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Large area recrystallization of polysilicon with tungsten-halogen lamps
484
Journal of Crystal Growth 63 (1983) 484—492 North-Holland Publishing Company
LARGE AREA RECRYSTALLIZATION OF POLYSILICON WITH TUNGSTEN-HALOGEN LAMPS McD. ROBINSON Bell Laboratories, Murray Hill, New Jersey 07974, USA
D.J. LISCHNER Bell Laboratories, Allentown, Pennsylvania 18103, USA
and Q.K. CELLER Bell Laboratories, Murray Hill, New Jersey 07974, USA Received 22 February 1983; manuscript received in final form 6 March 1983
This review describes the recrystallization of polysilicon films with uniform heating from a large array of tungsten—halogen lamps. Single crystal films have been grown by Lateral Epitaxial Growth over Oxide (LEGO), using a precursor structure in which regions of isolation oxide alternate with seeding windows. Polysilicon films from 10 to 180 ~smthick on this structure have been converted to single crystal over oxide islands as large as 3 mm. The defect density is low, except where solidification fronts meet, and small angle grain boundaries are absent. A cap oxide is required, and melt depth must be controlled to avoid distortion of the surface features during melting. The LEGO technique is compared with reports of silicon recrystallization using scanned lamps.
Contents 1. 2. 3. 4 5. 6. 7.
Introduction Tungsten—halogen heat source Sample preparation Results Discussion Other recrystallization experiments with lamp radiation Conclusions References
1. Introduction High quality single crystal silicon over a nOncrystalline insulating layer has long been a goal of semiconductor device designers and processors. Applications would range from flat panel displays to high speed bipolar and MOS circuits to high voltage devices, with the advantage deriving largely from eliminating the need for junction isolation between devices. Single crystalline silicon has been
grown on crystalline insulating substrates, notably aluminum oxide (sapphire). Despite great progress in silicon-on-sapphire (SOS) technology, lifetime and mobility of SOS silicon are lower than bulk silicon values, because of trapping states caused by interface mismatch stress, crystallographic defects, and impurities from the substrate. Silicon over amorphous SiO2 is a preferred alternative, because of the proven high quality of a clean single crystal Si/amorphous Si02 interface and the lower substrate cost compared with SOS. The major problem of silicon on insulating SiO2 (SOl) arises precisely because the SiO2 is noncrystalline. Silicon deposited onto Si02 by chemical vapor deposition (CVD) or other means is polycrystalline or amorphous, but not single crystal. In recent years there has been a surge of interest in finding ways to convert polycrystalline or amorphous silicon over Si02 to single crystal, as attested to by the other papers in this review volume.
0022-0248/83/0000—0000/$03.OO © 1983 North-Holland
McD. Robinson et al.
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Large area recrystallization ofpolysilicon
The most successful results have been on structures with a single crystal seeding region, where lateral overgrowth from the seed produces single crystal SO!. Lateral single crystal overgrowth has been reported using lasers, electron beams and scanned strip heaters as heat sources [1,2]. This paper discusses another approach, that of melting a precursor structure with an extended and uniform radiative heat source and controlling the heat flow with a pattern of openings in the isolation oxide, to cause Lateral Epitaxial Growth over the Oxide (LEGO) of the solidification front. The LEGO approach is unique in comparison with other SO! methods, in two ways. First, both the heat source and the sample are stationary, so that processing is simple and fast. Because the lamp array heats the sample uniformly, radial temperature gradients and associated stresses are minimized. Second, much thicker films have been recrystallized than in any other melt based technique. Polysilicon of 10—180 ~im thickness is recrystallized on a complete 3 inch wafer in 60 s. The thick, dielectrically isolated layers that can be fabricated in this way are of interest for high voltage integrated circuits. The LEGO process has not yet been characterized for thin (c 10 tim) silicon films.
2. Tungsten—hatogen heat source Although designed for incandescent lighting, tungsten—halogen lamps have been used for numerous heating applications such as domestic space heaters and commercial CVD systems. Cline and Anthony [3] used an array of tungsten—halogen lamps as a heat source to thermomigrate aluminum and other metals through silicon. A similar apparatus was used by Celler et al. for the first reported recrystallization of polysilicon using stationary tungsten—halogen lamps [4,5], and by Benton et al. for the first broad area lamp annealing of ion-implanted wafers [6]. The apparatus as described by Lischner and Celler [7] is shown in fig. 1. An array of tungsten—halogen lamps, 10 X 12.5 inches, is suspended below a gold-plated reflector and cooled by forced convection. Two fused quartz plates
485
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separate the lamp assembly from the sample chamber, so that the ambient gas around the samples can be controlled. The samples rest on quartz pins over a water-cooled aluminum base. Over 100 kW total power can be supplied to the lamps; more than enough to melt silicon in the sample chamber. The temperature cycle is microprocessor programmed. Heating time can be 10 s or less. If full power is applied instantly to the lamps, about three seconds is required for the filaments to reach maximum temperature. A few additional seconds are required for the wafers to reach maximum temperature. Fig. 2 shows a typical thermal cycle. The solid curve is lamp power versus time, and the dashed curve shows how the wafer temperature follows. In the example the power is ramped from zero to 100 kW in 10 s, held at maximum power for 5 s, then ramped back to zero power in 10 s. The wafer temperature is measured on the lower surface by an optical pyrometer, sighting through a small opening in the water-cooled oven chamber. Above 1000°C the pyrometer measurement is corrected for emissivity of the silicon wafer back. Below 1000°C the recorded temperature is uncalibrated. As fig. 2 shows, the wafer starts heating slowly from room temperature, then after a few seconds heats rapidly. The reason for this response is twofold. At low temperature, nondegenerately doped silicon is transparant to wavelengths longer than about 1.1 ~tm. The wafer therefore absorbs only a small fraction of infrared radiation emitted by the lamps as they first start to heat. As the filament temperature increases the radiation shifts to shorter
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an imbedded thermocouple across the furnace, one half inch above the oven floor. The ring was 2 inch outside diameter by 1.5 inch inside diameter by 2.5 mm thick, with the thermocouple inserted into a hole at the outside edge. Because of edge losses the temperature across a 3 inch silicon wafer is not as uniform as the figure might imply.
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R.T. TIME (SECONDSl
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Fig. 2. Plots of electrical power input and wafer temperature for a typical thermal cycle,
wavelength, while as the wafer heats up, its absorption in the infrared increases. The two effects combine to cause the wafer heating to accelerate rapidly in the first few seconds. Finally, the T4 dependence of radiative heat loss causes the rate 1250
/
240-
/ / / /
1220 ~
1210-
Features of the equipment that contribute to uniform and controlled melting are reproducible and accurate control of lamp power, ramp rates and heating time, and provision of the water-cooled base as a radiative heat sink below the wafers. For LEGO recrystallization of a thin surface layer the provision of the heat sink under the wafer is critical. The temperature gradient through the wafer can be simply estimated if heat loss from both wafer surfaces is assumed to be by radiation, with heat input from one side only. By conservation of energy, the gradient can be determined from the power emitted from the colder side, A eaT4
=
scA
—,
where A is the area of each side (~44 cm2), e the silicon emissivity (~0.6), a the Stefan—Boltzmann radiation constant (= 5.672 X 10— 12 W/cm2. K4), T the temperature of the wafer lower surface, ic the thermal conductivity of silicon (~ 0.2 W/cm. K at 1673 K), ~T the temperature difference between
~
1230
of heating to fall rapidly to zero as the wafer approaches its steady state temperature. Faster initial heating rates could be obtained with arc lamps that emit primarily in the UV and visible spectral ranges. A profile of the furnace temperature (fig. 3) shows that heating is uniform to ±5°Cacross the center 5 inches of the furnace at 1250°C. The profile was made by dragging a ring of silicon with
the top and bottom of the wafer, and thickn:ss(z~~ 0.05 cm).
t
the wafer
~ 11901180
-
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CENTER OF OVEN
EDGE
~
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2
i~,
3
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4
5
6
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Fig. 3. Temperature profile along the longer axis of the wafer chamber. Notice the greatly expanded temperature scale,
L~T—7°Cat 1400°C(1673 K). The calculation shows that the wafer back is less than 10°C below the melting point when melting begins at the top surface. Although the above-calculated vertical temperature gradient is comparable to the horizontal variation shown in
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Large area recrystallization ofpolysilicon
fig. 3, melt depth in actual samples is observed to be fairly uniform. This apparent contradiction may be explained by silicon’s substantial decrease in emissivity on melting [8]. The reduced light absorption that accompanies melting may serve to self-limit the melt penetration.
3. Sample preparation In most of the heating methods for melting and recrystallization of SOl using line or point heating, a horizontal temperature gradient is inherent in the mode of heating. However if the heat flux is uniform as with the lamp array described here, horizontal temperature gradients are determined by the microstructure of the sample. Fig. 4 shows a cross section of a typical LEGO precursor structure with the grain boundaries delineated by a 10 s Schimmel etch [9].This structure is formed by first patterning a thermal oxide into alternating windows and islands, then growing a silicon layer by CVD, and finally capping with a 2 ~itm layer of low pressure chemical deposited (LPCVD) oxide. In this example CYD silicon deposition was at 1050°C so that growth was epitaxial in the single crystal windows while polycrystalline silicon grew on the oxide. Before deposition the samples were etched briefly at 1200°C with HC1 vapor to re-
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487
move native oxide from the silicon in the windows. In most previous reports on recrystallization of thin silicon films, low temperature (600—700°C) LPCVD polysilicon has been deposited to form the precursor structure. We find the higher deposition temperature attractive for lamp recrystallization, both because the thick CVD silicon films can be grown at > 1 ~t/min, and because the silicon grows epitaxially in the windows at this temperature, increasing the chance of good quality seeding. It is possible, by choosing suitable deposition conditions, to grow single crystal silicon in the seeding windows while avoiding nucleation of polysilicon on the oxide [10]. Such selective epitaxy is the basis for epitaxial lateral overgrowth (ELO) reported by Jastrzebski [11]. However our best LEGO results have been with the epi/polysilicon precursor structure shown in fig. 4. Accordingly, we have grown the CVD silicon using conditions favoring simultaneous growth of epi in the windows and polysilicon on the oxide. Using LEGO we have grown single crystal silicon films from precursor structures as thin as 10 ~tm and as thick as 180 ~tm. We have also been able to increase the thickness of recrystallized layers by CVD epitaxial growth. Thick, dielectrically isolated films are of interest for high voltage integrated circuits [12].Thin (c 10 ~tm)films would also be of interest for many applications, but have not yet been investigated with the LEGO process.
~
Fig. 4. Optical micrograph of polished cross section after 10 s Schimmel etch. As-deposited.
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Large area recrystallization ofpolysilicon
Limit of melting
Limit of melting
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Fig. 5. Optical micrograph of polished cross section after 60 s Schimmel etch. Recrystallized with 67 W/cm peak power.
4. Results
section (60 s Schimmel etch) show that the precursor CVD polysilicon melted down to the isolation oxide, but that melting was incomplete at the poly/epi silicon interface at the window openings. The sample in fig. 6, by contrast, regrew as single crystal. Heating at 78 W/cm2 melted cornpletely through the CVD silicon and partly into the substrate. On cooling, epitaxial recrystalliza-
Figs. 5 and 6 illustrate the dynamics of recrystallization. For both of these samples the power was ramped up linearly for 10 s, held at maximum power 10 s, then ramped down over 60 s. Maximum power for the sample in fig. 5 was 67 W/cm2. The large grains in the polished and etched cross
Limit of melting ~
.
~
50 pm
~
Fig. 6. Optical micrograph of polished cross section after 10 s Schimmel etch. Recrystallized with 78 W/cm’ peak power.
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Large area recrystallization ofpolysilicon
tion proceeded up through the oxide opening and laterally across the isolation oxide. The recrystallized silicon is free of grain boundaries and has fewer than 1 X i0~dislocations per cm2. This result is significantly different from previous reports of recrystallization with scanned spot or line heat sources, in which low angle grain boundaries were observed in the direction of recrystallization. The
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absence of low angle boundaries after LEGO recrystallization is not understood, but may be related to the thickness of the recrystallized silicon layer. We assume that the low density of slip dislocations in LEGO recrystallized silicon can be attributed to the uniform illumination, resulting in low radial temperature gradients as shown in fig. 3. Since the rate of conductive heat transfer in the
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Fig. 7. TEM cross section of epitaxially sample. Electron diffraction from all three regions is combined into pattern shown in the inset.
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Large area recrystallization ofpolysilicon
Si and SiO2 is rapid compared with our ramped heating and cooling rates, thermal stresses during heat-up and cool-down should be no worse than those at constant maximum power. Fig. 6 shows via the etch (10 s Schimmel etch) that melting in this sample penetrated 10—15 ~tm below the isolation oxide. The CVD polysilicon was doped more heavily than the single crystal substrate, and the visible boundary results from the etch rate dependence on doping. This result also shows qualitatively that dopant was distributed throughout the molten region during the few seconds of melting. Fig. 7 is a transmission electron micrograph (TEM) cross section of the sample in fig. 6. The micrograph spans the LEGO recrystallized CVD silicon (top), the isolation oxide, and the substrate silicon (bottom). Only extinction contours are visible; no defects appear in the segment, located 50 ~tm from the nearest seeding window. The electron diffraction pattern of all three regions (inset) shows a single pattern of diffraction spots from the two crystalline regions, superimposed on a halo from the amorphous Si02. 5. Discussion Lateral epitaxial growth across the isolation oxide requires lateral temperature gradients in the recrystallizing silicon. In other approaches these gradients have been achieved with scanned heat sources such as electron and laser beams or a movable strip heater. In our lamp heated system the heat source is large and uniform, and the lateral gradients are provided by the silicon/insulator microstructure, and by the emissivity difference between the molten and solid phases. During recrystallization the lamp cooling rate is held low enough to assure that heat flow is always into the substrate. The rate of heat flow is higher through the windows than through the isolation oxide because silicon’s thermal diffusivity is about 100 times that of the oxide. The solidification front thus moves upward through the windows, then laterally over the oxide islands. The increase in emissivity over tha windows when the solid reaches the top surface further enhances the lateral temperature gradient.
We have observed lateral epitaxial growth over oxide islands as large as 3 mm. On islands this size or smaller we have never seen partial lateral epitaxity, i.e. individual islands that are part single crystal and part polycrystalline. Each recrystallized island is always either completely single crystal or completely polycrystalline. Because the wafers have a slight radial temperature gradient in the lamp furnace we have occasionally observed a sharp boundary concentric with the wafer perimeter, where all recrystallized islands toward the wafer center are single crystal and all those toward the edge are polycrystalline. The main process limitation in recrystallizing these films has been a tendency for the molten silicon to redistribute laterally. In extreme cases the silicon can pull completely away from an area, leaving the isolation oxide uncovered. This redistribution is minimized by deposition of a cap oxide and by control of melt depth. Similar to other methods of recrystallization, we have found that a capping layer is essential in preventing “balling up” of the molten silicon. A 2 ~tm oxide works well. At the melting temperature of silicon (1683 K), undoped SiO2 has a viscosity of about lO~poise. This provides enough flexibility to accommodate changes in volume and macroscopic shape when silicon is melted and recrystallized, yet the oxide is stiff enough to retain the microscopic roughness of the as-deposited polysilicon throughout the process. It should be pointed out that the capping layer contains no silicon nitride. One attempt to recrystallize with 500 A of plasma-deposited silicon nitride on top of 2 ~tm of oxide resulted in gaping holes in the cap after recrystallization, with loss of silicon from the exposed areas. Redistribution of the molten silicon is minimized if the melting depth is kept to the minimum necessary for epitaxial recrystallization. In extreme cases where melting penetrated well below the isolation oxide, both the molten silicon and disconnected islands of oxide have been displaced. Fig. 8 shows a polished and Schimmel etched, angle lapped section of the center of an island. The angle lap (which magnifies the vertical dimension by 20 X) shows that where the crystallization fronts meet there is high point in the silicon, and
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Large area recrystallization ofpolysilicon
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~~~1 __________________
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Fig. S Optical micrograph of angle-polished cross section after 2 peak power. 10 s Schimmel etch. Recrystallized with 78 W/cm
also an array of defects. Defects at intersecting melt fronts have also been reported in laser [13] and strip heater [14] recrystallization. These regions are probably unavoidable, but their location can be controlled by proper mask design. Wafer flatness is an important consideration in silicon processing. Bow measurements on a few wafers before and after lamp recrystallization show that wafer flatness is affected only slightly by the lamp processing. For example the bow of a 3 inch wafer with 30 tim of CVD silicon increased (in an absolute sense) from 14 p~mbefore to 2. 8(Bow tim after follows lamp recrystallization 75 positive W/cm means here the convention atthat concave on the CVD silicon side of the wafer.) We expect wafer bow before lamp recrystallization to increase with CVD polysilicon thickness [15]. —
—
6. Other recrystallization experiments with lamp radiation Several groups have demonstrated recrystallization of thin (<1 tim) silicon films .on SiO 2 with incoherent lamp irradiation [16—19].Conceptually, all these reports are similar as they attempt to improve the strip heater configuration by replacing the movable graphite filament with either a tungsten halogen lamp or an arc lamp. The advantages of lamp heating over graphite include
491
elimination of many impurities and better temperature reproducibility from one scan to another. Contamination of silicon with carbon, copper and other metals has occasionally been a problem in graphite strip heater experiments, while oxidation of electrical contacts, filament sagging and uneven sample contact with the lower area heater often cause nonuniform recrystallization. Karngar et al. [16,17] showed that radiant heating of an entire 3 inch wafer with a panel of six tungsten halogen lamps combined with zone melting with an additional motor-driven linear lamp achieved crystallization of silicon without causing any measurable contamination. The wafers had 0.5 ~tm of LPCVD silicon sandwiched between 0.5 ~tm polycrystalline of thermally grown isolation oxide and a capping layer of 2 ~tm of SiO 2 and 300 A of LPCVD silicon nitride. Subgrain boundaries were formed as in any other strip heater experiments, but the surface protrusions usually associated with these boundaries were absent, likely as a result of the film purity. Vu et al. [18] obtained similar results, but on smaller scale, by scanning a rectangular sample between two incandescent lamps, one to provide area heating, another tightly focused to achieve the molten zone of 7 mm length. Stultz et al. [19] built a hybrid system, with a conventional hot stage to elevate the samples to 1150°C, a mercury lamp They in anfabricated elliptical cavity as and a scanned line arc source. enhancement and depletion mode MOSFET’s in lamp recrystallized films and saw no dependence on the direction of channels relative to subboundaries. A 14 tim thick epitaxial film was also grown on a 0.5 j.tm recrystallized layer. The epitaxial film replicated the surface features and defects.
7. Conclusions Planar, stationary lamp arrays are excellent for recrystallization of relatively thick, 10—100 tim, silicon films on SiO2 when periodically spaced openings to the crystalline silicon substrate are provided for seeding and heat sinking. The recrystallized films are free of any grain boundaries
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including the small angle boundaries that are pervasive in other thin film recrystallization techniques. The density of dislocations is low except in the small “last-to-freeze” regions. These regions can be kept out of device areas by careful design of the photolithographic mask set. Bow-free recrystallization on 3 inch silicon wafers has been demonstrated and extension of this work to larger diameter wafers appears straightforward. Wafers are processed rapidly without any need to translate either the wafers or the heat source. This makes production-scale throughout feasible. While this paper has discussed recrystallization using stationary lamps, heating with scanned lamps also appears promising for applications where narrow zone melting is necessary, e.g. for recrystallization of thin films or where seeding openings are absent. All incoherent irradiation processes are limited to the use of substrates that can withstand high temperatures for several seconds. Crystallization of films on top of partially finished devices with shallow p—n junctions already in place will remain the domain of lasers and electron beams.
References [1] BR. Appleton and G.K. Celler, Eds., Laser and Electron Beam Interactions with Solids (North-Holland, New York 1982).
area recrystallization ofpolysilicon [2] J. Narayan, W.L. Brown and R.A. Lemons, Eds.,
[3] [4] [5] [6]
[7] [8]
Laser—Solid Interactions and Transient Thermal Processing of Materials (North-Holland, New York, 1983). H. Cline and T. Anthony, J. AppI. Phys. 49 (1978) 2412; US Patents 4,168,992; 4,170,490; 4,035,199. G.K. Celler, MeD. Robinson and D.J. Lischner, Appl. Phys. Letters 42 (1983) 99. G.K. Celler, MeD. Robinson, D.J. Lischner and T.T. Sheng, in ref. [2], pp. 575—580. J.L. Benton, G.K. Celler, D.C. Jacobson, L.C. Kimerling, D.J. Lischner, G.L. Miller and MeD. Robinson, in ref. [1], ~ 765—770. D.J. Lischner and G.K. Celler, in ref. [1], pp. 759—764. W.G. Hawkins and D.K. Biegelsen, Appl. Phys. Letters 42 (1983) 358.
[9] D.G. Electrochem. 126 (1979) [10] G.K. Schimmel, Celler, L.E.J.Trimble, MeD. Soc. Robinson, K.K.479. Ng and H.J. Leamy, Electronic Materials Conf., Fort Collins, CO, 1982. [11] L. Jastrzebski, J. Crystal Growth 63 (1983) 493. [12] MeD. Robinson, G.K. Celler and D.J. Lischner, Electrochem. Soc. Extended Abstracts 83-1 (1983) 147. [13] H. Tamura, M. Miyao and T. Tokuyama, Japan. J. Appl. Phys. 20 Suppl. 20-1 (1981) 43. [14] i.C.C. Fan, B.-Y. Tsaur and M.W. Geis, Appl. Phys. Letters 39 (1981) 308. [15] T. Suzuki, A. Mimura, T. Kamei and T. Ogawa, J. Electrochem. Soc. 127 (1977) 1537. [16] A. Kamgar and E. Labate, Mater. Letters 1(1982) 91. [17] A. Kamgar, G.A. Rozgonyi and R. Knoell, in ref. [2], pp. 569—574. [18] D.P. Vu, M. Haond, D. Bensahel and M. Dupuy, J. AppI. Phys. 54 (1983) 437. [19] T. Stultz, J. Sturm and J. Gibbons, in ref. [2], pp. 463—476.
Journal of Crystal Growth 63 (1983) 484—492 North-Holland Publishing Company
LARGE AREA RECRYSTALLIZATION OF POLYSILICON WITH TUNGSTEN-HALOGEN LAMPS McD. ROBINSON Bell Laboratories, Murray Hill, New Jersey 07974, USA
D.J. LISCHNER Bell Laboratories, Allentown, Pennsylvania 18103, USA
and Q.K. CELLER Bell Laboratories, Murray Hill, New Jersey 07974, USA Received 22 February 1983; manuscript received in final form 6 March 1983
This review describes the recrystallization of polysilicon films with uniform heating from a large array of tungsten—halogen lamps. Single crystal films have been grown by Lateral Epitaxial Growth over Oxide (LEGO), using a precursor structure in which regions of isolation oxide alternate with seeding windows. Polysilicon films from 10 to 180 ~smthick on this structure have been converted to single crystal over oxide islands as large as 3 mm. The defect density is low, except where solidification fronts meet, and small angle grain boundaries are absent. A cap oxide is required, and melt depth must be controlled to avoid distortion of the surface features during melting. The LEGO technique is compared with reports of silicon recrystallization using scanned lamps.
Contents 1. 2. 3. 4 5. 6. 7.
Introduction Tungsten—halogen heat source Sample preparation Results Discussion Other recrystallization experiments with lamp radiation Conclusions References
1. Introduction High quality single crystal silicon over a nOncrystalline insulating layer has long been a goal of semiconductor device designers and processors. Applications would range from flat panel displays to high speed bipolar and MOS circuits to high voltage devices, with the advantage deriving largely from eliminating the need for junction isolation between devices. Single crystalline silicon has been
grown on crystalline insulating substrates, notably aluminum oxide (sapphire). Despite great progress in silicon-on-sapphire (SOS) technology, lifetime and mobility of SOS silicon are lower than bulk silicon values, because of trapping states caused by interface mismatch stress, crystallographic defects, and impurities from the substrate. Silicon over amorphous SiO2 is a preferred alternative, because of the proven high quality of a clean single crystal Si/amorphous Si02 interface and the lower substrate cost compared with SOS. The major problem of silicon on insulating SiO2 (SOl) arises precisely because the SiO2 is noncrystalline. Silicon deposited onto Si02 by chemical vapor deposition (CVD) or other means is polycrystalline or amorphous, but not single crystal. In recent years there has been a surge of interest in finding ways to convert polycrystalline or amorphous silicon over Si02 to single crystal, as attested to by the other papers in this review volume.
0022-0248/83/0000—0000/$03.OO © 1983 North-Holland
McD. Robinson et al.
/
Large area recrystallization ofpolysilicon
The most successful results have been on structures with a single crystal seeding region, where lateral overgrowth from the seed produces single crystal SO!. Lateral single crystal overgrowth has been reported using lasers, electron beams and scanned strip heaters as heat sources [1,2]. This paper discusses another approach, that of melting a precursor structure with an extended and uniform radiative heat source and controlling the heat flow with a pattern of openings in the isolation oxide, to cause Lateral Epitaxial Growth over the Oxide (LEGO) of the solidification front. The LEGO approach is unique in comparison with other SO! methods, in two ways. First, both the heat source and the sample are stationary, so that processing is simple and fast. Because the lamp array heats the sample uniformly, radial temperature gradients and associated stresses are minimized. Second, much thicker films have been recrystallized than in any other melt based technique. Polysilicon of 10—180 ~im thickness is recrystallized on a complete 3 inch wafer in 60 s. The thick, dielectrically isolated layers that can be fabricated in this way are of interest for high voltage integrated circuits. The LEGO process has not yet been characterized for thin (c 10 tim) silicon films.
2. Tungsten—hatogen heat source Although designed for incandescent lighting, tungsten—halogen lamps have been used for numerous heating applications such as domestic space heaters and commercial CVD systems. Cline and Anthony [3] used an array of tungsten—halogen lamps as a heat source to thermomigrate aluminum and other metals through silicon. A similar apparatus was used by Celler et al. for the first reported recrystallization of polysilicon using stationary tungsten—halogen lamps [4,5], and by Benton et al. for the first broad area lamp annealing of ion-implanted wafers [6]. The apparatus as described by Lischner and Celler [7] is shown in fig. 1. An array of tungsten—halogen lamps, 10 X 12.5 inches, is suspended below a gold-plated reflector and cooled by forced convection. Two fused quartz plates
485
REFLECTOR
L ~
~
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0
~
a
_____________________________________________ QUARTZ PLATE 1
I
QUARTZ PLATE
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I
L WAFER Fig. 1. Schematic cross section of large area tungsten—halogen lamp heating apparatus.
separate the lamp assembly from the sample chamber, so that the ambient gas around the samples can be controlled. The samples rest on quartz pins over a water-cooled aluminum base. Over 100 kW total power can be supplied to the lamps; more than enough to melt silicon in the sample chamber. The temperature cycle is microprocessor programmed. Heating time can be 10 s or less. If full power is applied instantly to the lamps, about three seconds is required for the filaments to reach maximum temperature. A few additional seconds are required for the wafers to reach maximum temperature. Fig. 2 shows a typical thermal cycle. The solid curve is lamp power versus time, and the dashed curve shows how the wafer temperature follows. In the example the power is ramped from zero to 100 kW in 10 s, held at maximum power for 5 s, then ramped back to zero power in 10 s. The wafer temperature is measured on the lower surface by an optical pyrometer, sighting through a small opening in the water-cooled oven chamber. Above 1000°C the pyrometer measurement is corrected for emissivity of the silicon wafer back. Below 1000°C the recorded temperature is uncalibrated. As fig. 2 shows, the wafer starts heating slowly from room temperature, then after a few seconds heats rapidly. The reason for this response is twofold. At low temperature, nondegenerately doped silicon is transparant to wavelengths longer than about 1.1 ~tm. The wafer therefore absorbs only a small fraction of infrared radiation emitted by the lamps as they first start to heat. As the filament temperature increases the radiation shifts to shorter
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Large area recrystallization ofpolysilicon
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1360
120 1070 100
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an imbedded thermocouple across the furnace, one half inch above the oven floor. The ring was 2 inch outside diameter by 1.5 inch inside diameter by 2.5 mm thick, with the thermocouple inserted into a hole at the outside edge. Because of edge losses the temperature across a 3 inch silicon wafer is not as uniform as the figure might imply.
—
80
o ~
I /
60
-~
/
40
.1 I
/
20
0
R.T. TIME (SECONDSl
40
Fig. 2. Plots of electrical power input and wafer temperature for a typical thermal cycle,
wavelength, while as the wafer heats up, its absorption in the infrared increases. The two effects combine to cause the wafer heating to accelerate rapidly in the first few seconds. Finally, the T4 dependence of radiative heat loss causes the rate 1250
/
240-
/ / / /
1220 ~
1210-
Features of the equipment that contribute to uniform and controlled melting are reproducible and accurate control of lamp power, ramp rates and heating time, and provision of the water-cooled base as a radiative heat sink below the wafers. For LEGO recrystallization of a thin surface layer the provision of the heat sink under the wafer is critical. The temperature gradient through the wafer can be simply estimated if heat loss from both wafer surfaces is assumed to be by radiation, with heat input from one side only. By conservation of energy, the gradient can be determined from the power emitted from the colder side, A eaT4
=
scA
—,
where A is the area of each side (~44 cm2), e the silicon emissivity (~0.6), a the Stefan—Boltzmann radiation constant (= 5.672 X 10— 12 W/cm2. K4), T the temperature of the wafer lower surface, ic the thermal conductivity of silicon (~ 0.2 W/cm. K at 1673 K), ~T the temperature difference between
~
1230
of heating to fall rapidly to zero as the wafer approaches its steady state temperature. Faster initial heating rates could be obtained with arc lamps that emit primarily in the UV and visible spectral ranges. A profile of the furnace temperature (fig. 3) shows that heating is uniform to ±5°Cacross the center 5 inches of the furnace at 1250°C. The profile was made by dragging a ring of silicon with
the top and bottom of the wafer, and thickn:ss(z~~ 0.05 cm).
t
the wafer
~ 11901180
-
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CENTER OF OVEN
EDGE
~
I
2
i~,
3
I~,
4
5
6
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Fig. 3. Temperature profile along the longer axis of the wafer chamber. Notice the greatly expanded temperature scale,
L~T—7°Cat 1400°C(1673 K). The calculation shows that the wafer back is less than 10°C below the melting point when melting begins at the top surface. Although the above-calculated vertical temperature gradient is comparable to the horizontal variation shown in
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Large area recrystallization ofpolysilicon
fig. 3, melt depth in actual samples is observed to be fairly uniform. This apparent contradiction may be explained by silicon’s substantial decrease in emissivity on melting [8]. The reduced light absorption that accompanies melting may serve to self-limit the melt penetration.
3. Sample preparation In most of the heating methods for melting and recrystallization of SOl using line or point heating, a horizontal temperature gradient is inherent in the mode of heating. However if the heat flux is uniform as with the lamp array described here, horizontal temperature gradients are determined by the microstructure of the sample. Fig. 4 shows a cross section of a typical LEGO precursor structure with the grain boundaries delineated by a 10 s Schimmel etch [9].This structure is formed by first patterning a thermal oxide into alternating windows and islands, then growing a silicon layer by CVD, and finally capping with a 2 ~itm layer of low pressure chemical deposited (LPCVD) oxide. In this example CYD silicon deposition was at 1050°C so that growth was epitaxial in the single crystal windows while polycrystalline silicon grew on the oxide. Before deposition the samples were etched briefly at 1200°C with HC1 vapor to re-
~
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487
move native oxide from the silicon in the windows. In most previous reports on recrystallization of thin silicon films, low temperature (600—700°C) LPCVD polysilicon has been deposited to form the precursor structure. We find the higher deposition temperature attractive for lamp recrystallization, both because the thick CVD silicon films can be grown at > 1 ~t/min, and because the silicon grows epitaxially in the windows at this temperature, increasing the chance of good quality seeding. It is possible, by choosing suitable deposition conditions, to grow single crystal silicon in the seeding windows while avoiding nucleation of polysilicon on the oxide [10]. Such selective epitaxy is the basis for epitaxial lateral overgrowth (ELO) reported by Jastrzebski [11]. However our best LEGO results have been with the epi/polysilicon precursor structure shown in fig. 4. Accordingly, we have grown the CVD silicon using conditions favoring simultaneous growth of epi in the windows and polysilicon on the oxide. Using LEGO we have grown single crystal silicon films from precursor structures as thin as 10 ~tm and as thick as 180 ~tm. We have also been able to increase the thickness of recrystallized layers by CVD epitaxial growth. Thick, dielectrically isolated films are of interest for high voltage integrated circuits [12].Thin (c 10 ~tm)films would also be of interest for many applications, but have not yet been investigated with the LEGO process.
~
Fig. 4. Optical micrograph of polished cross section after 10 s Schimmel etch. As-deposited.
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Large area recrystallization ofpolysilicon
Limit of melting
Limit of melting
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Fig. 5. Optical micrograph of polished cross section after 60 s Schimmel etch. Recrystallized with 67 W/cm peak power.
4. Results
section (60 s Schimmel etch) show that the precursor CVD polysilicon melted down to the isolation oxide, but that melting was incomplete at the poly/epi silicon interface at the window openings. The sample in fig. 6, by contrast, regrew as single crystal. Heating at 78 W/cm2 melted cornpletely through the CVD silicon and partly into the substrate. On cooling, epitaxial recrystalliza-
Figs. 5 and 6 illustrate the dynamics of recrystallization. For both of these samples the power was ramped up linearly for 10 s, held at maximum power 10 s, then ramped down over 60 s. Maximum power for the sample in fig. 5 was 67 W/cm2. The large grains in the polished and etched cross
Limit of melting ~
.
~
50 pm
~
Fig. 6. Optical micrograph of polished cross section after 10 s Schimmel etch. Recrystallized with 78 W/cm’ peak power.
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Large area recrystallization ofpolysilicon
tion proceeded up through the oxide opening and laterally across the isolation oxide. The recrystallized silicon is free of grain boundaries and has fewer than 1 X i0~dislocations per cm2. This result is significantly different from previous reports of recrystallization with scanned spot or line heat sources, in which low angle grain boundaries were observed in the direction of recrystallization. The
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absence of low angle boundaries after LEGO recrystallization is not understood, but may be related to the thickness of the recrystallized silicon layer. We assume that the low density of slip dislocations in LEGO recrystallized silicon can be attributed to the uniform illumination, resulting in low radial temperature gradients as shown in fig. 3. Since the rate of conductive heat transfer in the
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~BS~E,
~1ELWI’*ON
DIFFRACTION
Fig. 7. TEM cross section of epitaxially sample. Electron diffraction from all three regions is combined into pattern shown in the inset.
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Large area recrystallization ofpolysilicon
Si and SiO2 is rapid compared with our ramped heating and cooling rates, thermal stresses during heat-up and cool-down should be no worse than those at constant maximum power. Fig. 6 shows via the etch (10 s Schimmel etch) that melting in this sample penetrated 10—15 ~tm below the isolation oxide. The CVD polysilicon was doped more heavily than the single crystal substrate, and the visible boundary results from the etch rate dependence on doping. This result also shows qualitatively that dopant was distributed throughout the molten region during the few seconds of melting. Fig. 7 is a transmission electron micrograph (TEM) cross section of the sample in fig. 6. The micrograph spans the LEGO recrystallized CVD silicon (top), the isolation oxide, and the substrate silicon (bottom). Only extinction contours are visible; no defects appear in the segment, located 50 ~tm from the nearest seeding window. The electron diffraction pattern of all three regions (inset) shows a single pattern of diffraction spots from the two crystalline regions, superimposed on a halo from the amorphous Si02. 5. Discussion Lateral epitaxial growth across the isolation oxide requires lateral temperature gradients in the recrystallizing silicon. In other approaches these gradients have been achieved with scanned heat sources such as electron and laser beams or a movable strip heater. In our lamp heated system the heat source is large and uniform, and the lateral gradients are provided by the silicon/insulator microstructure, and by the emissivity difference between the molten and solid phases. During recrystallization the lamp cooling rate is held low enough to assure that heat flow is always into the substrate. The rate of heat flow is higher through the windows than through the isolation oxide because silicon’s thermal diffusivity is about 100 times that of the oxide. The solidification front thus moves upward through the windows, then laterally over the oxide islands. The increase in emissivity over tha windows when the solid reaches the top surface further enhances the lateral temperature gradient.
We have observed lateral epitaxial growth over oxide islands as large as 3 mm. On islands this size or smaller we have never seen partial lateral epitaxity, i.e. individual islands that are part single crystal and part polycrystalline. Each recrystallized island is always either completely single crystal or completely polycrystalline. Because the wafers have a slight radial temperature gradient in the lamp furnace we have occasionally observed a sharp boundary concentric with the wafer perimeter, where all recrystallized islands toward the wafer center are single crystal and all those toward the edge are polycrystalline. The main process limitation in recrystallizing these films has been a tendency for the molten silicon to redistribute laterally. In extreme cases the silicon can pull completely away from an area, leaving the isolation oxide uncovered. This redistribution is minimized by deposition of a cap oxide and by control of melt depth. Similar to other methods of recrystallization, we have found that a capping layer is essential in preventing “balling up” of the molten silicon. A 2 ~tm oxide works well. At the melting temperature of silicon (1683 K), undoped SiO2 has a viscosity of about lO~poise. This provides enough flexibility to accommodate changes in volume and macroscopic shape when silicon is melted and recrystallized, yet the oxide is stiff enough to retain the microscopic roughness of the as-deposited polysilicon throughout the process. It should be pointed out that the capping layer contains no silicon nitride. One attempt to recrystallize with 500 A of plasma-deposited silicon nitride on top of 2 ~tm of oxide resulted in gaping holes in the cap after recrystallization, with loss of silicon from the exposed areas. Redistribution of the molten silicon is minimized if the melting depth is kept to the minimum necessary for epitaxial recrystallization. In extreme cases where melting penetrated well below the isolation oxide, both the molten silicon and disconnected islands of oxide have been displaced. Fig. 8 shows a polished and Schimmel etched, angle lapped section of the center of an island. The angle lap (which magnifies the vertical dimension by 20 X) shows that where the crystallization fronts meet there is high point in the silicon, and
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Large area recrystallization ofpolysilicon
—
~~~1 __________________
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__________
_____
__________________________________________
Fig. S Optical micrograph of angle-polished cross section after 2 peak power. 10 s Schimmel etch. Recrystallized with 78 W/cm
also an array of defects. Defects at intersecting melt fronts have also been reported in laser [13] and strip heater [14] recrystallization. These regions are probably unavoidable, but their location can be controlled by proper mask design. Wafer flatness is an important consideration in silicon processing. Bow measurements on a few wafers before and after lamp recrystallization show that wafer flatness is affected only slightly by the lamp processing. For example the bow of a 3 inch wafer with 30 tim of CVD silicon increased (in an absolute sense) from 14 p~mbefore to 2. 8(Bow tim after follows lamp recrystallization 75 positive W/cm means here the convention atthat concave on the CVD silicon side of the wafer.) We expect wafer bow before lamp recrystallization to increase with CVD polysilicon thickness [15]. —
—
6. Other recrystallization experiments with lamp radiation Several groups have demonstrated recrystallization of thin (<1 tim) silicon films .on SiO 2 with incoherent lamp irradiation [16—19].Conceptually, all these reports are similar as they attempt to improve the strip heater configuration by replacing the movable graphite filament with either a tungsten halogen lamp or an arc lamp. The advantages of lamp heating over graphite include
491
elimination of many impurities and better temperature reproducibility from one scan to another. Contamination of silicon with carbon, copper and other metals has occasionally been a problem in graphite strip heater experiments, while oxidation of electrical contacts, filament sagging and uneven sample contact with the lower area heater often cause nonuniform recrystallization. Karngar et al. [16,17] showed that radiant heating of an entire 3 inch wafer with a panel of six tungsten halogen lamps combined with zone melting with an additional motor-driven linear lamp achieved crystallization of silicon without causing any measurable contamination. The wafers had 0.5 ~tm of LPCVD silicon sandwiched between 0.5 ~tm polycrystalline of thermally grown isolation oxide and a capping layer of 2 ~tm of SiO 2 and 300 A of LPCVD silicon nitride. Subgrain boundaries were formed as in any other strip heater experiments, but the surface protrusions usually associated with these boundaries were absent, likely as a result of the film purity. Vu et al. [18] obtained similar results, but on smaller scale, by scanning a rectangular sample between two incandescent lamps, one to provide area heating, another tightly focused to achieve the molten zone of 7 mm length. Stultz et al. [19] built a hybrid system, with a conventional hot stage to elevate the samples to 1150°C, a mercury lamp They in anfabricated elliptical cavity as and a scanned line arc source. enhancement and depletion mode MOSFET’s in lamp recrystallized films and saw no dependence on the direction of channels relative to subboundaries. A 14 tim thick epitaxial film was also grown on a 0.5 j.tm recrystallized layer. The epitaxial film replicated the surface features and defects.
7. Conclusions Planar, stationary lamp arrays are excellent for recrystallization of relatively thick, 10—100 tim, silicon films on SiO2 when periodically spaced openings to the crystalline silicon substrate are provided for seeding and heat sinking. The recrystallized films are free of any grain boundaries
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including the small angle boundaries that are pervasive in other thin film recrystallization techniques. The density of dislocations is low except in the small “last-to-freeze” regions. These regions can be kept out of device areas by careful design of the photolithographic mask set. Bow-free recrystallization on 3 inch silicon wafers has been demonstrated and extension of this work to larger diameter wafers appears straightforward. Wafers are processed rapidly without any need to translate either the wafers or the heat source. This makes production-scale throughout feasible. While this paper has discussed recrystallization using stationary lamps, heating with scanned lamps also appears promising for applications where narrow zone melting is necessary, e.g. for recrystallization of thin films or where seeding openings are absent. All incoherent irradiation processes are limited to the use of substrates that can withstand high temperatures for several seconds. Crystallization of films on top of partially finished devices with shallow p—n junctions already in place will remain the domain of lasers and electron beams.
References [1] BR. Appleton and G.K. Celler, Eds., Laser and Electron Beam Interactions with Solids (North-Holland, New York 1982).
area recrystallization ofpolysilicon [2] J. Narayan, W.L. Brown and R.A. Lemons, Eds.,
[3] [4] [5] [6]
[7] [8]
Laser—Solid Interactions and Transient Thermal Processing of Materials (North-Holland, New York, 1983). H. Cline and T. Anthony, J. AppI. Phys. 49 (1978) 2412; US Patents 4,168,992; 4,170,490; 4,035,199. G.K. Celler, MeD. Robinson and D.J. Lischner, Appl. Phys. Letters 42 (1983) 99. G.K. Celler, MeD. Robinson, D.J. Lischner and T.T. Sheng, in ref. [2], pp. 575—580. J.L. Benton, G.K. Celler, D.C. Jacobson, L.C. Kimerling, D.J. Lischner, G.L. Miller and MeD. Robinson, in ref. [1], ~ 765—770. D.J. Lischner and G.K. Celler, in ref. [1], pp. 759—764. W.G. Hawkins and D.K. Biegelsen, Appl. Phys. Letters 42 (1983) 358.
[9] D.G. Electrochem. 126 (1979) [10] G.K. Schimmel, Celler, L.E.J.Trimble, MeD. Soc. Robinson, K.K.479. Ng and H.J. Leamy, Electronic Materials Conf., Fort Collins, CO, 1982. [11] L. Jastrzebski, J. Crystal Growth 63 (1983) 493. [12] MeD. Robinson, G.K. Celler and D.J. Lischner, Electrochem. Soc. Extended Abstracts 83-1 (1983) 147. [13] H. Tamura, M. Miyao and T. Tokuyama, Japan. J. Appl. Phys. 20 Suppl. 20-1 (1981) 43. [14] i.C.C. Fan, B.-Y. Tsaur and M.W. Geis, Appl. Phys. Letters 39 (1981) 308. [15] T. Suzuki, A. Mimura, T. Kamei and T. Ogawa, J. Electrochem. Soc. 127 (1977) 1537. [16] A. Kamgar and E. Labate, Mater. Letters 1(1982) 91. [17] A. Kamgar, G.A. Rozgonyi and R. Knoell, in ref. [2], pp. 569—574. [18] D.P. Vu, M. Haond, D. Bensahel and M. Dupuy, J. AppI. Phys. 54 (1983) 437. [19] T. Stultz, J. Sturm and J. Gibbons, in ref. [2], pp. 463—476.