Procedimientos

Before starting the fabrication sequence of the cells, the wafer are thinned by using the mechanical grinder tool at ISE, Freiburg. After this, wet chemical etching and cleaning steps are applied in order to solve surface contaminations from the substrates. In our process the first cleaning step is a HNF-cleaning using HNO3 for surface oxidation and

HF for etching the oxide. Thereby, most contamination is removed.

The next cleaning process step is the RCA cleaning. The RCA cleaning is based on a two-step wet-oxidation and a complexing treatment in aqueous H2O2-NH4OH and

H2O2-HCL mixture at 75-80 °C for ten minutes [34]. Between the two-step wet

oxidation, two one-minute HF (1%) clean steps are applied in order to remove the formed oxide films.

The chemical principles of the RCA cleaning are

• H2O2 at high pH-values is a powerful oxidant which decomposes to H2O and O2.

• step 1: NH4OH is a strong complexant for many metals.

• step2: HCL in H2O2 forms soluble alkali and metal salts by dissolution and/ or

complexing.

• the mixtures are formulated not to attack silicon. Step1:

• due to the wet oxidation organic surface films are removed and due to NH4OH the

surfaces are exposed for the desorption of trace metals (Au, Ag, Cu ...).

• due to the wet chemical oxidation and the subsequent HF dips, the samples keep forming and dissolving of native oxide films.

Step 2: Using HCL

• alkali ions and hydroxide of Al+3, Fe+3, Mg+3 are dissolved.

• residual metals are desorbed by complexing.

• a protective oxide film is left.

After these wet chemical cleaning steps the substrates should be free of contamination and the actual fabrication process of the cells starts.

Diffusion

The sequence of the RLCC cell fabrication process is determined by the different diffusion profiles of the dopant materials. The deep doping profiles for highly- efficient solar cells, were optimised by S. Sterk [35]. In that work, it is shown that very deep doping profiles (5 µm, 32 Ω/) of boron underneath the contact windows lead to the

desired low contact resistance and to the best open-circuit voltages. Such a local boron diffusion is the first dopant diffusion process in the fabrication of the RLCC cell. After this, a deep phosphor diffusion (2.7 µm, 19 Ω/) follows underneath the n-contacts. The third dopant diffusion process is the shallow emitter diffusion (1.4 µm, 125 Ω/) for the floating emitter on the front side and for the pn-junction on the rear side [36]. All dopant diffusions used here are two-step diffusion processes. First in a pre-deposition step, a diffused layer is formed under constant-surface–concentration condition. The dopants are introduced by using liquid sources (BBr3, POCL3) and they are transported

to the semiconductor surface using nitrogen. Then a 30 min long drive-in diffusion is followed under a constant-total-dopant condition [37]. So, all dopant diffusion processes lead to a Gaussian dopant profile. An additional drive-in diffusion of the boron and phosphor dopants is applied in processing the masking silicon dioxide layers, simultaneously. In doping the wafers a thin boron or phosphorus oxide layer is formed on the silicon surface, which has to be etched by a 30 sec wet chemical process step, called SiO-etch step, using a mixture of ammonium chloride (NH4Cl) and hydrofluoric

acid (HF) in water.

Oxide layer

The silicon dioxide layer on the front side, used as a masking layer or as an antireflection layer, and the silicon dioxide layer on the rear side are fabricated by the thermal oxidation process [37]. Additionally, wettish chlorine (DCE) is introduced into the oxidation ambient to reduce the process time and to remove impurities at the Si- SiO2 interface by transforming them into volatile chlorides. The reaction time and the

temperature of the thermal oxidation depends on the silicon dioxide thickness. The silicone dioxide layer can then be locally etched for the diffusion processes by using the SiO-etch.

Metallisation

For the rear side metallisation the electron beam evaporation is used [38]. A deposit thickness of 5 µm can be reached. A disadvantage of the process is the generation of X- rays by the e-beam which causes damage of oxide surface layer. For reducing the damage, the samples are sintered.

Sintern

At the end of the fabrication process all samples are sintered by an inert gas (N2H2) at

450°C. While sintering the wafers (25 minutes) the passivation of the SiO2 layer is

strongly improved. There are two different theoretical models for this effect [39]. Building in atomic hydrogen in the SiO2 layer the open dangling-bonds of the surface

4 Design and technology of the rear-contacted silicon concentrator cell

48

hydrogen displaces the very active recombination centres from the middle of the band gap leading to a lower recombination rate.

Photolithography

Due to the small dimensions of the RLCC cells in contrast to other highly-efficient solar cells, the challenges in processing the cells were the optimisation of the different photo lithographic transfer processes and the development of the rear-side metallisation. For this purpose several new resists were tested. A detailed description of the single photo lithography steps is given in the Appendix.

positive photoresist SiO2 Si substrate photomask light dissolution of the exposed resist etching of the insulating SiO2-layer stripping of the resist Process Condition - spin speed - spin-on time

- front or rear side first - exposure time - mixture of developer - developing time - etching time - plasma etching or acetone

Figure 4.12: The optical lithographic transfer process in detail using a positive photoresist.

Using the optical transfer process, patterns of geometric shapes of the RLCC cell on a mask are first transferred to a thin radiation sensitive resist and then transferred to the

insulating SiO2 layer. The patterns define the diffusion regions and the contact windows

of the RLCC cell. The choice of the right photoresist depends strongly on the dimensions of the defined regions. The resist parameters as for example the baking temperature or the baking time must be optimised for every single photoresist step. A typical photo lithography process, using a positive resist, with a following dissolution of the resist and an etching of the insulating SiO2 layer is presented in Figure 4.12.

After forming an insulating layer of SiO2 on the substrate surface, the photoresist is

applied to the wafer using a spin-on coating system. The spin speed and the spin time depends on the used photoresist and the desired thickness of the photoresist film. After the spinning step a pre-exposure baking step of the resist is done in order to remove the solvent and to improve the adhesion to the wafer. In an optical lithography system the wafer is aligned to the photomask and the opaque patterns of the mask are exposed to UV-light. The exposed areas of the positive resist are then dissolved in a developer. Using the SiO-etch step the exposed areas of the insulating layer are etched and the not exposed areas are not attacked. At the end the positive resist is stripped using plasma etching or acetone and the insulating layer is left as the inverse image of the opaque mask pattern.