Masanori Akatsuka

Effect of Rapid Thermal Annealing on Oxygen Precipitation Behavior in Silicon Wafers

The effect of rapid thermal annealing (RTA) on oxygen precipitation behavior in Czochralski silicon wafers was investigated with an emphasis on the RTA ambient, temperature and cooling rate. It was found that (i) anomalous oxygen precipitation (AOP) was observed in the case of RTA temperature at 1240°C with cooling rates of 25°C/s in Ar and in the case of RTA temperature at 1200°C with cooling rates of 5°C/s in N2, while AOP was not observed in O2, (ii) an M-like depth profile of precipitate density appeared for the cooling rates of 50°C/s in Ar, and 25°C/s in N2, and (iii) the width of the precipitate denuded zone was larger than the width of outdiffused oxygen in the case of RTA in Ar. The relationships of thermal equilibrium concentrations CJ* and diffusion constants DJ were estimated to be CI* DV (I: interstitial, V: vacancy) from the experimental results of RTA in Ar and the calculated results using the Voronkov model.

Effect of Heavy Boron Doping on Oxygen Precipitation in Czochralski Silicon Substrates of Epitaxial Wafers

The effect of heavy boron doping on oxygen precipitation in Czochralski silicon substrates of epitaxial wafers has been studied with transmission electron microscopy observations and a preferential etching method. Prolonged isothermal annealing between 700 and 1000°C for up to 700 h was performed on p/p+ (5–20 mΩ cm) and p/p− (10 Ω cm) wafers. It was found that, with an increase in boron concentration, the precipitate density increased, and the precipitates could nucleate at a higher temperature. The growth process of platelet precipitates was also investigated and compared with the process in polished p− wafers. It was confirmed that precipitate growth rate in p/p+ wafers was higher than that in p− wafers, and precipitate nucleation in p/p− wafers was delayed compared with p/p+ wafers. The precipitate growth in p/p+ wafers was determined to be reaction‐limited, which differed from the diffusion‐limited growth in p− wafers.

Dependence of Mechanical Strength of Czochralski Silicon Wafers on the Temperature of Oxygen Precipitation Annealing

Dependence of mechanical strength of Czochralski silicon (CZ-Si) wafers on the temperature of oxygen precipitation annealing has been studied both experimentally and theoretically. Thermal stress was applied to CZ-Si wafers after oxygen precipitation annealing at 1100°C or 1000°C after preannealing at 800°C. The warpages and the densities of slip dislocations in the wafers annealed at 1100°C are much higher than those in the wafers annealed at 1000°C, nevertheless each precipitate density is almost equal. Transmission electron microscopy observations of the 1100°C samples showed that both platelet and polyhedral precipitates were generated, but very few of these precipitates actually generated punched-out dislocations. In contrast, in the 1000°C samples, only platelet precipitates were generated, many of which generated punched-out dislocations. Further studies showed that slip dislocations formed only from platelets which did not punch out dislocations, i.e., slip dislocations formed only in the 1100°C samples. The mechanism of the generation of slip dislocation by oxide precipitates is discussed with calculated results of the system energy change due to slip dislocation generation.

High-sensitivity two-dimensional thermal- and mechanical-stress-induced birefringence measurements in a Nd:YAG rod.

A novel polarimeter for measuring the two-dimensional (2D) thermal- and mechanical-stress-induced birefringence in solid-state laser materials such as Nd:YAG is proposed. Using this device, we could sensitively measure the direction of the principal birefringence axis as well as the phase shift δ with sign when δ < π/4. The 2D thermal- and mechanical-stress-induced birefringence in a laser-diode-pumped Nd:YAG rod was successfully measured with the proposed polarimeter. We also found an active quarter-wave Nd:YAG phase retarder.

Pinning Effect of Punched-Out Dislocations in Carbon-, Nitrogen- or Boron-Doped Silicon Wafers

The pinning effect of punched-out dislocations in carbon-, nitrogen- or boron-doped Czochralski-grown silicon wafers was investigated using an indentation method. The size of a rosette pattern which corresponds to the distance of dislocation movement was measured after heat-treatment without any thermal stress. It was found that the rosette size decreased by carbon, nitrogen and boron doping with a concentration of 2.0×1016–1.4×1017, 5.3×1013–5.3×1014 and 2.0×1018–2.0×1019 atoms/cm3, respectively. The rosette size was approximately proportional to the power -1/3 of carbon, -1/10 of nitrogen and -1/10 of boron concentration, which differed from the reported power of -2/3 of oxygen concentration.

Effect of Oxide Precipitate Sizes on the Mechanical Strength of Czochralski Silicon Wafers

The effect of oxide precipitate sizes on the mechanical strength of Czochralski silicon (CZ-Si) wafers has been studied with emphasis on the mechanism of slip dislocation generation by oxide precipitates. Thermal stresses, which are larger than those set up in wafers during actual device processes, were applied to two-step annealed wafers. It was determined by X-ray topography and transmission electron microscopy observations that both platelet and polyhedral precipitates can generate slip dislocations when their size is larger than approximately 200 nm. With further experiments, it is concluded that the precipitates cannot generate slip dislocations during actual device processing when the precipitate size is smaller than 200 nm, and this conclusion is independent of the precipitate density. The stress concentration of compressive thermal stresses applied to oxide precipitates should be the cause of slip dislocation generation. Three critical stress curves of slip dislocation generation were obtained for the wafers, in which the platelet sizes are approximately 70 nm, 200 nm and from 330 nm to 490 nm.

Pinning Effect on Punched-Out Dislocations in Silicon Wafers Investigated Using Indentation Method

The mechanical strength of silicon wafers was investigated using the indentation method. Sizes of rosette patterns L, generated during annealing at 900° C for 30 min, were measured for as-grown floating zone (FZ) and Czochralski (CZ) silicon wafers. It was found that (1) the rosette size L of FZ wafers was larger than that of CZ wafers and (2) L decreases in proportion to the -2/3 power of interstitial oxygen concentration ([Oi]) for CZ wafers ([ Oi]=3.7–15.5×1017 atoms/cm3). From the experimental results, it was concluded that the wafers, in which [Oi] was larger than approximately 2×1017 atoms/cm3, had the ability to pin on dislocation movements. The pinning effect on dislocations by oxide precipitates or stacking faults was also investigated using the indentation method. It was found that precipitates, of which the density was approximately 1×109 /cm3 and the average size was lower than approximately 500 nm, did not affect the rosette sizes L. On the other hand, stacking faults, of which the density was approximately 1×107 /cm3 and the average size was approximately 50 µ m, have shown the pinning effect.

Slip Length in Silicon Wafers Caused by Indentation during Heat Treatment

The relationship between the length of slip dislocation and applied stress in silicon wafers was investigated. Czochralski (CZ) silicon wafers [boron-doped, interstitial oxygen concentration [Oi]=(13.8–14.0)×1017/cm3], indented by a Vickers hardness tester, were thermally stressed by insertion into and withdrawal from a horizontal furnace. The length of slip dislocations generated during the heat treatment was measured by X-ray topography (XRT). To estimate the applied thermal stress during the heat treatment, temperature distribution in the wafer was measured with a thermocouple. From the results of XRT observations, slip dislocations were found to be generated at peripheral and central regions during the insertion and the withdrawal, respectively. The length of slip dislocations was calculated using the experimentally estimated thermal stress. It was found that the length of slip dislocation could be explained well by the model, with consideration of the applied stress and the dislocation velocity.

Parametric Studies on the Laser-Diode-Pumped, Thermal-Lensing-Compensated, Mode-Locked, Q-Switched Nd:YAG Laser

The parametric studies of a laser-diode (LD)-pumped, acousto-optic (AO)-mode-locked and Q-switched Nd:YAG laser were carried out. Adequate compensation for the thermal lensing in the longitudinally pumped laser rod was provided by analyzing the spot size in the astigmatically compensated cavity. The width of the mode-locked pulse with 0.5 nJ output energy per pulse was elongated to 910 ps by means of an intracavity etalon. Good stabilities of the output energy (±3%) and the peak power (±5%) were obtained, which are comparable to those of the flash-lamp-pumped mode-locked Q-switched lasers.

Demonstration of high energy extraction efficiency in a laser‐diode pumped high gain Nd:YAG regenerative amplifier

High energy extraction efficiency of 48±4% for the cavity mode cross section (17.5%±1% for the entire lasant cross section) and high energy gain of 71±1 dB have been demonstrated simultaneously, for the first time, in a laser‐diode pumped Nd:YAG regenerative amplifier (RA). The RA was operated at an initial small‐signal gain of 3.15±0.20, an input pulse energy of 0.5±0.1 nJ (pulse width of 910±12 ps), an amplified output pulse energy of 6.0±0.3 mJ (pulse width <1 ns), and a repetition rate of 50 Hz. The dependence of energy extraction efficiency on initial small‐signal gain of the RA was shown to be well fitted with the theory of Lowdermilk and Murray [J. Appl. Phys. 51, 2436 (1980)].