Centre for Sustainable Energy Systems (CSES) Facilities

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Two Varian deposition systems

These machines are thermal evaporators used for deposition of metals on the sample surface. The metal to be deposited is held in vacuum in a tungsten boat and is resistively heated. The surface to be coated is held on top of the boat and the chamber enclosing the metal source and the target (sample to be coated) is pumped down to a pressure of ~ 3-5 ×10-6 Torr. Evacuating the deposition chamber helps reduce contaminations and increases the mean free path of the metal vapors. The metal vapors are deposited on the sample (on the inner walls of the entire chamber) and the thickness is measured using a crystal monitor (the deposited thickness is calculated by measuring the frequency of vibration of the crystal).

Photolithography

The machine has three different sized chucks that can hold 2", 3" or 4" wafers. This machine is routinely used for patterning Si wafers for making slivers (lateral dimensions ~12 µm). 

Furnaces

8 furnaces that can handle 4" wafers and 3 furnaces that can hold 2" wafers are currently operational. The furnaces capable of holding 4" wafers are used for P/B diffusions or for oxidation. The temperature can be varied between 200oC and 1100oC, but are usually operated between 700oC and 1100oC.

The furnaces capable of holding 2" wafers are used for annealing. N2/O2/forming gas ambient can be used for annealing.  Temperature can be varied between 400 and 1100oC. Some furnaces cannot be used for metals (to avoid cross-contamination).

Spin rinser

This machine is used for cleaning wafers. Cassettes holding 25 of 4" wafers can be cleaned at a time. There are 2 chambers, one for metals and one for samples free of any metals. The samples are rinsed with DI water and dried.

Fume cupboards

One fume cupboard is exclusively used to handle cyanides and 7 other fumehoods are available for handling acids and solvents.

Laser scriber

This machine is usually used for patterning Si wafers for making slivers. IR or green laser pulses are produced that scribe a pattern on a coating used for wafer protection and a layer of dielectric beneath it. Wet etching is then used to etch through the wafer along the pattern opened up in the dielectric by the scriber. The scribe depth can go up to 500 µm and the minimum lateral dimensions that can be patterned depend on how well the laser pulse is focused on to the wafer. Lateral scribe dimensions of ~ 5µm can be achieved.

2 dicing saws (DISCO and K & S)

These dicing saws are used for cutting Si or glass. Wafers with a maximum thickness ~1.6 mm can be cut with a lateral resolution of ~ 150 µm. The lateral resolution can be increased by choosing appropriate blades, but blades that can cut smaller lateral dimensions are usually brittle and can only be used for dicing thinner wafers. Both machines are semi-automated.

Screen printer

Screen printing is used for depositing metal pastes on the semiconductor surface. The sample is then annealed to make electrical contact between the metal and the semiconductor. This is a labor intensive task (issues with cleaning for use of different metals, issues with alignment) and is not viable for laboratory studies. The metal pastes used usually have an expiry period of 3 months and additional solvents need to be used to be able to continue using the pastes. Features with minimum dimensions ~100 µm can be printed using this technique.

Roth & Rau AK 400 PECVD and RIE

The PECVD system has provision for silane, nitrous oxide and ammonia anc can be used for deposition of Si3N4, SiO2 and amorphous Si.

The RIE machine is currently being tested for etch uniformity and resolution. The machine has provision for CF4 and H2 gas sources. The system has two power sources, microwave source for creating plasma and an RF source for biasing the substrate.

APCVD

The SierraTherm 5k6 atmospheric pressure chemical vapor deposition system essetailly is a 12' long beltfurnace, with a CVD injector head in the middle, the injector head sprays a mist of tetra-isopropyl-titanate (TPT) and water vapor on to samples passing on the belt below. Nitrogen gas sheets prevent the reactants mixing until they intercept the top of the passing samples where they mix hydrolysing the TPT creating TiO2 films. Parameters that can be modified to control the properties of the deposited film are belt speed, reactant / nitrogen carrier ratios, reactant / reactant ratios, total flow rate, and temperature. . Once the system has been calibrated and suitable reactant / reactant ratios have been calculated to control the thickness of the film the belt speed and total gas flow are adjusted. The temperature range of the APCVD can be controlled to between 100 and 500 oC, with the effect of changing the crystallographic structure of the TiO2 film and it optical properties.

This system is capable of depoitiong uniform TiO2 films between 20 and 200nm thick with refractive index ranging from 1.9 - 2.5 depending on the deposition temperature. The system has also been modified to include forming gas in the CVD injector head.

LPCVD * 2: CVD-LPCVD and Tempress LPCVD

These two machines are used for depositing Si3N4 on wafers. The machines are usually operated at 775oC with fixed flows for Ammonia and dichlorosilane, which results in a deposition rate of ~6 nm/min. The Tempress machine is a relatively new machine that is automated is safer to use.

Sliver plating

Filmetrics F20

Filmetrics F20 is used for characterizing thin films. Film thickness ranging from 150 Å to 50 µm can be measured. Refractive index and extinction coefficient can be measured for film thickness ranging between 1000 Å and 5 µm for wavelengths from 400 nm to 1000 nm.

Flash IV tester

Flash tester is used to measure the IV response of solar cells. The cell temperature is controlled using a Peltier cooler and can go upto a maximum of 70oC. The flash intensity can be varied between 1 sun and a maximum of 100 suns.

Sinton lifetime testers

The equipment measures the effective carrier lifetime that can be used to deduce the bulk lifetime, metal contaminations (Fe/Cr) and the quality of surface passivation. All the software has been set up for Si. The machine can also be used as a backup for resistivity measurements.

Photoluminescence lifetime mapper (on order)

Kelvin probe

Quicky solderer

This is a vapor phase soldering machine and is used for making metal interconnects on the solar cells. The metal to be soldered is deposited on the cell surface using a stencil and the cell is then placed inside the solder chamber. The chamber contains a Teflon type fluid, which can be heated up to temperatures of ~215oC. The vapors surround the entire cell with metal pattern on its surface, raising its temperature and thus soldering. The equipment is easy to clean (rinsed with water). The equipment avoids exposure of samples to O2 and can handle six 50 mm×40 mm samples at a time.

Spot welding

Is used for soldering Cu wires to interconnect the solar cells.

Selective wave solderer (EBSO SPA 300-F)

This is a fully automated machine that is used to interconnect individual solar cells to produce a linear concentrator module.

Four point probe measurement

Spectral response unit

Unitemp RTA

The machine has 2 chambers, one intended for metals and the other chamber is RCA cleaned. The chambers can hold a maximum wafer area of 5"×5".The maximum temperature this machine can handle is 1000oC (for 1 s). N2/forming gas/O2 ambient can be used while annealing.