Introduction

Although there have been a few predecessors [1, 2], the invention of laser annealing [3] and flash lamp annealing (FLA) [4] is usually dated 1975 and 1978, respectively. The invention was done in the environment of semiconductor research, and further development was driven by the needs of microelectronics. With the continuously ongoing miniaturization of microelectronic devices, the tolerable diffusion lengths became shorter and shorter, which required an adequate reduction of annealing times [5]. Since the 1980s, rapid thermal processing (RTP) has been established as the dominant annealing technique in microelectronics with ca. 0.5 s as the shortest annealing time in case of spike annealing [6], but around 2000 FLA was gradually introduced for cases in which even this limit was not sufficient anymore. Along with this development, the technology of FLA has advanced, and there is now much better control over the annealing process and a much better adaption to industrial processes. Thus, FLA has moved into new areas of application such as photovoltaics [7], flexible electronics [8], printed electronics [9], and energy materials [10, 11].

Today, FLA is a modern annealing technique that offers a couple of advantages. Similar to ion implantation, it is a thermal non-equilibrium process, which is why FLA can achieve material properties that are not possible to achieve in thermal equilibrium. Examples are hyperdo** (do** above the solubility limit) or the synthesis of metastable phases. Metastable in this context means that the material is stable under room-temperature (RT) and operation conditions but is vulnerable to further annealing steps. Because of the short time scale, the bandwidth of temperature-sensitive materials, which nevertheless can be exposed to high temperature during FLA, is larger than for RTP and furnace annealing. Finally, FLA offers energy and process time savings, which makes this technique suitable for roll-to-roll and conveyor belt applications.

However, these advantages come with a price in form of additional issues to be considered and with challenges to be met. Temperature is now much more difficult to estimate as the temperature profile within a sample depends on the material properties, thermal stress has to be managed, and additional measures have to be taken in order to ensure a high reproducibility and homogeneity. Thus, the first part of this review gives a short overview of FLA technology concerning these issues. The second part discusses some special semiconductor application in which FLA was used to achieve specific material properties.

The technology of flash lamp annealing

In general, an FLA system consists of an energy storage system and a flash chamber. The energy storage system comprises a capacitance (made of one or several capacitors) to store the energy, an inductance to form the pulse, a charging device, and the required electronics including high-power switches for control. The flash chamber is mainly composed of one or several flash lamps, a reflector to harvest a maximum of light, and protection windows made of quartz to prevent the deposition of material onto the flash lamps and other harmful processes. Many tools are also equipped with a preheating system in order to minimize thermal stress or to achieve even higher temperatures. The preheating system itself can be a bank of flash lamps for longer pulses [12], a bank of halogen lamps similar to RTP, or a hot plate. The basic circuitry of an FLA system is given in Fig. 1a, and more details about the setup can be found in [13,14,15,16].

Fig. 1
figure 1

Basic scheme of an FLA circuitry comprising a capacitance C, and inductance L, the flash lamp F and a switch S (a), and the working scheme (b)

Figure 1b displays the working scheme of an FLA tool that roughly follows the flow of energy through the system and the sample. The charging voltage V0 and the capacitance C determine the stored energy, and a switch S usually separates the charging device. In case of active pulse sha**, i.e., for direct control of the lamp current, additional high power, controllable switches such as gate turn-off thyristors, metal-oxide semiconductor, field-effect transistors (MOSFETs), and insulated-gate bipolar transistors can be used [17,18,19,15, 21]. More details are given in the text