Abstract
Flash lamp annealing (FLA) is a non-equilibrium annealing method on the sub-second time scale which excellently meets the requirements of thin-film processing. FLA has already been used in microelectronics, mostly after ion implantation, to activate dopants, to recrystallize amorphous semiconductor layers, and to anneal out defects. Another field of application is the formation of silicide and germanide materials for contact fabrication. However, in the last twenty years, FLA has opened up new areas of application like thin films on glass, sensors, printed electronics, flexible electronics, energy materials, etc. For two years, the Helmholtz Innovation Blitzlab aims to transfer this technology to industry and application-related research. After a short introduction, a brief overview of FLA is given, discussing the advantages and challenges of this technology. The main part displays various examples from the literature and from our own research, in which FLA has been applied to semiconductors, namely to Si, Ge and GaN. In detail, the do** close to or even above the solubility limit of dopants, the crystallization of Ge during FLA, the formation of NiGe for contacts, and p-type do** in GaN are addressed.
Graphical abstract
![](http://media.springernature.com/lw685/springer-static/image/art%3A10.1557%2Fs43580-022-00425-w/MediaObjects/43580_2022_425_Figa_HTML.png)
Similar content being viewed by others
Avoid common mistakes on your manuscript.
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].
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
The optical spectrum of the light pulse extends from the ultra-violet (UV) to the near-infrared (Fig. 2b) and is composed of the broad thermal emission of the hot plasma and a couple of discrete spectral lines originating from bound-to-bound emissions of the noble gas. As seen in Fig. 2b, a flash lamp filled with Kr gives a somewhat higher output in the UV (3–5 eV), whereas the Xe lamp has a higher output in the visible spectral region. In fact, lower atomic masses of the filling noble gas lead to higher plasma temperatures and, thus, to a blue-shift of thermal radiation, but at the expense of power conversion [13]. Furthermore, an increase in the applied voltage will also lead to higher plasma temperatures and a corresponding blue shift of the spectrum. The enhancement of the UV part is important if FLA is applied to materials that are nearly or fully transparent in the visible, such as TiO2 or transparent conducting oxides. Nevertheless, most flash lamps are filled with Xe due to the high conversion efficiency.
Finally, the design of the flash chamber and the reflector determines the light energy density ED, typically given in Jcm−2, which is delivered to the surface of the sample. From that point, the energy entry into and the temperature profile within the sample strongly depend on the sample properties. Many opaque samples are made of a thick substrate and thin films on top. In a rough approximation, the thin films mainly determine reflectivity and, thus, the amount of energy that is absorbed, whereas the thickness and thermal conductivity of the substrate govern the distribution of heat within the sample and, thus, the temperature profile. Figure 3 displays a typical temperature profile on the front and the backside of a sample as simulated by the COMSOL Multiphysics® software for a 3 ms flash with ED = 100 Jcm−2 applied to a 525-µm-thick Si wafer. The temperature at the surface starts at T0 (RT or the preheating temperature), strongly increases to a maximum Tmax, and decays to a quasi-equilibrium temperature Tequ. In contrast, the temperature on the backside monotonously increases to Tequ. During the flash, surface temperature rises as long as the entry of energy by absorption exceeds the outflow of energy from the surface region via heat conduction, which is why the rising time of the temperature peak and, thus, the effective annealing time can be longer than the pulse length. The temperature dependencies are discussed in detail in [13, 14]. In summary, the increase in sample thickness will usually lead to a decrease in Tmax and Tequ, whereas an increase in thermal conductivity causes a decrease in Tmax and a flattening of the temperature profile.
Finally, there is the question of whether the temperature must be known, and if so, how to determine it. From an application point of view, the knowledge of temperature is not required as long as the FLA process can be optimized concerning certain material properties, e.g., to achieve a certain electrical conductivity or a certain degree of dopant activation without diffusion. However, if the temperature is of interest, it can be either measured by pyrometry or determined by a combination of calibration and simulation [22]. Temperature measurement during FLA by pyrometry is possible but has to meet a couple of challenges, namely the fast time scale, the change of the emissivity curve during FLA and the fact that the flash light outshines that of thermal emission by far. The latter problem can be solved by filtering the main water absorption band at usually 2.73 μm and placing the diagnostic wavelength of a pyrometer there. To do so, the flash light has to pass a quartz plate enriched with OH groups or a water film which concurrently cools the lamp. This concept was realized in a couple of tools [19, 20, 23] but remains a complex issue. Further details about temperature measurements during FLA can be found in [24].
Alternatively, the temperature can be estimated by a combination of calibration and simulation. In order to perform a simulation in the case of thin films on a substrate, the temperature-dependent heat capacity and thermal conductivity of the substrate as well as the absorbed energy density Eabs have to be known. The latter can be estimated by
where RS is the effective reflectance of the sample, TS is the effective transmittance of the sample, and Γ is a correction factor slightly higher than 1 that considers the contribution of multiple wall reflections. RS and TS can be determined by measuring the reflectance and the transmittance as a function of photon energy and convolute it with the flash lamp spectrum. If the latter is not known, it can be approximated by standard spectrum as given in [25]. Γ can be roughly estimated by a melting point comparison. In this case, a thin piece of Si or Ge (or another suitable material) is flashed with increasing intensity up to the point where first melt seeds appear at the surface. Γ is then deduced from the difference between the two values of ED where the melting temperature is reached in the experiment and simulation. Further details about this type of calibration can be found in [22].