Vol. 124 (2013) ACTA PHYSICA POLONICA A No. 2

Special Anniversary Issue: Professor Jan Czochralski Year 2013  Invited Paper Czochralski-Grown for Microelectronics A. Bukowski Institute of Electronic Materials Technology, Wólczy«ska 133, 01-919 Warsaw, Poland The of 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). Silicon has a unique characteristics of forming, on its surface, an ultrathin passivating lm. Ther- In the era of 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 (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 was made (1948) was (Ge). In a leading material in electronics due to its energy gap order to produce a germanium , 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, and lead. At the time there was no need whatso- creasing :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: (B), phosphorus (P), anti- acteristics of the method, contributing to its success are mony (Sb) and (As), discussed, taking into consideration the mechanisms of crystallization and technological progress observed in the • 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 (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 (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 (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: -free crystal. ters. Such a large distance ensures a strong reduction of • Usual 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 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 ( 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, 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. There are the magnetic elds described in (c) and (e), are called a few directions of improvement of the growth process of second-generation magnetic elds. large crystals. Up to now, the studies of magnetic eld application The rst one is a group of improvements without have been mostly focused on the congurations (a), (b) changing the construction of the puller, for example using and (c). Their characteristics can be summarized as more than one heater, ameliorating the quartz crucibles, follows. using new types of thermal insulation, constructional im- 4.1.1. Steady magnetic elds  horizontal (a) provements of heating elements, etc. When applying a relatively strong magnetic eld such Other improvements are more complex. Two of them as 0.60.8 T, a signicant decrease of oxygen content in are described below: the crystal is attained. If such strong magnetic eld is used, the processes are dicult to carry out because of • Magnetic-eld-applied Czochralski (MCZ), leading problems related to the ow of heat from the heater to to signicant reduction of the scale of turbulences, the crystallization area. The costs related to the appli- and consequently causing an improvement of single- cation of such magnetic eld are high. When a weaker -crystal quality. magnetic eld is applied (up to 0.05 T) thermal convec- tion was absent due to the rotation of the crucible and Continuous liquid feed method (CLF method), re- • the which is a signicant advantage of this sulting in an improvement of axial homogeneity of conguration [13]. the single crystal. The method enables to reduce the mass of molten silicon. 4.1.2. Axial magnetic eld (b) The temperature layers under the crystallization front are more symmetrical than in conguration (a). Due to 4.1. The Czochralski method with the application this, the growth processes are easier to carry out. The of a magnetic eld homogeneity and oxygen reduction eects are dicult to achieve. Slowing down the oxygen diusion to the The use of a magnetic eld in silicon single crystal surface leads to an increased oxygen content in the grown growth has a long history [1012]. Research in this area crystals. Application of this conguration does not bring and the rst results of experiments were attained for sin- any advantages. gle crystals of 76100 mm diameter. The main aim, a re- 4.1.3. Unipolar magnetic eld (c) duction of the amount of oxygen in the silicon single The lines of the magnetic eld are orthogonal in rela- crystals, has been achieved. For a long time, a steady tion to the bottom of the crucible and to its side. Due magnetic eld set horizontally (perpendicularly to the to this, the largest magnetohydrodynamic pressure oc- crucible axis) to the charge was used. In order to achieve curs at the crucible walls, whereas small magnetic forces the intended results, the intensity of the magnetic eld act on the area around the crystallization front. The was relatively large and amounted to T which 0.6−0.8 drop of oxygen value to the level of 17 at./cm3 was required an additional electric power comparable to the 2×10 achieved with magnetic eld of 0.1 T, which is the best heating power of the furnace, despite using moderate-size result so far. chambers of Czochralski furnace. Besides the above mentioned eects, application of the The method was further developed using dierent con- magnetic eld leads to a visible calming of the liquid gurations of magnetic eld, i.e.: surface. When the charge is large, vibrations of the ex- (a) Horizontal magnetic eld (mentioned above); tended liquid surface occur as a harmful phenomenon. Elimination of the vibrations is an additional advantage (b) Axial magnetic eld (magnetic ux is parallel to of magnetic eld application. the axis of the furnace and of the growing crystal); 4.2. Continuous liquid feed method (c) Unipolar magnetic eld (the lines of the magnetic eld are perpendicular to the walls and to the bot- Research regarding this method originates from the tom of the quartz crucible). This eect was at- needs to achieve savings in the use of the quartz cru- tained thanks to the interaction of two independent cible [12, 14]. Continuous feeding of the crucible with axial elds; silicon leads to crystals of a mass larger than the mass of original charge. Theoretically, when using the continu- (d) Magneto-hydro-dynamic-rotor (MHD); ous feeding, the crystal length is limited only by the time during which the quartz crucible remains intact. In prac- (e) Steady (axial) magnetic eld with a simultaneous tice, several silicon single crystals of total 100−200 kg interaction of MHD conguration. The aim is to mass are grown in a series of processes proceeded with- break down the cells of ow circulation which are out cooling the furnace. The furnace may be charged formed in the axial eld using the MHD congura- with polycrystalline rods, granules or silicon powder as tion. well as with molten silicon. 238 A. Bukowski

A practical problem that arose, regarded the process pected to take place soon, as multiple possible magnetic- of a continuous feeding of melted silicon which caused -eld settings (especially the second generation elds) perturbations in the process of crystal growth. So far, give a big choice of congurations. Moreover, for mag- this method is used to produce silicon for solar applica- netic eld generations, superconducting magnets at liq- tions, where the requirements concerning crystal quality uid helium temperature can be used with a reduced power are not high. If the perturbation problem was solved, this cost. Good quality of silicon crystals produced using the method would become of interest for the single-crystal MCZ method prove that its application is eective. In growth task. Furthermore, besides savings in the use the CLF method the main eorts regarding its imple- of the quartz crucible, this method opens the door to mentations and improvements are focused on the use of achieve a high homogeneity in axial distribution of the new materials, including, in particular, new crucible ma- dopant. The silicon feed which does not contain a dopant, terials. will dissolve the concentrated dopant in the liquid found in the crucible. In the case of dopants with a decom- References position coecient k < 1, a constant concentration of, e.g., phosphorus in silicon crystal may be expected. This [1] Handbook of Semiconductor Silicon Technology, Eds. phenomenon had been conrmed by experimental data. W.C.O. Mara, R.B. Herring, L.P. Hunt, Noyes Pub- Crystallization at the conditions of strongly reduced lications, Park Ridge (NJ) 1990. volume of molten silicon in the crucible may become an [2] H.R. Hu, J. Electrochem. Soc. 149, S35 (2002). even more valuable modication of the method. Here, the [3] L. Arnberg, M. Di Sabatino, E. Øvrelid, JOM, 38 crucible will be fed with a silicon feed. In this case, the (October 2011). volume of the molten material will be a classical zone [4] G. Fisher, M.R. Seacrist, R.W. Standley, Proc. IEEE and the crystallization will be subject to zone crystalliza- 100, 1454 (2012). tion instead of conventional Czochralski directional crys- [5] P. Tomaszewski, On Jan Czochralski, Atut, Wrocªaw tallization. Consequently, the dopant distribution will be 2012 (in Polish, the English translation in prepara- determined by a dierent law. Up to now, such a version tion). of the CLF method has not been reported, but the pre- [6] Elementary Crystal Growth, Ed. K. Sangwal, Saan requisites of such a method seem to be justied. The Publ., Lublin 1994. advantage of this version would be to achieve ve times [7] D.T.J. Hurle, B. Cockayne, in: Handbook of Crystal smaller volume of liquid in the crystallization area than Growth, Vol. 2A, Basic Techniques, Prentice-Hall, while using the conventional method. Englewood Clis 1994, Ch. 3. Such or other improvements are expected to be devel- [8] A.A. Wheeler, in: Handbook of Crystal Growth, Vol oped in future. They can be concentrated around new 1b. ed. D.T.J. Hurle, North-Holland, Amsterdam crucible materials (i.e. silicon nitride, carbon based ma- 1993, p. 683. terials, other materials and their combinations, including [9] W.G. Pfann, , Wiley, New York 1958. modied quartz). Using such new materials may be fruit- [10] M. Ohwa, T. Higuchi, E. Toji, M. Watanabe, ful for the success of the CLF method. K. Homma, S. Takasu, in: Semiconductor Silicon, Eds. H.R. Hu, T. Abe, B. Kolbesen, The Electro- chemical Society, Pennington, NJ 1986, p. 117. 5. Summary [11] L.N. Hjellming, J.S. Walker, J. Fluid Mech. 182, 335 (1987). In the article, the currently applied Czochralski tech- [12] A. Bukowski, P. Zabierowski, Elektronika 40, 10 nique of silicon single crystal production is presented. (1999) (in Polish). Multiple advantages of the method led to production of [13] A. Bukowski, P. Zabierowski, W. Grzejszczyk, such crystals at an industrial scale. The increase of di- R. Nowak, Method of Obtaining the Silicon Sin- ameter of produced single crystals from 76100 mm to gle Crystals in the Czochralski Crystalization Device, 200300 mm has been accompanied by various improve- Patent A1(21) 288359 (22) 90 12 20 5(51) C33B ments, such as magnetic eld application and continuous 15/00, 13.01.1994 (Polish patent). liquid feeding. It is expected that such improvements [14] Leybold System Presentation  Silicon'96, Fifth Sci- will be introduced into commercial production practice. entic and Bussiness Conf., Roºnov pod Radho²t¥m In particular, the introduction of magnetic elds is ex- (Czech Republic) 1996.