reactors at Fraunhofer ISE

APCVD reactor designs are explained in much detail in books such as [23, 45, 57] and others. The reactors are generally grouped by the wall temperature (hot and cold wall), the method of heating (resistive, inductive and optical) and their geometry. The most common geometries of reactors are horizontal, barrel, true vertical and pancake reactors. All reactors have in common that they are quite sophisticated yet have a low gas yield. The thickness homogeneity required for microelectronic applications is much severer than for solar cells, where it seems

that 10% uniformity is sufficient [14]. The crucial point of a CVD system for solar cells is a low-cost reactor with maximum gas conversion and high throughput. The silicon layers needed for EpiWE are quite thick in comparison to typical silicon layers for microelectronic devices. Therefore, a high growth rate is necessary to avoid excessive deposition times [14]. In order to increase the efficiencies in terms of gas yield, throughput and apparatus complexity, a new reactor design was developed at Fraunhofer ISE [58]. Two lab-type reactors and one prototype industrial CVD reactor are currently used at Fraunhofer ISE using this special deposition principle. Their main characteristics are presented showing a brief history of the up-scaling in size and throughput of the reactors.

3.3.1 Deposition principle

The idea is to inject the process gas between the samples, so that the silicon is only deposited on the wafers. Figure 3-8 shows a schematic view of the larger lab-type CVD reactor at Fraunhofer ISE (RTCVD160). The process gas is injected into an inner reaction chamber, whose left and right walls are formed by the samples themselves. The quartz carrier seals the top and bottom walls (not shown). Additionally, the inner volume is surrounded by the quartz tube, which is purged with hydrogen. Therefore, only a little parasitic deposition occurs and the reactor design can be simplified. No complex side wall cooling or specific wafer heating is necessary. The reactor can be heated by halogen lamps or by resistance heating, which are situated on the sides of the quartz tube.

The thickness homogeneity perpendicular to the gas flow on the samples is achieved by different gas injection systems. A small over pressure can be built up with shutters, which force the gas to homogenise in the vertical direction. In contrast to the over pressure, a diffusing shower can disperse the laminar gas flow in the same direction. Alternatively, several injection lines situated in the vertical direction can be used. In addition, a depletion of the silicon precursor gas in the gas flow direction occurs as the reaction kinetics are very fast at these temperatures. In order to level the inhomogeneity in the gas flow direction a continuous deposition is performed by moving the samples in or against the gas flow direction.

Figure 3-8: Deposition principle of the lab-type reactor RTCVD160.

Figure 3-9: The RTCVD160 during a process.

3.3.2 RTCVD100

The RTCVD100 is the first generation CVD reactor at Fraunhofer ISE with the deposition principle as described above [48, 58]. The ‘100’ in the name signifies the diameter of the reactor quartz tube⁴, which is 100 mm. The wafer carrier is relatively simple as the wafers are placed horizontally and are stacked between quartz rods. Lower and upper wafer rows are therefore formed, separated by two quartz slats on the side and quartz sheets with holes for the gas injection and exhaust at the front and at the end of the inner reaction chamber. The reactor has only one row of halogen lamps situated above the quartz tube so only one row of wafers can be used for silicon deposition with an optimised temperature profile.

This results in a homogeneous silicon deposition zone of only 10 x 5 cm². The CVD processes are usually run at high Cl/H ratios of 0.75 [47] with growth rates between 6 and 10 µm/min. The resultant gas conversion yield is approximately 15%, which can be increased for lower Cl/H ratios. The RTCVD100 is the

‘workhorse’ of the CVD laboratory and has now been running for more than 10 years. This tool has proven that quite a simple reactor and process achieve sufficiently high quality and homogeneous silicon layers for solar cell purposes.

Many depositions for solar cells and etching experiments presented in Chapter 4 and 6 were performed in this reactor.

4 The same applies to the RTCVD160, where the reactor tube has a diameter of 160 mm.

3.3.3 RTCVD160

The goals of building the larger lab-type RTCVD160 reactor were to increase the throughput and to optimise the layer homogeneity [59]. The main differences from the RTCVD100 are the vertical orientation of the wafers and that two rows of halogen lamps perform the heating. A schematic view of the RTCVD160 was already shown in Figure 3-8 as well as a photo of the reactor during deposition in Figure 3-9. Due to the increased size and gas flows, the silicon precursor gas concentration depletes strongly in the gas flow direction. Therefore, the samples are moved in the horizontal direction so that a continuous deposition process is performed, levelling the thickness inhomogeneities of the deposited layer. The vertical homogeneity has been achieved by several different gas distribution systems during the last three years. The first approach was to use shutters and showerhead plates to hinder and redirect the gas flow. Pressure is built up prior to the apertures, forcing the gas to flow uniformly in the vertical direction. The development of the gas distribution system has already been described in [60, 61]. Very homogeneous layers were deposited: a thickness homogeneity of better than 90% is shown in Figure 3-10-A. However, the CVD reactor experienced frequent down-times as many shutters became over-grown with silicon, broke or bent during processes. Therefore, a simpler gas distribution system was developed (described in [62]), whereby the injected process gas is distributed by a diffusor. In both distributing concepts the gas flows are laminar [63].

Figure 3-10: Layer thickness homogeneity in the RTCVD160 performed by either showerheads (A) or a diffusor (B).

Thickness homogeneities up to 70% have been achieved by adjusting the deflectors in the diffusor [64, 65]. Figure 3-10-B shows the actual deposition homogeneity. The inferior homogeneity was accepted in compromise for a better process control and machine up-time. Of course the homogeneity must be further improved by optimising the gas injection prior to the diffuser, stabilising the carrier and making it gas tight. However, the RTCVD160 became the new primary tool for silicon CVD deposition at Fraunhofer ISE and many layers for solar cells presented in Chapter 4 and 5 were deposited in this reactor.

3.3.4 ConCVD

The crucial process requirement for the production of cSiTF solar cells is an economical silicon deposition. Until now, no reactors fulfilling high-throughput and high gas conversion efficiency are available. Other research groups are developing a batch type reactor based on thermal convection (COCVD) [66] or a stacked epitaxial reactor (SER) with process gas recycling [67]. Hurrle et al.

designed the ConCVD, a prototype of a high-throughput and continuous deposition CVD reactor [68]. The deposition principle from the lab-type reactors was assigned, however heating is provided by resistance heating. Two parallel rows of wafers are continuously fed through the reactor, passing through gas curtains at either end. Figure 3-11 shows a schematic view of the gas curtains.

Nitrogen is injected between the two sample carriers in order to hinder the entry of oxygen from the laboratory atmosphere into the reactor tube. Near the reactor the out-diffusing hydrogen is pumped away, as well as the oxygen, which may diffuse through the nitrogen flow. With such a gas curtain the atmospheres of the laboratory and the reactor are separated. The reactor is able to deposit layers with a throughput of more than 1 m²/h [34]. Figure 3-12 shows an epitaxial

Reactor H₂ Laboratory

O₂ Sample carrier

N₂

N₂

Pump

Pump

Figure 3-11: Schematic view of a gas curtain, separating the laboratory and reactor atmospheres.

Figure 3-12: Epitaxial dep-osition performed in the ConCVD.

deposition of 48 samples deposited in 90 minutes. The machine still has several problems with process stability, as well as homogeneity and quality problems.

However, the first solar cells with efficiencies up to 12.5% have been fabricated from epitaxial layers deposited in this machine, proving that good results are possible.

3.4