Semiconductor, Epitaxy Process
Semiconductors – The Epitaxy Process
The word epitaxy is derived from the Greek “epi” meaning surface and “taxi” meaning arrangement and refers to the process of growing complex structures atom-by-atom arranged onto the surface of a substrate. Based in Cardiff, IQE is the largest pure-play supplier of outsourced epitaxy services world-wide. The company has built up an impressive customer base since it was formed was formed in 1988 and now employs more than 400 people at its three UK and one US manufacturing sites. IQE offers a choice of leading technologies for producing epitaxial structures and although they work closely with customers on the development of new products and services, their expertise and IP lies in the knowledge of how to achieve the complex structures defined by the customer. Their clear focus on materials ensures that IQE does not have a conflict of interests with their customers. In many cases, they work alongside existing epitaxy experts within their customers businesses, allowing customers to concentrate their efforts on device properties and R&D whilst IQE concentrates on the materials characteristics. Many of their customers do not have any epitaxy expertise and rely totally on IQE to provide all their materials requirements. There are two leading epitaxial growth techniques: Metal Organic Vapour Phase Epitaxy (MOVPE) and Molecular Beam Epitaxy (MBE). The first technique (MOVPE) employs a reaction chamber into which ultra-pure gases containing the elements required in the structure are introduced under precisely controlled flow-rate, temperature and pressure conditions. The second method (MBE), utilises heated sources evaporated within an Ultra High Vacuum system. IQE has a total of 32 MOVPE and MBE reactors installed at its main manufacturing sites in Wales and Pennsylvania, USA, with plans to expand further over the coming years. Using these techniques, structures can be produced that comprise up to 300 layers, each of which may only be a few nanometres in thickness. Most silicon based technology relies on multi-stage processing where the functionality is added in the form of layers and coatings on top of a base substrate. In the case of compound semiconductors, the epitaxy process is used to modify the fundamental electronic, optical and physical properties at the surface of the material and the functionality of the devices is then built into the structure. As a result, the epitaxial processing stage normally represents a significant proportion of the overall device costs. With increasing complexity of the epitaxial structures such as VCSELs this proportion is expected to further increase over the coming years.
Conventional semiconductor lasers emit an elliptical beam of light from the edge of the epitaxial material structure as in the figure below.
Each laser measures approximately 0.25mm x 0.25mm which means that a 75mm (3”) diameter epi-wafer could yield up to 9,000 individual devices. However, the wafer needs to be fabricated and cleaved from the epi-wafer before it can be tested and packaged as a discrete device. The cleaved edge of each device forming a partial mirror which is essential to the operation of the LASER. The Vertical Cavity Surface Emitting Laser or VCSEL, is a laser diode where light is generated then propagates vertically rather than laterally through the structure. The resulting light output is from the surface of a fabricated wafer which and offers significant advantages when compared to the edge-emitting lasers VCSELs can be fabricated efficiently on a 3-inch diameter wafer. The ability to manufacture these lasers using standard microelectronic fabrication method allows integration of VCSELs with other components without requiring pre-packaging and allows arrays of lasers to be fabricated. Light is generated in an active region that is sandwiched between two mirrors that are constructed from Bragg Reflectors. These comprise hundreds of layers, each of which is just a few tens of atoms thick. The resulting devices offer a number of significant advantages when compared with traditional edge-emitting lasers. Benefits include: - operating current approx one third that of conventional edge-emitting laser. lower current requirements of VCSEL are ideally suited for battery-driven applications
- series resistance more than 5x that of conventional edge-emitting laser which eases design and enhances performance
- modulation bandwidth better than 10GHz compared with less than 2GHz for of conventional edge-emitting laser, permitting higher data rates and faster information transfer
- power dissipation around one fifth that of conventional edge-emitting laser leading to better temperature stability
- threshold current less than one fifth that of conventional edge-emitting laser which means less power input to produce laser output
- “wallplug efficiency” much greater than that of conventional edge-emitting laser resulting in higher output per unit input currentbeam angle “circular” and smaller than that of conventional edge-emitting laser, Allowing for simpler optics and greater sensing distances. The narrow, symmetric beam improves sensing accuracy & coupling through optical fibres.
In short, VCSELs are emerging as the technology of choice and IQE is the first epi-wafer foundry to supply commercial VCSEL structures.
This article was supplied by IQE plc and has featured in the graduate magazine “Silicon Futures”
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