Pulsed Laser Dopant Activation for Semiconductors and Solar Cells

Abstract

The constantly decreasing size of semiconductor structures during the 1990’s and early 2000’s led to a reduction of the thermal budget for high temperature activation processes. This reduction was realized by decreasing process duration, coupled with strongly enhanced wafer heating and cooling ramps. Standard lamp technology finally achieved processes as short as a few seconds only with ramps of up to 300 K/s.

The next technology step was achieved when flash lamp annealers and laser devices using continuous wave (cw) lasers were introduced between 2000 and 2005. Flash lamp annealers with their array of high peak power flash lamps can thermally process an entire wafer at once within a period of 1–20 ms. Thus they are able to reduce the process duration by 3 orders of magnitude, compared to standard rapid thermal processing equipment. Annealers using cw lasers shape the laser beam into a narrow line with typical dimensions of 10–20 mm×50–100 μm. To achieve process durations of 1 millisecond or less these lines are scanned across the wafer surface with a velocity of 100–500 mm/s. The semiconductor industry soon began evaluating these tools and started to manufacture devices with 28 nm nodes in mass production.

Other devices, however, require much shorter processes—in the microsecond range or below. These devices are not characterized by even smaller dimensions, but by thermally sensitive structures like metal contacts in close vicinity to the layers which require dopant activation. Localized sub-microsecond processes benefit from the short thermal diffusion length of a few micrometers only which allows keeping nearby structures at reasonably low temperature. In contrast to continuous wave laser annealers for millisecond processes, sub-microsecond activation is realized by using pulsed lasers with pulse durations in the suitable range. In order to achieve high activation rates these processes have to be conducted in the melt regime, where the diffusion rate of impurities in silicon is around seven orders of magnitudes higher than in the solid phase. Examples of devices which benefit greatly from microsecond processes are power transistors like IGBT’s (insulated gate bipolar transistor) and backside illuminated CMOS image sensors which can be found in modern mobile devices like smart phones. Another product range where sub-microsecond processes are presently introduced with technical and commercial success are crystalline solar cells, where localized dopant activation to create so-called selective emitters increases the cell efficiency.

This paper describes how modern electronic devices benefit from sub-microsecond localized thermal processes and explains the demands which processes like IGBT and CMOS sensor backside annealing make. It also describes the laser and optics technology which was developed in order to meet these demands. The laser technology advanced to a level where it can offer additional “free” parameters for process optimization like variable pulse duration, combined with extremely small pulse-to-pulse fluctuations. The duration of a pulsed laser process strongly affects the heat diffusion length and thus the depth of the thermal process. Optical beam parameters like spot size, spot shape and wavelength also allow control of the heat flow and hence better optimization of the thermal processes. The creation of selective emitters of crystalline solar cells using pulsed lasers is a good example to demonstrate how creative optical concepts can help to meet the high throughput demands which the solar cell industry makes.