The advantages and disadvantages of both pulsed and cw scanned laser processing for Si devices are discussed. In particular, adaptions of laser processes for MOSFETs and bipolar structures are described from the viewpoint of active layer annealing. Moreover, MOSFETs fabricated in laser induced grown Si films on Si substrates having SiO2 patterns by bridging epitaxy, are discussed, as a function of sample structures, film formation conditions and laser irradiation conditions.

Over the past ten years, the performance of bipolar transistors in high speed integrated circuits has been steadily improved by fully exploiting the complex solid state diffusion behaviour of dopants in silicon to make successively closer approaches to the ideal dopant profiles in the emitter and base regions of the transistor. Further optimisation requires a new approach, and in this paper an entirely new method of shaping the dopant profiles by exploiting the large ratio (about 108) of diffusion coefficients in liquid and solid silicon near the melting point is described Melting is effected by irradiation with a homogenised pulse from a Q-switched ruby laser. Completely new features of the technique are the controllable fabrication of both quasi–Gaussian and quasi-rectangular dopant profiles independent of dopant (arsenic or boron), dopant concentration (1013−1016atom cm−2) or presence of other dopants. This allows for the first time independent optimisation of emitter profile shape, emitter doping level, and base profile in bipolar transistor structures.

N- and p-channel enhancement-mode MOSFETs have been fabricated in Si films prepared by zone-melting recrystallization of poly-Si deposited on SiO2-coated Si substrates. The transistors exhibit high surface mobilities, in the range of 560–620 cm2/V−s for electrons and 200–240 cm2/V−s for holes, and low leakage currents of the order of 0.1 pA/μm (channel width). Uniform device performance with a yield exceeding 90% has been measured in tests of more than 100 devices. The interface between the Si film and the SiO2 layer on the substrate is characterized by an oxide charge density of 1–2 × 1011 cm−2 and a high surface carrier mobility. N-channel MOSFETs fabricated inSi films recrystallized on SiO2-coated fused quartz subtrates exhibit surface electron mobilities substantially higher than those of single-crystal Si devices because the films are under a large tensile stress.

To improve the dielectric properties of thin oxides grown on polysilicon, a matrix of conventional process variables such as phosphorus doping level and oxide growth temperature, and the inclusion of laser recrystallization of the polysilicon have been investigated. For conventionally processed wafers, highest dielectric strengths were obtained at the highest oxidation temperature (1150°C) and a simultaneous low doping level in the polysilicon. Similarly high dielectric strengths were obtained after cw-Ar laser recrystallization of the polysilicon at much lower temperatures (900°C), independent of the doping level in the polysilicon.

We report an investigation of the effects of laser processing on the thermal oxides of polysilicon. LPCVD polysilicon, 500 nm thick, deposited on 500 nm thermal oxide of single crystal silicon was laser processed at various stages in the process sequence for device fabrication. Effects of CW Ar+ and pulsed 1.06 and 0.53 μm laser processing were investigated. Laser annealed polysilicon was oxidized in a steam ambient. Using a second level of polysilicon, guard ring diode and capacitors were fabricated. Electrical characterization revealed an improvement in breakdown field strengths of these oxides without deleterious effects on any of the associated interfaces.

Phosphosilicate glass reflow to improve step coverage of metallizations in integrated circuits is usualy made by a high temperature heat treatment. A new technique is proposed to achieve this annealing by a transient localized heating with a tunable CW CO2 laser. A selective coupling of the laser power is obtained by choosing a 9.25 μm wavelength which matches the Si-O absorption band at 1100 cm−1. Focusing and scanning the laser beam makes the P-glass flow without excessive substrate heating.

Silicon can be prepared in a unique morphological form by anodic dissolution in HF. The resultant material contains pores of 10–100Å diameter in numbers sufficient toproduce a high surface area, low density single crystal. We have investigated the potential of both pulsed and CW laser processing in this material for dielectric isolation applications. Pulsed processing at λ = 532 nm yields fused surface layer structures that are epitaxial. The porous Si that underlies this surface film is undisturbed and can beoxidized to produce vertical isolation from the substrate.

A characterization of Q-switched Nd-YAG laser induced backside damage in various stages of a bipolar process is presented. TEM-analysis shows the occurence of microcracks, low angle boundaries and dislocations in as-irradiated wafers, of which the microcracks anneal out during an initial oxidation at 1050°C. The other damage is very stable and remains even after a heat treatment at 1200°C. This is quite in contrast with mechanically or Ar implantation induced backside damage, which anneal out completely at such high temperatures. Side effects of laser induced backside damage such as surface roughness and influence on wafer strength are found to be of no consequence within a limited range of laser pulse energy densities where the gettering efficiency is found to be very promising.

Continued study [1] of pulsed electron beam annealing (PEBA) of thin silicon films deposited by LPCVD on silicon substrates has shown that a thin SiO2 interface remains intact during melt. PEBA of ion-implant damage in silicon has been shown to correlate dopant activation with melt depth. Also, PEBA was superior to thermal annealing of ion-implant damage in processing solar cells in some types of polycrystalline silicon material.

CW scanned Argon laser annealing is used to form photo diodes in single crystal Boron implanted germanium. The starting material is doped n–type to 4 × 1015 cm−3 and oriented 2° off the <100>. For annealing, a substrate temperature of 250°C is employed in conjunction with a beam spot diameter of 50 μm and scan speeds in the vicinity of 1 cm/sec. Pyrolytic oxides are deposited to provide a protective and antireflective coating. Devices are fabricated and display dark leakage characteristics of the order of ∼ 10−4 A/cm2 which is comparable to diodes made by more conventional techniques.

CW laser annealing techniques were incorporated into standard MOS/SOS transistor fabrication procedures and found to be advantageous as compared to conventional furnace methods for electrical activation of ion–implanted source/drain dopants for both N- and P–MOS transistors. Static electrical characteristics of 2.4 μm channel–length transistors are similar for both types of annealing, whereas the speed of devices with cw laser annealed source–drain regions is increased 10 to 40%, depending on the operating voltage.

Schottky-diodes on laser recrystallized polycrystalline silicon are studied and compared to diodes fabricated on conventional, fine grained poly-Si. A strong dependence of reverse current on grain size is found and can be quantitatively explained by carrier generation at grain boundaries. In order to reduce the series resistance of the diodes, various methods to create a n+n doping profile in the recrystallized layer are investigated.

Planar p-n junctions with Pt silicide contacts have been formed by laser irradiating p-type Si/120Å Sb/400Å Pt composite thin film structures. The samples were prepared in a standard e-beam evaporation system. They were never ion implanted. Laser processing was performed with either a scanning cw Ar laser or a Q-switched Nd:YAG (λ = 0.53 μm) laser which provided 100 nsec long pulses. After the irradiations were performed, a p-type Si/n+Si/Pt-Si structure was present. The n+ region resulted from Sb incorporated in the Si lattice. When the same initial composite was furnace annealed, the Sb diffused to the surface and sublimated, leaving a p-type Si/PtSi near-ohmic contact. Depth profiles of the samples were determined with Auger electron spectroscopy. The structure of the Pt-Si layer was studied using wide film Debye X-ray diffraction. The electrical properties of the junctions were examined by measuring their I-V and C-V characteristics and minority carrier lifetimes. Differences in the structural and electrical properties that resulted from the two types of laser irradiation will be discussed.

This review article provides an overview of the current understanding of the physical mechanisms involved in the annealing of ion implanted GaAs by thermal techniques and by CW and pulsed laser and electron beams. The successfulness of these techniques is evaluated in terms of the electrical and optical properties of the annealed layers. Promising areas of investigation for the improvement of the electrical properties of n-type layers produced by pulsed annealing are identified.

A Q-switched ruby laser has been used to anneal gallium arsenide which has either been implanted with a donor or coated with a thin layer of material containing the donor. Silicon nitride (∼500Å), germanium (∼50Å), tin (∼50Å) and silicon (∼500Å) have been used and in all cases laser annealing produces n-type doping levels in excess of 1019cm−3. Non alloyed ohmic contacts have been made on these heavily doped layers with specific contact resistances as low as 1.4 × 10−6Ω cm2. These contacts have been used in the fabrication of fine geometry coplanar mixer diodes. Ideality factors of 1.1 have been measured from the d.c. current voltage characteristics and diodes with a total capacitance of O.03pF have series resistances below 10 ohms. Matched pairs of these devices have given a 4.8dB conversion loss at 35GHz in a fin line balanced mixer.

A study of the pulsed laser annealing of AuGe films on GaAs using a Nd:YAG laser has revealed differences between the surface and subsurface morfhologies. At laser energy densities lower than 1.1 J/cm2 , the surface retained the smooth, “golden” appearance of deposited AuGe films, while evidence of damage was observed below the surface. At higher energy densities, surface damage was observed. SIMS profiles of Ga and As in the AuGe layer and a laser heating model have been used to explain the presence or absence of damage in terms of the outdiffusion of As and Ga through the laser created melt which leads to the presence or absence of Ga and As at the surface and below the surface.

The diffusion of Si into GaAs from a thin source has been investigated. Electron beam evaporated Si films were deposited on chromium–doped GaAs wafers. Pulsed irradiation from a Q–switched Nd:YAG laser resulted in the formation of n+ layers. After metal evaporation these layers displayed nonalloyed ohmic contact behavior of very low contact resistance (5×10−7 Ω−cm2 ) and good thermal stability at 300°C. SIMS analysis of these layers revealed a steplike Si profile and significant chromium redistribution within the laser induced molten layer. A major improvement in surface morphology was obtained using Si02 encapsulants.

The surface regions of (100), (110) and (111) oriented single crystals of GaAs have been investigated by the combine techniques of LEED, AES and RBS subsequent to their irradiation in UHV with the output of a Q–switched, ruby laser (0.15−0.8 J/cm2, 15 × 10−9 sec). Clean surfaces, as determined by AES, were obtained after Ar+ sputtering followed by laser irradiation. LEED observations indicate that the degree of disorder in the outermost surface layers remaining after irradiation depends on the crystal orientation. Although the relative intensities of Ga and As Auger transitions in spectra obtained from sputtered and laser annealed surfaces are similar, the differences in lineshape in these spectra of the M2,3M4,5M4,5 Ga Auger transition indicate that in the laser annealed case there are local regions which are nonstoichiometric. These observations are confirmed by RBS results, and this technique has been used to determine the stoichiometry and to characterize the damage in the subsurface region for all orientations.

In an effort to understand the origin of defects earlier found to be present in p–n junctions formed by pulsed laser annealing (PLA) of ion implanted (II) semiconducting GaAs, photoluminescence (PL) studies have been carried out. PL spectra have been obtained at 4K, 77K and 300K, for both n–and p–type GaAs, for laser energy densities 0 ≤ El ≤ 0.6 J/cm2. It is found that PLA of crystalline (c−) GaAs alters the PL spectrum and decreases the PL intensity, corresponding to an increase in density of non-radiative recombination centers with increasing El. The variation of PL intensity with El is found to be different for n– and p–type material. No PL is observed from high dose (1 or 5×1015 ions/cm2 ) Sior Zn-implanted GaAs, either before or after laser annealing. The results suggest that the ion implantation step is primarily responsible for formation of defects associated with the loss of radiative recombination, with pulsed annealing contributing only secondarily.

Experimental results obtained by multiply-scanned electron beam annealing (MEBA) of Si implanted semi-insulating GaAs are presented and compared with results obtained by thermal annealing. A tentative model for Cr redistribution during MEBA treatment is presented which can explain the 100% activation without compensation threshold and the high drift mobility obtained for low Si doses, almost independent of the Cr content of the crystal.