In III-V MBE we direct a molecular beam at a heated substrate. The substrate is located within a sample holder (platen) for ease of mounting and manipulation within the MBE system. Hence whilst intentional epitaxial deposition is taking place upon the substrate, any exposed metal will partake in unintentional amorphous deposition. The shiny new platen soon becomes coated in a blue/purple film. This film readily adsorbs moisture and readily oxidizes in air, thus it is a significant source of contamination to an MBE system. The substrate and platen are typically outgassed to around 400°C in a buffer chamber. A clean platen and epiready substrate need only 30 minutes to reach the mid 10-9 mBar range. After some 30 growth runs, the same substrate and platen take 90 minutes. Clearly the epiready substrate still requires 30 minutes, but the contaminated platen needs 90 minutes. The thicker the film, the longer the outgas procedure. It seems we are wasting time outgassing our platen and in so doing we are unnecessarily contaminating our vacuum system. Also the III-V layer significantly alters the platen’s thermal response and actually lowers the maximum temperature you can achieve! What to do?
Most platens are composed of Molybdenum (Mo). Occasionally stainless steel (SS) may be utilized for low temperature (<550°C) substrates (InAs, InP, InSb) to save on material and machining costs, but Mo is standard. Mo is a very robust element, which means it can be rather aggressively cleaned without worrying about damaging it with chemical etching. Outlined below is a procedure to remove all III-V deposits from Mo platens. Note full breathing apparatus is advised and work should only be performed under a fume hood!
Firstly you will need two relatively coarse grades of wet & dry abrasive sheets. Automotive wet and dry is sufficient. I use 80 and 120 grit. Using a container of DI-water, submerge the contaminated platen and then rub it with the 80 grit. During this phase you can touch the platen with your gloved hands. 80 grit cuts through the III-V material relatively swiftly, whilst only inflicting minor scratches on the Mo platen. Working with the platen submerged in water will minimize III-V dust, however if you prefer not to work underwater regularly wet the platen to reduce the depth and number of scratches. Once all the material is removed, sand down the scratches with the 120 grit. Repeat with 240 grit if you want to eliminate all scratches. Finally thoroughly rinse the platen with DI-water and dispose of the III-V contaminated water by your facilities established practices. From this point on only handle the platen with clean tweezers.
Next you will need to submerge the platen in undiluted (38%) HCl for several hours. The Mo is impervious to HCl, though any wet & dry residue will be etched away. I deliberately avoid using Aqua Regia since this tarnishes the Mo. After the etching, rinse again with DI-water and this time blow dry with dry N2 gas. Finally soak the platen in isopropanol (IPA) for 5 minutes and blow dry for the last time.
You should hold in your tweezers a very shiny, silvery Mo platen free from aqueous IPA residue. Before you place the platen in your MBE system you need to bake off the dried on IPA residue. To do this place the platen in a 120°C baking cupboard overnight. Once removed and cooled the platen can be stored in a desiccator until needed.
The final phase is to outgas the platen in vacuo. It is best to load an appropriately sized tantalum (Ta) disc in place of a wafer. A Si wafer is a viable alternative to Ta. The Ta/Si is required in order to outgas the platen to the maximum heater temperature, which is around 100-200°C hotter than one would typically take a III-V wafer in an MBE system. Outgas the platen in two stages. First heat it to the maximum you can reach in the system’s preparation chamber outgas stage (~600°C), then take the platen to the maximum operation temperature of the growth chamber’s sample heating stage (~800°C). The maximum temperature will vary from system to system. It should however be at least 100°C higher than you would otherwise normally grow. When outgassing the platen keep an eye on the background pressure and avoid letting it exceed the mid 10-7 mBar range. Aim to hold the platen at maximum temperature for an hour. The entire procedure may take 8 hours per platen to complete. I perform the procedure manually the first few times, then write a simple recipe of substrate temperature ramps to perform the outgas automatically overnight thereafter.
Each platen may need cleaning as often as once a month or as infrequent as once a year. It depends on the system usage and the cocktail of materials deposited. Keep an eye out for the telltale lengthening of normal outgassing times. It is good practice to purchase a number of spare platens to have a few clean ones in reserve whilst the contaminated ones are “out for cleaning”.