Characterization of Polycrystalline Silicon Particles Produced via Cvd from Monosilane in a Fluidized Bed Reactor

Characterization of polycrystalline silicon particles produced in a fluidized bed reactor via CVD from monosilane was carried out It was observed that part of the hydrogen from monosilane remained in the particles. Most of hydrogen was bonded to silicon and temperatures as high as 1300 K were required to dehydrogenate the silicon particles. Particles were contaminated with metal elements which were diffused from the reactor wall, suggesting that the material used in constructing the fluidized bed reactor wall also needed to be controlled to improve the purity of product. Polycrystalline silicon has conventionally been produced by using the bell-jar type reactor in a batch-wise manner. It takes about two weeks to industrially produce polycrystalline silicon via CVD on a Si substrate from SiHC13, leading to low thermal efficiency. Continuous production of polycrystalline silicon particles via CVD from SiHC13 or SiH4 has been proposed to increase thermal efficiency and to develop a continuous CZ furnace system. Several types of fluidized bed reactors for production of polycrystalline silicon particles via CVD have recently been developed/ 1 -7/. In the present study, poly-Si particles were produced in a laboratory-scale fluidized bed reactor in a step-wise manner and properties of produced poly-Si particles were investigated. In particular, characterization in relation to the amount and chemical state of remaining hydrogen was carried out because hydrogen contained in poly-Si particles creates a splashing problem in a crucible for pulling out a single.silicon crystal. Polycrystalline silicon particles were produced in a fluidized bed reactor via CVD from monosilane. The reactor was a 50 mm i.d. tube made of stainless steel. Detailed apparatus and experimental procedure were reported elsewhere/5,7/. CVD experiments to grow particles were carried out at 870 700 K. Ten vol% of SiH4 diluted with hydrogen was supplied into the fluidized bed. The flow rate of feed gas was adjusted to 5 Umf in each run; the actual flow rate was increased with particle growth. Morphology of produced particles was observed by means of Secondary electron microscopy (SEMI. Structural characteristics were measured by X-ray diffraction (XRD) and FT-IR. Temperature-programmed desorption (TPD) experiments for hydrogen remaining in polyArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1991259 C2-484 JOURNAL DE PHYSIQUE IV Si particles were conducted in the temperature range from 773 to 1373 K at four different heating rates of 2.5, 5.0, 7.5 and 10 K min-1 in a stream of argon. Hydrogen was determined by gas chromatograph with a TCD detector. Depth distribution of hydrogen was measured by means of secondary ion mass spectrometry (SIMS, Hitachi IMA-2) using Ar+ ion. The particles were stuck on a sample holder with adhesive silver paste. Then it was mounted in a UHV chamber which was evacuated to better than torr before ion bombardment. The primary ion beam energy was 10 keV and incident Ar+ ion beam flux was 5x103 A m-2. 3.-Results and discussion Poly-Si particles were produced in the fluidized bed via CVD on seed poly-Si particles from SiH4. Particles were taken out from the reactor after each run of 2.5 6 h to remove fine powders by sieving. CVD of silicon on seed particles was repeatedly carried out 9 times. Figure 1 illustrates the growth of poly-Si particles in 9 times of experiments. Seed poly-Si particles having the average particle diameter of 339 pm was grown to 545 pm, namely 3/4 of the product came from CVD by monosilane decomposition. Density of produced poly-Si particles were 2.3 19(+0.00 11x1 03 kg m-3, which was slightly less than 2 . 3 3 0 ~ 1 0 3 kg m-3 of the density of Si metal. Cumulative Reaction Time [hl Figure 1 Growth of Polycrystalline Silicon Particles by CYD from Monosilane. Figure 2(a) shows a typical SEM image for cross section of the produced poly-Si particles. The central part showing in dark contrast was a seed particle and the surrounding part showing in bright contrast was the layer of silicon deposited . Figure 2(b) presents morphology of the boundary between seed particle and deposited silicon. It was apparent that the layer of deposited silicon consisted of assemblages of tiny silicon particles. As typically shown in Figure 2(c), the accumulation of tiny particles was observed on the surface of produced particles as well. FT-IR spectra for 4 kinds of silicon particles are compared in Figure 3. Stretching vibrations of Si-H bond were observed at around 2000 2100 cm-l for the surface of poly-Si particles produced, ,whereas no signal indicative of Si-H bond was detected both for seed poly-Si particles and for poly-Si produced in a conventional bell-jar type of reactor. Another FT-IR spectrum was taken for a sample which was obtained by finely crushing produced poly-Si particles in order to observe the chemical structure of the bulk of the