Multilevel Approaches Toward Monitoring and Control of Semiconductor Epitaxy
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ABSTRACT Various optical techniques have been developed over the last few years to allow real-time analysis of regions of importance for semiconductor epitaxy, in particular the unreacted and reacted parts of the surface reaction layer (SRL) and the near-surface region of the sample. When coupled with emerging microscopic methods of calculating optical properties, these approaches will allow several levels of control beyond that which has been currently demonstrated. INTRODUCTION Electronics and optoelectronics technologies are based on artificial materials and structures, which are becoming increasingly complex as new levels of performance are achieved [1,2]. This trend will clearly continue, with devices becoming smaller and more complex, tolerances becoming more stringent, and materials being tailored according to function rather than compatibility with a particular growth process. Present requirements also include lateral patterning, selective-area deposition, and scale-up to industrial production levels. This has generated a second trend away from physical deposition techniques such as molecular beam epitaxy (MBE) toward chemical-beam methods such as organometallic chemical vapor deposition (OMCVD) and chemical beam epitaxy (CBE), where the extra dimension of chemistry can be used for greater capabilities and flexibility [3,4]. At the same time, yields must be maintained at acceptably high levels, These trends have created a strong incentive to develop a better understanding of growth processes and better methods of monitoring growth, ideally leading to sample-driven closed-loop feedback control of the growth process itself. Much present effort is being directed toward accurately measuring process parameters and accurately modeling growth, mainly because numerous probes are available to provide a relatively detailed assessment of the ambient. However, this strategy is clearly limited in its capability to deal with complex nonlinear processes, as for example OMCVD where the surface plays an integral role in decomposing precursor species and small changes of ambient compositions can affect growth substantially [3,5]. The device that best represents current challenges is the vertical-cavity surface-emitting laser (VCSEL) [6]. VCSELs may contain hundreds of layers of varying compositions and thicknesses, which are grouped into the distributed Bragg reflectors (DBRs) at the top and bottom and an active region in the middle. At deposition rates of 1 lim/h about 6 h are required to grow a VCSEL device structure. The probability that deposition rates determined at the beginning of growth will be the same at those at the end is relatively small, especially for MBE. For this reason considerable effort has been invested in adapting and extending reflectance monitoring techniques [7] that were developed over the last 20 years by the optical thin films industry to 451 Mat. Res. Soc. Symp. Proc. Vol. 448 01997 Materials Research Society
the growth of VCSEL substructures, specifically DBRs [8-10]. However, while reflectometr
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