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Czochralski-Grown Silicon Crystals for Microelectronics A

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Czochralski-Grown Silicon Crystals for Microelectronics A Vol. 124 (2013) ACTA PHYSICA POLONICA A No. 2 Special Anniversary Issue: Professor Jan Czochralski Year 2013 Invited Paper Czochralski-Grown Silicon Crystals for Microelectronics A. Bukowski Institute of Electronic Materials Technology, Wólczy«ska 133, 01-919 Warsaw, Poland The Czochralski method of crystal growth is used since 1950s in scientic and industrial laboratories for growth of single crystals of large size and high quality. The article presents the general characteristics and selected improvements of the Czochralski method, and discusses its meaning and advantages in growth of silicon single crystals playing a key role in microelectronics. DOI: 10.12693/APhysPolA.124.235 PACS: 81.10.Fq, 85.40.e 1. The Czochralski method of silicon resources (the primary resource of silicon is SiO2). crystallization Silicon has a unique characteristics of forming, on its surface, an ultrathin passivating oxide lm. Ther- In the era of semiconductor electronics, monocrys- mal conductivity of silicon (140163 W/(m K)) is higher talline silicon remains a basic and widely used material in comparison to other semiconductors (GaAs 46 [14]. It will probably continue to dominate in the world 55 W/(m K), SiC 1655 W/(m K)). This property is of of electronics for many years to come. The rst material particular signicance as it leads to (advantageous) heat in history, though, on the basis of which the invention of dissipation at semiconductor junctions. Silicon became a transistor was made (1948) was germanium (Ge). In a leading material in electronics due to its energy gap order to produce a germanium single crystal, the molten (1.1 eV at 300 K) which guarantees the work of semicon- germanium was held in a graphite crucible, and the crys- ductor devices at high temperatures. tal grew from a seed initially immersed in the liquid and later slowly pulled upwards. This method of crystalliza- 2. Growth process of silicon single crystals: tion was described in 1916 by Jan Czochralski, a scien- basic parameters tist and inventor (for his life and scientic achievements, see [5]). During the rst years after invention, Czochral- From the point of view of crystallization, the Czochral- ski focused his studies on determination of the rate of ski method is classied as a directional crystallization metal crystallization. In later investigations, he proved process. During growth, the total mass of the mate- that materials produced using this method are single rial (liquid plus crystal) is conserved. If the crystal is crystals. They were not single crystals in today under- doped, then the dopant concentration varies along its standing, though, they were threads of such metals as axis. The distribution of the dopant together with in- zinc, tin and lead. At the time there was no need whatso- creasing solid:liquid proportion is described by the fol- ever for monocrystaline materials, as all materials indus- lowing formula [9]: k−1 trially exploited at that time were polycrystalline. Prop- C = kC0(1 − g) ; erties were studied such as textures, martensitic struc- where dopant concentration, coecient of tures; recrystallization, mostly in order to improve the C k dopant distribution, concentration of the dopant mechanical properties. C0 before the beginning of the process, g the crystallized Returning to the subject of this article, it must be material to charge ratio. pointed out that despite technological progress and de- General characteristics of commercially grown silicon velopment in the examination of semiconductor devices, single crystals includes the data on typical dopants, crys- the Czochralski crystallization method applied to semi- tal orientation, growth rate and other technological pa- conductor materials, in particular to silicon, remained es- rameters as well as on the quality and purity: sentially the same since the 1950s. It is commonly called Technological parameters: the Czochralski method (for a detailed description of the method see, e.g. [68]). On the following pages, the char- • Typical dopants: boron (B), phosphorus (P), anti- acteristics of the method, contributing to its success are mony (Sb) and arsenic (As), discussed, taking into consideration the mechanisms of crystallization and technological progress observed in the • Crystal growth directions: h111i, h100i, h110i, last decades. The discussion is focused on crystallization h112i, of silicon, a material which became irreplaceable in elec- • Atmosphere: most often argon (Ar). Gas ow rate tronics and microelectronics. At the present time, silicon of 1550 l/min most often in exhaust vacuum of crystals are produced in tens of thousands of tons, annu- 10200 Tr. ally. The broad use of silicon is due to both, its physical and technological characteristics as well as availability of • Rate of crystallization: 0.52.0 mm/min, (235) 236 A. Bukowski • The charge and crystal diameter: up to 150 kg and such as: high vacuum technology, methods of pure argon 300 mm, respectively, production, methods of tightness assessment of installa- tions and furnaces, electronic solutions of temperature • Homogeneous temperature distribution: achieved stability processes, stability of movement parameters of through both, the crucible rotation (usually the pulling rod, computerization of processes etc. 320 rpm) and rotation of pulling rod (with a Si Taking into consideration the above requirements the seed attached) rotation (usually 530 rpm), fulllment of which is not easy, the question remains: what are the advantages of the Czochralski method? • Crucible material: usually pure melted quartz (rarely synthetic quartz), There are only three methods of crystallization: the Czochralski method (Cz), the oating zone (FZ) method • Material of heater and container for quartz crucible: and the Verneuil method (MV), which fulll the con- high purity graphite, dition that the growing crystal has no contact with the crucible material. Among these three methods of crystal- • Liquid level during growth: typically, automated lization, two (Czochralski and oating zone) are useful for maintenance of a xed level of the liquid is applied. growth of silicon single crystals and ensure full repeata- bility of the growth process. The area of contact with the Characteristics of grown Si crystals: wall of the crucible is usually outlying from the area of crystallization and varies from a few to over ten centime- • Typical structural quality: dislocation-free crystal. ters. Such a large distance ensures a strong reduction of • Usual carbon content: nucleation centers formation. < 2×1016 at./cm3, Another particular feature of the Czochralski method is the control of the diameter of the single crystal not only • Usual oxygen content: by changing the power of the heater, but also by chang- 17 17 3 5×10 < O2 < 9×10 at./cm . ing the pulling rate or by changing the rotation speed of crucible or of the crystal. In the case of changing the pulling rate, the crystallization heat is emitted or ab- 3. Silicon crystal growth process requirements. sorbed, inuencing the diameter of the growing crystal. Characteristics of the Czochralski method This phenomenon does not occur when material crystal- lizes using a dierent method based on direct crystalliza- Silicon (melting point 1415 ◦C) reacts with oxygen and tion in a crucible. Appropriate manipulation or applica- water vapor if they are present, even in trace amounts, in tion of programming of the above-listed parameters not the furnace atmosphere. It also enters into a live reaction only changes the diameter of the growing crystal, but with crucible materials. also activate or limit the nucleation centers. In order to avoid oxidation of the charge, it is neces- sary to use a protective atmosphere, i.e. argon or he- lium with minimized oxygen and water vapor content. 4. Improvements of the Czochralski method Molten silicon is an aggressive material, which dissolves applied to silicon the quartz crucible. The quartz crucible is a source of oxygen created due to the reaction of molten silicon with Modern microelectronics requires silicon single crystals quartz. The dissolved oxygen partially leaves the liq- of a standard 200 mm diameter (the state of the art tech- uid and moves to the furnace atmosphere, and forms the nologies are starting to use the crystals of 300 mm diam- silicon monoxide. During the growth process, precipi- eter). The mass of 1 m long crystals exceeds 50 kg. The tate nucleation centers from foreign phases (i.e. silicon quartz crucibles used in the growth process have a diam- monoxide) aect the growth process. When such centers eter of 182400 (≈ 450−600 mm). With increasing silicon exist, the crystal continues to grow as a polycrystal. To charge, disadvantageous phenomena tend to occur, such ascertain a successful growth process, the formation of as: the nucleation centers must be minimized. A particular solution which reduces the nucleation centers formation • Thermal turbulences appear in molten silicon of a is the direction of argon ow through the furnace. The large mass; gas is forced through the furnace against the convection Overheating of quartz crucibles is observed for cru- stream, it is injected as if the smoke into a chimney and it • cibles of increased diameter and when the distance is removed from the bottom part of the furnace using the from the wall of the crucible to the side of growing suitable vacuum pumps. This is a particular solution the single crystals is large; aim of which is to remove the nucleation centers (due to the presence of silicon vapor, silicon monoxide, and vapor • The large amount of molten silicon in itself has a of dopants such as phosphorus, antimony and other con- negative inuence on the crystal puller; taminations occurring at the crystallization front) from the area of crystallization. Facing the above dened dif- • Large electric power is needed to maintain a large culties requires application of various high-technologies mass of silicon in molten state. Czochralski-Grown Silicon Crystals for Microelectronics 237 These factors entail large costs and cause diculties in Due to the use of two congurations simultaneously, maintaining the high quality of single crystals.
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