Characterization of pyrolytic boron nitride for semiconductor materials processing

6

Journal of Crystal Growth 106 (1990) 6—15 North-Holland

CHARACTERIZATION OF PYROLYTIC BORON NITRIDE FOR SEMICONDUCTOR MATERIALS PROCESSING A.W. MOORE Union Carbide Coatings Service Corporation, Parma Technical Center, 12900 Snow Road, Parma, Ohio 44130, USA

A description is given of the properties of pyrolytic boron nitride (PBN) which make it an attractive material for the processing of semiconductor materials. These properties include purity, chemical and thermal stability, and the thermal and mechanical properties and their anisotropy. Present knowledge of the structure—morphology—property relationships in PBN has enabled us to improve the performance of this anisotropic material in PBN containers for crystal growth and metal evaporation. Boron nitride decomposes into nitrogen and boron at about 2500°C; but at lower temperatures, the dissociation of nitrogen from PBN is much less than that expected from thermodynamic equilibrium. This permits PBN to be heated to high temperatures in ultra-high vacuum without decomposition. Ill—V and Il—VI semiconductor compounds of very high purity can be grown in PBN because of its purity and thermal stability. Carbon is the principal impurity (50—250 ppm), but its concentration can be reduced by an order of magnitude if necessary by baking the PBN in ultra-high vacuum. The total metallic impurity content is usually less than 10 ppm. The favorable properties of PBN have led to a number of important applications in the processing of semiconductor materials. New information is being obtained on the purity and structure/property relationships in PBN which will permit the manufacture of even better products for future needs.

1. Introduction The structure, properties, purity, and chemical inertness of pyrolytic boron nitride (PBN) make it an attractive container material for elemental purification, compounding, and growth of cornpound semiconductor crystals. Examples include crucibles for liquid-encapsulated Czochralski (LEC) and vertical gradient freeze (VGF) growth of GaAs and other 111—V and lI—VI compounds, and evaporation crucibles for deposition of metals and dopants at high temperatures and ultra-high vacuum by molecular beam epitaxy (MBE). Recently, PBN has been used as a container for growth of GaAs crystals by a liquid encapsulated vertical zone melting process. GaAs crystals with extremely low carbon content have been produced In LEC furnaces where the graphite furnace parts were coated with PBN. A PBN—PG resistance heater has been developed which extends the capabilities for growing compound semiconductor crystal layers by rapid thermal processing in metal organic chemical vapor deposition (MOCVD) and other epitaxial processes. In view of the successful use of PBN in these applications, it seems ap0022-0248/90/$03.50 © 1990

—

propriate to review the latest information on the properties and purity of PBN so that crystal growers may achieve maximum benefit from using this material. The present paper provides some new information on the structure, properties, and purity of PBN used in semiconductor materials processing.

2. PBN deposition process Although many papers and patents have been published on the preparation of PBN, the presentday commercial processes are based on the original patent by Basche [1,2]. Existing processes and techniques have been developed mainly by empirical methods. There have been some studies of the preparation of PBN in cold-wall furnaces, i.e., on heated substrates in thermally uninsulated furnaces [3,4], However, commercial production of large quantities and sizes of PBN requires a hotwall, i.e., thermally insulated furnace, in which the reactant gases are fed into a graphite hot zone which is heated by resistance or induction.

Elsevier Science Publishers B.V. (North-Holland)

A. W Moore

/ Characterization

of PBN for semiconductor materials processing

7

PSNICVD

GAS

VACUUM ~UM~AGE

GRUIIITE MANO~L

.

p~T

ELECTRICAL POWER

IA

___ Fig. I. Schematic of PBN deposition process.

A simple diagram of the PBN deposition process is shown in fig 1. The usual reactant gases for preparation of PBN are boron trichloride and ammonia. Nitrogen diluent gas is often used. Other boron compounds, such as boron trifluoride or diborane, are sometimes used for PBN preparation. Deposition temperatures for commercial PBN are usually in the range 1800—1900°C, and pressures are kept below 1 Torr. There are a few cases where PBN is produced at lower temperatures, such as the coating of ceramic fibers at temperatures below about 1100°C to improve the toughness of ceramic composites [5], but these applications are in the early development stages.

3. Structure and morphology of pyrolytic boron nitride Boron nitride, like graphite, exists as a hexagonal crystal with a large difference in the separation of atoms in the layers (1.45 A) and between the planes (3.33 A). The boron nitride structure consists of alternating B and N atoms in the hexagonal planes and also alternating B and N atoms along the crystallographic c-axis. The layer stack-

ing (aaaa...) differs from that of graphite (abab...) [6]. Boron nitride and graphite made by chemical vapor deposition (e.g., 1700—1900°Cfor PBN and 1900—2200°C for graphite) exhibit stacking disorder which results in an average interlayer spacing 2%—4% higher than that of the ideal hexagonal crystal, i.e., 3.42—3.44 A versus 3.354 A for graphite [7] and 3.40—3.46 A for PBN versus 3.33 A for ideal hexagonal BN [3]. These materials also show a high degree of preferred orientation of the layer planes which are predominantly parallel to the deposition surface. This is observed from X-ray preferred orientation diffraction (rocking curve) measurements. Until recently, it was accepted that PBN and pyrolytic carbon were analogous in their structure and morphology. However, Matsuda et al. [3] using a cold-wall CVD reactor, and Moore and Strong [8] using hot-wall reactors of the type used for commercial PBN production, identified among PBN samples made at or near normal deposition conditions two structure/ morphologies which have never been observed in as-deposited pyrolytic carbons. The three types of structure/ morphology which have been observed in PBN

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A. W. Moore

/ Characterization

of PBN for semiconductor materials processing

made from mixtures of boron trichloride and ammonia are shown in fig. 2. One of the structures (arbitrarily labelled Type I) is purely turbostratic (c0/2> 3.40 A) like that of pyrolytic carbon. A second structure (Type II) is more crystalline (c0/2 = 3.34—3.35 A)3). andBoth is found in PBN high of these PBNoftypes density (>2.20 g/cm exhibit a single Gaussian-shaped peak in the intensity of the X-ray preferred orientation distribution or rocking curve as it is commonly known. A third type of PBN (Type III) shows two or three peaks in the rocking curves, in which the outer peaks are separated by 70°—75°. These outer peaks are due to a columnar crystalline component in the PBN, probably caused by twinning [8,9]. Under certain conditions, it is possible to obtain a strong enhancement of the crystallinity and orientation of PBN with this morphology as shown in fig. 3. The above-mentioned structure/ morphology variations in PBN cause variations in PBN properties as might be expected because of the highly anisotropic nature of this material. For example, the thermal expansion of PBN normal to the deposition surface, i.e., predominantly parallel to the hexagonal c-axis, is much larger than the expansion parallel to the deposition plane. PBN exhibiting two or three peaks in the X-ray rocking curve is effectively more misoriented than the other types of PBN and, therefore, exhibits a higher thermal expansion coefficient in the deposiTYPE I STRUCTURE DT 1680°C 1.94 g/cc

TYPE ifi STRUCTURES DT 1890°C DT 1880°C 2.17 glee 2.14 glee

TYPE II STRUCTURE

110°

2 8°

82° 8°

1 ~°

I 0° +45°

Fig. 3. X-ray (002) rocking curve for experimental sample 8511.

tion plane direction. The delamination and fracture behavior of PBN are also affected by the structural and morphological variations. Although most PBN is made up of a mixture of Type III + I or Type III + II structure/morphologies, the vanations determine whether a curved PBN shape, such as a crucible, is more likely to delaminate or to form cross-plane cracks as a result of thermal cycling.

DT 2000°C 2.22 g/cc

Col2~°3.42ACo/2=3.38A Co/2’°3.39A Co/2=3.34A

60°

73°

110°

4. Properties of PBN The properties of PBN most relevant to its uses in growing and doping semiconductor crystals are its chemical stability, thermal stability, and its thermal and mechanical properties and their anisotropy. Table 1 shows that the thermal and

61

mechanical properties of PBN are highly aniso-45

0 +45

45

~

•45

~~45

•45

45

,45

DEGREES TILT FROM DEPOSITION PLANE

Fig. 2. X-ray preferred orientation diffraction ((002) rocking curves) in Lab-grown PBN.

tropic, as would be expected based on the crystal structure of boron nitride and the high degree of preferred orientation in PBN. This anisotropy provides advantages and disadvantages. For ex-

A. W. Moore

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Characterization of PBN for semiconductor materials processing

ample, the basal planes of the hexagonal BN crystallites are predominantly parallel to the deposition surface so the PBN surface (e.g., a cruci ble wall) exhibits high chemical and mechanical stability compared with the edge surface or the surface of hot-pressed BN. On the other hand, the anisotropic thermal expansion of PBN can cause large stresses in curved pieces at temperatures below the deposition temperature. A study on pyrolytic graphite (PG) [10], a material with stmi lan structure and properties, showed that the tensile stress parallel to the layers at the inside of a PG tube with wall thickness/radius ratio of 0.1 could

I -_____________

T

Table I Properties of pyrolytic boron nitride ________________________________________________ Property Units a Direction c Direction Thermal expansion (RT to 1500°C) Thermal conductivity at RT at 800°C Tensile strength at RT Tensile strength at 2200°C Flexural strength atRT Young’s modulus at RT

%

W/m.K W/m.K

0.20—0.65

5.0

63 63

1.5 2.9

MPa psi

41 6000

2.8 410

MPa psi

103 15000

No data

MPa rsi

83 12000

No data

22 3

No data

GPa 10~psi

TABLE ~ AFML DATA ~ DREGER DATA ~GEND ______

______

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______

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// ______

_____

_____

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______

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______

______

______

______

______

______

______

______

0~ ______

o z

______

______

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______

be as high as 70 MPa (10,000 psi). An analysis of the mechanical properties of PG shows that a weaker failure mode is due to basal plane shear stress or radial tension near the inside surface, leading to delaminations instead of cross-plane cracking [11]. The mechanical and thermal properties of PBN suggest an upper limit of thickness/ radius ratio of about 0.08 to prevent delaminations and/or cracking. Of the fracture modes, controlled delaminations are preferred if crucible life is to be maximized. Controlled delaminations are sometimes deliberately induced during the

9

_____

_____

______ ______

CI

o

/

/

~‘

~

1300

0 ) 0 ______

o~ ~0 °

1600

1900

2200

2500

TEMPERATURE (DEG K) Fig. 4. Nitrogen pressure over boron nitride.

2600

growth of PBN crucibles [12,13] to reduce thermal stresses and increase the flexibility of the PBN crucibles. The problem of residual stress and strain due to anisotropic thermal expansion in PBN has been analyzed by Naito and Hsueh [14]. Calculalions showed that radial tension and combined tangential tension and compression exist in PBN, in the same way as in PG. Boron nitride does not sublime but decomposes into the elements boron and nitrogen at elevated temperatures. Fig. 4 shows nitrogen dissociation pressure data for both equilibrium (from thermodynamic data) and nonequilibrium conditions. The equilibrium data from the JANAF Tables indicate a decomposition temperature of about 2765 K at I atm. A decomposition temperature of about 2500°C was indicated from uniaxial hot-pressing experiments to convert PBN into a highly oriented form [15]. Based on the equilibrium data, decomposition of PBN might be expected under the most extreme conditions of use, i.e., in an MBE system operating at 10~0 Torn at 1600 K, where

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A.W. Moore

/ Characterization of PBN for semiconductor materials processing

an equilibrium nitrogen pressure of 5 x 10~ Torn is indicated. However, a nitrogen pressure of about 5 x 10~ Torn at 1600 K was measured by Dreger et al. [16] using the Knudsen method. (Expenimental data of McLaine and Coppel [17] using a static method yielded equilibrium data as shown in fig. 4.) This work shows that even hot-pressed boron nitride, which presents a relatively high proportion of edge planes to the surface, has a deficiency of surface sites for nitrogen vaporization. The fraction of such available sites in PBN could be still lower because the PBN surface is predominantly the BN basal plane. In practice, PBN crucibles have been vacuum baked at 1600°Cand 5 X 10 Torr for 1 h without decomposition [18]. Evaporation rate calculations using the nonequilibnium data of Dreger et al. suggest that sufficient BN would decompose at this heat-treatment condition to leave a 3 ~im layer of boron on the surface.

Table 2 Impurities in pyrolytic boron nitride Element ppm Analytical method Na <1 1 Mg <1 2 Al <5 3

PBN ts a very pure material because the byproduct HC1 helps to volatilize many metals under deposition conditions of high temperature and low pressure, in a way similar to the chlorination process which makes spectroscopically pure graphite. The total concentration of metallic impurities in all commercially available PBN is typically only a few ppm. Table 2 lists the upper limits of metal impurity concentration in PBN as determined by such techniques as atormc absorption, mass spectrometry, and induction coupled plasma emission spectroscopy. For most of the metallic impurities, the true concentrations may be much lower than those indicated in the table. Many of the analysis techniques require a means for getting the PBN into solution. For example, measurement of most of the elements by atomic absorption requires fusion of the PBN with alkali so the purity of the alkali itself becomes an important issue. Measurements by secondary ion mass spectroscopy and spark source mass spectroscopy are underway in an effort to more accurately determine the concentration of metallic impurities in PBN. Although the concentration of metallic impurities is not very accurately known at present, it

1 1

~ Ca Ti Cr Fe Co Ni Cu Zn Cl

<2 <2 <2 <5 <1 <3 <2 <3 2

3 4 3 3 4 3 3 2 5

~

3 240-270 5- 20 60—70

5

Ca)

0

5. Purity of PBN

Reference

1 1 1 1 1 1 1 1 2 2 2

7 7

Analytical method (1) Water extraction and atomic absorption (2) Flame atomic absorption . (3) atomic absorption (4) Alkaline Inductionand coupled plasma-emission spectroscopy (5) Mass spectrometry (6) IR, RF combustion with Cu (7) LECO with microretort R (1) Union Carbide data (2) Chambers et al. [18] a)

-

After vacuum bake at 1600 C and 5 x 10 ~ Torr for 1 h.

appears to be too low to adversely affect the properties of semiconductor crystals grown or doped using PBN containers. Because of exposure to air and handling, the concentration of certain impurities at the surface of PBN will be greater than in the bulk. Procedures for surface cleaning have been outlined in Technical Information Bulletins issued by Union Carbide Corporation. Carbon and organic cornpounds can be removed by heating the PBN in air to 700—750°C for 15—30 mm. Salts and most metals can be removed by (1) boiling in HC1 or Aqua Regia, (2) firing for 1 h at 1200°Cin pure dry nitrogen and 1% gaseous anhydrous HC1, or

A. W. Moore

/ Characterization

of PBN for semiconductor materials processing

(3) baking at high temperature in ultra-high vacuum. Carbon is the most abundant impurity found in PBN. Combustion analysis by the LECO method of PBN specimens from any manufacturer typically yields concentrations of 50—250 ppm. Carbon can be co-deposited with PBN over the entire compositional range from pure boron nitride to pure carbon [19]. Although this carbon may originate from exposed graphite surfaces necessarily employed in PBN furnaces, Fortucci et al. [20] recovered approximately 3 ppm C (as hydrocarbons), 4 ppm H, and 2 ppm 0 (as CO) from a PBN specimen after two 10 mm treatments at 1000°C and 108 Torn. Chambers et al. [18] reduced the carbon content of MBE crucibles from values cited above to 5—20 ppm by heat treatments up to 1600°Cat pressures as low as 5 x i0~ Torr. It can be concluded that carbon is incorporated at least in part as hydrocarbons thermally stable under deposition conditions, that much of the carbon can be released by thermal treatments, and the processes, such as oxygen evolution, may be involved in carbon removal during high-temperature treatments [21].

11

The presence of carbon and of hydrocarbons In PBN has different implications for growth of cornpound semiconductors and for molecular beam epitaxy. In growing GaAs by the LEC process, where the PBN crucibles are not vacuum baked before use, a 200 ppm of carbon in the PBN is unlikely to affect the concentration in the crystal. If 1 g of PBN containing 200 ppm carbon was to dissolve into a 4 kg boule of GaAs, it would increase the concentration of carbon in the GaAs by 1.3 X 1016 atoms/cm3. In practice GaAs crystals containing less than 1015 atoms/cm3 of carbon have been produced by the LEC process using PBN crucibles, showing that any carbon from PBN dissolution is negligible. Boron contamination may be slightly higher when PBN is used, but there is no evidence that boron is electrically active in GaAs. The PBN crucible weight losses in the LEC process, typically about 2 g per run in 150 mm diameter crucibles, occur by delamination (peeling) of the crucible wall after the boric oxide melt freezes and sticks to the crucible wall. Among the other nonmetals, oxygen has been found in appreciable quantities (60—70 ppm), but

Fig. 5. Large nodule in PBN caused by nucleation from filamentous growth of carbon from graphite substrate. Marker represents 0.2

mm.

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A. W. Moore

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Characterization of PBN for semiconductor materials processing

this oxygen is mainly confined to the surface, Surface oxygen is removed by vacuum baking before MBE use and in unimportant in LEC crucibles where the GaAs is enclosed in liquid boric oxide encapsulant.

6. Inhomogeneously distributed impurities in PBN Several types of inhomogeneously distributed impurities can be found in PBN if proper care is not taken in setting up the runs and in maintaining the equipment for PBN deposition. For example, impurities on the graphite substrate can nucleate filamentous growth of carbon which later becomes coated with PBN in the form of large nodules or clusters of nodules (see fig. 5). Proper selection of mandrel materials and rigid specifications regarding nodule size and quantity can be imposed to prevent production and delivery of crucibles with such obvious defects. Inclusions of carbon or metals are infrequently found em-

Fig. 6. Metallic inclusion in PI3N.

bedded in the PBN (see fig. 6). The presence of those inclusions can be traced to the failure of some component of the deposition equipment. Such inclusions can be readily observed by visual inspection so the crucibles can be discarded following inspection. Long experience in the production PBN has enabled us to control inclusions and nodules to the point where their occurrence is now a rare event.

7. Performance of PBN in semiconductor materials processing 7.1. LEC growth of GaAs and other Ill—V cornpounds The liquid encapsulated Czochralski (LEC) process using PBN crucibles has proven to be a very efficient and reliable technique for producing large single crystals of GaAs [22—25],InP [26], and GaP [27]. The use of PBN containers was promoted by Swiggard et al. [28], and the first commercial LEC production of undoped semi-insulating (SI) GaAs in PBN crucibles began in the early 1980s. A key advantage of PBN over quartz containers is the prevention of silicon contamination in the GaAs crystals, eliminating the need for compensatory doping with chromium [291 to achieve SI quality. Another advantage of PBN in the LEC process is its lower cost per GaAs crystal because each PBN crucible can be used many times, but each quartz crucible can be used only once. To better understand the performance of PBN in the LEC growth of GaAs, we developed a boric oxide melt test in PBN crucibles which simulates the LEC process. In this test, a quantity of pure anhydrous boric oxide powder is melted in the PBN crucible at a temperature similar to that used in the LEC process. The material is then cooled and the boric oxide lump is removed from the PBN crucible. Bonding of the boric oxide to the PBN and the greater shrinkage of boric oxide on cooling after freezing causes thin layers of PBN to peel away from the wall of the crucible. Typical weight losses from such tests are shown in fig. 7 which gives data for two 150 mm diameter crucibles each tested through 24 cycles. These values

A. W. Moore

/

Characterization of PBN for semiconductor materials processing

the stresses which cause polycrystalline GaAs growth instead of single crystal growth. However,

6” BORALLOY PBN CRUCIBLE WEIGHT LOSS

PBN boats have occasionally been used in the HB process to grow SI GaAs [31] Resistivities of about ~? cm were obtained, with etch-pit densities than 10,000/cm2. Von Neida and Jordan [32] reported on GaAs crystals grown in PBN boats using a horizontal gradient freeze technique. The crystals were semi-insulating with resistivities typi-

6

iO~ less 3

I

2

3

4

5

6

7

0

9

10 Ii 12 13

13

14 15 16

17

18

19

20 ~l

22 23 24

• K TEST NUMBER • A Fig. 7. Weight loss of PBN crucibles in boric oxide melt tests.

are similar to those experienced by manufacturers using similar crucibles for the LEC growth of GaAs. The tests, combined with a knowledge of the structure and morphology of PBN as reported earlier, have enabled us to produce PBN crucibles with optimum characteristics, resulting in extended life and a further lowering of crucible cost per GaAs crystal. Large GaAs crystal pullers using the LEC method contain many graphite furnace parts. These can absorb moisture or oxygen and transfer carbon to the GaAs by forming oxides. Coating the graphite parts with PBN and careful degassing of the system enabled Inada et al. [30] to lower the carbon content of their LEC GaAs by an order of magnitude to 1 x 1015 atoms/cm. 7.2. Horizontal Bridgman process Horizontal Bridgman (HB) technology is at present the principal method for growing semiconducting GaAs with low dislocation content. A low concentration of dislocations, as determined by etch pit counts, is necessary for good performance of light-emitting diodes, laser diodes, and other optoelectronic devices. Using HB technology, it is possible to routinely make GaAs wafers having less than 2000 etch pits/cm2. Quartz is the preferred container for the HB growth of GaAs because its surface can be roughened to make it nonwetting, an important feature in eliminating

cally>1O7f~cm and etch ptt densities were only Because of purity and chemical inertness, PBN boats are useful for purification of elemental and compound polycrystalline semiconductor matenials for subsequent bulk crystal growth and epitaxy. For example, PBN boats for presynthesis purification of the component elements enabled growth of high-purity crystalline large-grain InP using the horizontal gradient freeze technique [33]. 7.3. Vertical Bridgrnan process The vertical Bnidgman (YB) process is a relatively new technique which can produce controlled diameter Ill—V semiconductor compound crystals with both low impurity content and low dislocation densities. These properties will be important in future miniaturized devices and also in integrated optoelectronics. High-quality GaP, InP, and GaAs crystals have been grown using the vertical gradient freeze (VG) method [34—36]. GaAs crystals with > 10~S2 cm resistivity and <1000 etch pits/cm2 have been made using this method. In the VGF method, PBN solved both the contamination problem due to quartz and allowed the GaAs to expand while freezing without causing dislocation-producing stresses. The PBN crucible can be used repeatedly in this process as in the LEC process because only a thin layer peels from the inside wall of the crucible and seed well after each run. Boric oxide encapsulation has been employed in some VB processes as a convenient method to control melt composition and to prevent formation of grain boundaries which can occur when the growing crystal is in direct contact with the wall of the PBN crucible [37]. Boric oxide encapsulant was also used to control the composition of InP

14

A. W. Moore

/

Characterization of PBN for semiconductor materials processing

crystals grown in PBN using a dynamic gradient freeze growth method [38]. 7.4. Liquid encapsulated vertical zone melt process

Swiggard [39] has developed a vertical zone melting (VZM) process for growing GaAs crystals using boric oxide encapsulation. A special furnace with a thermal spike creates a molten zone in a bar of GaAs, and furnaces on either end of the spike adjust the length of the molten zone and control the thermal gradient at the growth interface. The GaAs crystal is grown in a PBN crucible shaped with a seed well at the bottom. After ramped cooling to 1100—1200°C,the crucible and crystal are transferred to a separate furnace where the crucible is inverted and the bonc oxide is slowly drained off. This permits easy removal of the crystal and minimizes damage to the crucible, extending its life. GaAs crystals grown using this method are reported to have resistivities of about 5 x i0~ ~Q cm with etch-pit densities of 2000— 5000/cm2. Efforts are underway to scale up this process. 7.5. PBN crucibles for metal evaporation PBN crucibles for evaporation of metals, such as aluminum and aluminium—copper onto semiconductor materials (e.g., on silicon wafers for computer memory chips) have been manufactured for more than 20 years. The high purity, chemical inertness, relative flexibility, and electrically insulating properties of PBN are valuable in this application. PBN crucibles exhibit excellent durability in metal evaporation, but a good knowledge of the thermal and mechanical properties is required for manufacturing crucibles which perform optimally. 7.6. PBNfor thin film processes

Molecular beam epitaxy (MBE) and other thin film deposition techniques have become important processes for the growth of epitaxial layers on wafers of Ill—V and Il—VI semiconductor crystals. PBN is the crucible of choice for containing the element on compound to be evaporated in MBE

Fig. 8. Boralcctric~ PG,/PBN resistanie heaters: (a) bare: (h) PBN-encapsulated. (Boralectnc i 51 is a trademark of Union Carbide Coatings Service Technology Corporation.)

systems because of its purity and thermal and chemical stability [23,40]. Vacuum baking of the PBN crucibles before use in MBE eliminates surface impurities which could contaminate the epitaxial layers. PBN is used as a lining material for quartz vapor phase epitaxy reactors and as a coating on machined graphite parts for liquid phase epitaxy and for metalorganic chemical vapor deposition (MOCVD). Resistance heaters machined from a layer of PG deposited on a PBN substrate have now been developed [41]. Power outputs in these heaters can exceed 45 W/cm2 and heater life is exceptionally tolerant of rapid and frequent thermal cycling. An optional encapsulating layer of PBN can be deposited on the heaters to provide electrical insulation and minimize carbon contamination (see fig. 8). These PG/PBN heaters are finding wide-spread use as source heaters for metal evaporation, semiconductor epitaxy, and CVD. A recent example describes the use of such a heater in a MOCVD reactor for growing epitaxial films on a GaAs substrate [42].

8. Conclusions Pyrolytic boron nitride has proven to be a valuable yet economic container material for the elemental purification, compounding, and growth

A. W. Moore

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Characterization of PBN for semiconductor materials processing

of semiconductor crystals. Optimization of PBN performance in these applications not only requires material of highest possible purity but also an accurate knowledge of the structure and thermal and mechanical properties. New information is being obtained on the purity and on the structure/property relationships in PBN which will allow us to provide even better products for future needs.

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