Yarıiletken silisyum üretim teknolojisi

Abstract

Yarıiletken malzemelerin üretimi, bugün "kristal büyütme" adı altında incelenmektedir. Elektronik sanayiinin can daman olan yarıiletkenlerin kullanıla cakları alana göre sahip olmaları gereken kristal yapı özellikleri, kristal büyütme işlemini etkileyen tüm parametrelere bağlı olmaktadır. Gelişen teknoloji ile bu malzemelerin giderek daha yüksek özelliklere sahip olmaları istenmektedir. Kristal büyütme işleminin fiziğinin incelenmesi ve bu doğrultuda yapılan deneyler, üretilen malzemenin teknolojik özelliklerinin arttırılması yönünde yapılan çalışmalardır. Bu çalışmada, en çok üretilen yarıiletken malzeme olan silisyum başta olmak üzere yarıiletken malzemeler, kullanım alanları ve üretim yöntemleri ele alınmıştır. Yarıiletken kristal üretim yöntemlerinin fiziği incelenmiş ve üretim teknolojisindeki son gelişmeler aktarılmıştır. Ayrıca en çok uygulanan üretim yöntemi olan katılaş tırmanın incelenmesinde büyük fayda sağlayan bazı sonlu elemanlar yaklaşımlarına ve Czochralski kristal çekme yönteminin bir sonlu elemanlar analiz programı olan ANSYS ile yapılmış olan işlem esnasındaki sıcaklık ve ısıl gerilme analizine de bu çalışmada yer verilmiştir.

Semiconductor materials belong to the most progressing part of engineering materials. Since their application to the electronic technology 40 years ago, many achievments have been made and the application field of these materials becomes larger steadily. Integrated circuits, different kind of diodes, solar energy conversion and optoelectronic application are some of these fields. The discovery of semiconductors is recognized as one of the biggest impacts in the technological development of the 20th century in the creation of the solid state electronic industry. First microwave (radar) detectors were fabricated, directly followed by electrical diodes. Then the invention was made of the transistor effect in 1949 by Bardeen and Brattain, which, together with the theoretical work of Schokley gave insight into the role of electron and holes in the solid. Todays electronics is almost exclusively based on devices made from silicon- both in low power and in high power electronics. Silicon has also a number of qualities that make it one of the most desirable materials for terrestrial photovoltaic applications. Some of these are its abundance, established technology base, favorable electrical transport properties, relative salubrity, unique oxidation characteristics and demostrated high photovoltaic energy conversion efficiencies. In order to use a semiconductor for an electronic application, there are some principal characteristics, which have to be fullfilled by the semiconductor material. According to this fact, the objective of the manufacturing techniques is to obtain a single cristal with uniformly distributed solutes a tolerable range of impurities and a low number of defects in its crystallographic structure. The techniques for manufacturing silicon single crystal can be devided in four main groups: 1) Chemical Vapor Deposition 2) Vapor-Liquid-Solid Technique 3) Solution Growth 4) Solidification xiv But before the application of these techniques, metallurgical grade silicon has to be purified to make it applicable for them. Metallurgical grade silicon is obtained from the are furnace, where silicon is reduced by carbon according to the reaction: Si02 + 2C -> Si + 2CO The arc furnace process produces metallurgical grade silicon 98-99% pure. Semiconductor-grade silicon requires purity levels at least in the low ppma range, and many impurities must be in the ppba range. Therefore, purification of the silicon is necassary. To purify it, the metallurgical grade silcon is first converted to trichlorsilane (SiHCl3) by reacting it with HC1 in a fluidized bed at temperature of about 300°C : Si + 3HC1 -> SİHCI3 + H2 The liquid trichlorsilane has a boiling point of 33 °C and can be purified by distillation easily. The purified liquid trichlorsilane must now be converted back to high purity elemental silicon. This is done by passing it and hydrogen over the surface of hot silicon rods where decomposition and chemical vapor deposition take place according to : SİHCI3 + H2 -» Si + 3HC1 This process is carried out at temperatures in the 900-11 00 °C range. The silicon forms in a cylindrical shape at a rather slow deposition rate (< lmm/n). Major residual impurities in such "polyrods" are carbon and oxygen at about 1018 atoms/cm3. Long rods of this type are used as feed material for floating zone crystal growth, while large diameter pieces and broken chunks are used in Czochralski growth. 1) Chemical Vapor Deposition (CVD) Technique : CVD is a technique by which a solid material can be deposited on a support, from a gaseous state. Although a lot of devices are made on a silcon wafer by indif fusion, ion implantation, proton bombardement and oxidation, there are still a number of applications where CVD is useful, as, for instance in high performance diodes. The growth mechanism can be explained briefly in such way (Fig.l): First the silane-hydrogen gas mixture is transported through a cell to a hot zone. Diffusion takes place through the gas phase to the hot crystal surfaces coupled with chemical reactions as the gas is heated up. A chemical equilibrium mixture is formed close to the surface. Then adsorption on the crystal surface, surface diffusion to step sites and incorporation in the lattice takes place. Finally the reaction products are carried away from the crystal surface. xv SİH4 2 I Diffusion SIH4 -» H2 ? SIHj Fig. 1. Schematic view of the growth of silicon from silane by CVD. For a proper growth following conditions are essential. The transport should be laminar, otherwise not only diffusion of reactants will determine the growth rate but also convective flow. This will give rise to growth non-uniformity. Crystal growth should take place by a step growth mechanism, i.e., by surface diffusion of the adsorbed growth species over the low index plane to the steps on the surface. Only in this case defectfree growth and a controllable doping can be attained. Finally the coverage of the crystal surface by impurities should be low. The growth of silicon by CVD is still open for improvements. Lower growth temperatures are prefered requiring alternative growth techniques. High vacuum growth, plasma and laser assisted growth are serious possibilities here although their mechanisms are still not fully understood. 2) Vapor-Liquid-Solid (VLS) Growth : The VLS technique, which was developed by Wagner, contains a liquid and a solid phase reaction. The production of whiskers has been performance by -this method. Being quite a new mechanism for crystal growth, experimental work has been attained to grow silicon single crystal. One of its advantages is to permit the growth of crytals on localized parts. 3) Solution Growth : In order for solution growth to occur the solute species must move from the crystal surface, move to a step, and finally move to a kink on the step. If ionic xvi species are involved then hydration and chemical reaction may also be required. The latent heat of crystallization must also move back into the solution, since in solution growth the crystal is generally surrounded by the solution. The movement of solute from the bulk solution to the crystal surface occurs by a combination of molecular diffusion and convection (solution movement) under the general title of mass transfer. The processes that occur on the surfaces are commonly named "surface integration". In order to characterise a solution growth process, these two steps have to be studied. The conventional solution growth technology is based on some techniques, which can be devided into three main groups, namely seeding, producing supersaturated solutions and stirring the solution. There has been made progress on growing silicon from solutions on amorphous and on patterned amorphous substrates. 4) Growth from melt : Growing single crystals from melt through solidification is todays the leading technique in the production of single crystal silicon. There are two main methods which use solidification, namely the floating zone technique and Czochralski technique. The high melting point of silicon (1412°C) is a main problem for crystal growth, because at these temperatures it significatly reacts with all container materials. In this case the floating zone technique offers an advantage for there is not any need for a crucible in this method. A molten zone is passed through the material so that the polycrystalline part becomes single crystal. The molten zone is stabilized by the surface tension force. In order to reduce the gravitational effects, the growth direction is downwards. Due to the fact that most of the solutes in silicon melt have less solubilities in the solid state silicon, solute is rejected from crystal into the melt during the solidification process. After a single pass of the molten zone the solute distribution in the crystal will have an increasing character. This enables the zone melting techniques like the floating zone technique to be used also as a purification method. Commercial floating zone crystals are grown dislocation-free in lengths of 1 m and with diameters over 10 cm. The mostly used solidification technique is still the Czochralski method. The basic principle of the pulling of single crystals by the Czochralski method is illustrated in Fig. 2. A melt is held in a crucible, a seed crystal is first dipped into the melt and is then slowly withdrawn vertically to the melt surface, whereby the liquid silicon crystallizes at the seed. This method is named after J. Czochralski who established it in 1916 to determine the crystallization velocity of metals. The heating can be applied by resistive heating of carbon electrodes or by RF induction, where a carbon succeptor must support the crucible, which is usually made from vitreous silica, in order to couple with the RF field. The pulling atmosphere is usually Argon gas and the pressures vary between 10-1000 mbar. After melting xvi i Fig. 2. Czochralski crystal pulling. crystal is dipped onto the melt. However, at each dipping stage, dislocations are generated in the seed by thermal shock and surface tension effects. These dislocations are subsequently eliminated by using Dash technique, where the growth speed is highly increased to obtain a thin crystal in the beginning stage, so that dislocations slip to the periphery of the crystal and then decreasing the speed to obtain the regular growth. The crucible position relative to the heater, the pulling speed, the rotation of crucible and crystal respectively are the mostly affecting parameters which influence the quality of the produced crystal. Today, commercial Czochralski crystals are grown up to 200 mm in diameter, 2 m in length and up to 50 kg in weight. The grown crystals have to be measured to obtain their characteristics. The orientation of the crystal, dopant and impurity leves and distribution, defects in the crstallografic structure are the characteristics of a silicon crystal which have to be measured. Optical methods, X-ray diffraction and different electron beam techniques including electron microscopy are the most conventional testing methods. The objective of the design and control of a solidificaiton system such like floating zone or Czochralski metod, is the growth of a constant radius crystal with uniformly distributed dopants and impurties and a low number of crystal defects. The interactions of heat transfer, interfacial phenomena, hydrodynamics in the melt and dopant transport that occur in the system all influence these objectives. Because the high temperatures of typical solidification systems prevent the use of many types of sensors during crystal growth, large-scale numerical simulations of the transport processes in such systems provide the only means for probing these complex couplings. The finite element method offers a quite applicable solution for treating the numerical simulations. In the past 15 years, many works have been performed based on this idea, especially in modelling the floating zone and Czochralski growth systems. The restricting assumptions in the first works become less steadily by time. xvi n The source of dislocations in a Czochralski silicon is mostly the stresses due to the thermal gradients. In this work the temperature and thermal stress distribution with increasing crystal radii is investigated with a finite element analysis software program, ANSYS. The heat transfer in the melt and in the crystal is assumed pure conduction. Heat loss from crystal and melt surface occurs with convection and radiation from the free surfaces. It is obtained from the results that increasing crystal radii, give an increase on the radial temperature differences and a decrease on the axial temperature differences. The equivalent stresses are calculated according to von Mises criterion and it is observed that increasing crystal radii give an increase on the thermal stresses, which have a maximum on the outer periphery of the crystal. The equivalent stresses are in a range of 0-15 MPa and the maximum stress increases with increasing crystal radii. This shows us that the probability of the dislocation formation at the outer periphery of the crystal becomes higher with increasing crystal radii. Furtherwork may be done by adapting the coupling of heater temperature and crystal radius, defining the shape of the interface and by adapting mass transfer and melt flow to the model.