Southampton Nanofabrication Centre
Southampton Nanofabrication Centre
 

The Centre contains a wide range of standard fabrication options, with a standard wafer size of 150mm, but many machines can accommodate 200mm wafers. The core fabrication is based on several EVG optical aligners with a minimum resolution of 0.5 micron and alignment to <1 micron. The EVG Infinity in particular can accept 200mm wafers and also offers a nano-align mode of <0.1 micron. For smaller geometries, optical lithography can be mixed with electron beam lithography, but allowance should be made for any systematic mis-alignments between the two types of aligner.

Both dry and wet etch processes are available for pattern transfer after photolithography. Wet etching is simple and highly selective, but generally gives little control over the etch profile. The usual array of wet etches and cleans are available, including hydrofluoric acid for oxide etching, fuming nitric acid for resist strip and the RCA clean for wafer cleaning prior to furnace or rapid thermal processing. Dry etching allows the etch profile to be varied from anisotropic to isotropic, but is not always highly selective and the etch chemistry is much more complex. A range of materials can be handled: ICP for oxide and metal etching, as well as deep silicon etching; RIE for general etching; and ion beam milling for hard materials.

Furnace and rapid thermal processing is performed in Tempress diffusion furnaces and two Jipelec Jetfirst 200 rapid thermal processors (RTP). Furnaces are used for long anneals or oxidations of typically 30 minutes or more, whereas RTP is used for short anneals or oxidations of typically 30 seconds. RTP would be used where dopant diffusion needed to be minimised. Silicon-based processing is very sensitive to contamination and hence some furnaces are reserved for clean processes, including a stack of four 150mm furnaces and a stack of three 200mm furnaces and one of the RTP systems. A stack of three 150mm furnaces and the second RTP system are available for other processes.

Insulator deposition can be performed using either low pressure chemical vapour deposition (LPCVD) or plasma enhanced chemical vapour deposition (PECVD). Silicon nitride can be deposited by LPCVD in one of the 200mm Tempress furnaces and both silicon nitride and silicon dioxide can be deposited by PECVD in an OPT System 100 LS. The System 100 offers the option of depositing silicon dioxide either using gases (SiH4 and N2O) or a liquid source (TEOS). For general silicon processing, oxidation is the preferred method of producing a silicon dioxide layer because it offers high throughput, a high quality oxide and a uniform thickness. However, oxidation has the disadvantage of requiring high temperatures of typically 900C or above. Where temperature is a constraint or silicon is not the substrate, silicon dioxide can be deposited by LPCVD or PECVD, with PECVD generally offering the lowest deposition temperatures (and lowest quality oxide) because some of the energy required for the chemical reaction is provided by the plasma. TEOS oxide is mainly used as a planarising layer prior to metallisation because it smooths the surface and prevents breaks in metal tracks. Amorphous and polycrystalline silicon can either be deposited by LPCVD in a Tempress poly furnace or by PECVD in the OPT System 100. Only undoped material is available in the LPCVD furnace, whereas undoped, p-type (boron) and n-type (phosphorus) material is available in the PECVD System 100. Amorphous and polycrystalline germanium and silicon-germanium can also be deposited in the PECVD System 100, again with or without dopant. For photovoltaic applications, a Plasmaquest PECVD system is available for the low temperature deposition of amorphous silicon.

Metals, metal oxides and metal nitrides can be deposited either by sputtering or evaporation. Sputtering is performed on the Leybold Helios, which has a dual magnetron and hence allows co-sputtering from two different targets at the same time. An equivalent capability is also available on the Leybold Lab 700 e-gun evaporator, which allows co-evaporation from two different targets at the same time. The e-gun on the Lab 700 provides precise control of the evaporation process and the planetary rotation system provides precise control of the metal profile over etched steps. The Lab 700 is therefore ideal for metal lift-off processes. Sputter targets are much bigger (and hence much more expensive) than evaporation targets and hence expensive metals such as gold and platinum are only deposited by evaporation. A Leybold Bak 600 and an Edwards evaporator are also available for general evaporation.