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Properties of polycrystalline silicon grown on insulating substrates by electron beam gun evaporation
Thin Solid Films, 155 (1987) 97-113
97
PREPARATION AND CHARACTERIZATION
P R O P E R T I E S OF P O L Y C R Y S T A L L I N E S I L I C O N G R O W N O N I N S U L A T I N G S U B S T R A T E S BY E L E C T R O N BEAM G U N EVAPORATION N. K. ANNAMALAI
RADC-ESR, Hanscom Air Force Base, MA 01731 (U.S.A.) NARA MEYYAPPAN AND A. N. KHONDKER
C/arkson University, Potsdam, N Y 13676 (U.S.A.) (Received September ! 7, 1986; accepted June 4, 1987)
Growth of polycrystalline silicon on insulating substrates such as glass, silicon dioxide and fused quartz was studied using an electron beam gun evaporation technique. Growth characteristics were studied as a function of substrate temperature. The results of scanning electron microscopy, secondary ion mass spectrometry and X-ray diffraction studies on various films are presented. Hall mobility, resistivity and carrier concentration measurements are also presented. Growth of polycrystalline films (as determined by X-ray diffraction studies) on glass substrates at as low a temperature as 525 °C were observed. Below this substrate temperature, films became amorphous. The grain size increased with the increase in the substrate temperature. The highest value of the Hall mobility measured was about 10 cm 2 V - 1 s 1. Both n-type and p-type films were obtained.
1. INTRODUCTION
The CdS thin film transistor (TFT) was developed by Weimer ~ in 1961. A T F T is a field effect transistor made up of a polycrystalline semiconductor deposited on an insulating substrate. The current is modulated on the same principle as that of a metal oxide semiconductor transistor (MOST). A T F T has an insulating substrate and an M O S T has a semiconducting substrate. Hence, a T F T has a lower capacitance than an MOST, and it is possible to construct high speed circuits with TFTs. This polycrystalline layer is of high resistivity owing to grain boundaries. This in effect increases the threshold voltage VT required to turn on the transistor in the case of the T F T as opposed to the M O S T 2'3. A number of researchers 4 9 have fabricated TFTs using semiconductors such as silicon, PbS, PbSe, CdS, InSb and CdSe. Salama and Young 4 studied evaporated p-type silicon on a sapphire substrate. Kramer's investigation 5 revealed that PbS and PbSe TFTs were stable with A120 3 as the gate insulator and that PbTe TFTs were unstable. Erskine and Cserhati 6 developed vacuum-deposited CdS TFTs with sputtered SiO2 as the gate insulator. Baudrand e t al. 7 produced InSb TFTs and developed a theory of currentvoltage characteristics. Luo e t al. 8 reported on the characteristics ofCdSe TFTs they fabricated. The polycrystalline silicon (polysilicon) was deposited by low pressure 0040-6090/87/$3.50
© ElsevierSequoia/Printed in The Netherlands
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N. ANNAMALAI, N. MEYYAPPAN, A. KHONDKER
chemical vapor deposition (LPCVD) at 625 °C on fused quartz or alumina or oxidized single-crystal silicon 9. A 120 nm gate oxide was grown at 1050 °C and both n-channel and p-channel FETs were fabricated. Luo I o has reported on CdSe TFTs being successfully utilized in large-area TFT-addressed liquid crystal and electroluminescent display panels. Flat panel displays up to 0.1 m x 0.1 m with 250 x 250 elements have been demonstrated 1°. CdSe TFTs have exhibited acceptable electrical characteristics, stability and reproducibility for display applications. The main purpose in developing a T F T on an insulating substrate such as glass is to produce an active matrix flat panel display, especially a television screen. The important application of T F T s is as a switching element for active matrix fiat panel displays. An active matrix, according to Brody 11, is one which contains gainproducing, switching and/or memory elements at every mesh point, as opposed to a passive matrix, which is normally composed of two sets of parallel conductors, oriented at right-angles to each other, with the display medium usually sandwiched between them. To produce large area display panels (1 m x I m), it is necessary that the semiconductor can be deposited on a glass or a transparent substrate. The device layer being on top of an insulator, the device is radiation hardened and is suitable for high speed applications. The material selected for T F T fabrication is silicon. It is possible to grow silicon by electron beam gun evaporation. Silicon, being an elemental semiconductor, poses no stoichiometric problems as opposed to compound semiconductors. Silicon is cheap and pure compared with other T F T materials such as CdSe and CdS. Even though silicon could be deposited by L P C V D on an insulating substrate, it is not possible to deposit it on a large area substrate. Vacuum deposition of silicon on a large area substrate is merely an engineering problem. Vacuum deposition of materials on large area panels or substrates has been carried out previously for solar cell applications and for defense purposes 1°'11. It is possible to deposit various layers and materials through proper masks mounted on a rotatable carousel to fabricate a coplanar device in a single pump-down. Therefore, in this work we studied the growth properties of polysilicon on an insulating substrate, and the goal is to fabricate TFTs which are useful to produce flat panel displays, especially flat panel televisions. 2. EXPERIMENT
2.1. Polysilicon deposition A Veeco VE 747 high vacuum evaporator system equipped with a CTI Cryogenics Cryo-Torr 10 p u m p was used for polysilicon deposition. The system has a 32 in stainless steel water-cooled bell jar and 14 kW electron beam gun, model VB-14, with a power supply. In this study, three types of insulating substrates were used: (1) a 3 in x l in Corning microslide known as flint glass; (a) a thermally oxidized silicon wafer; (3) 1 in x I in Dynasil 6000 fused quartz, 1 m m thick, which is equivalent to Corning code 7940. A specially designed and built heater assembly was used for uniform heating of the substrate and to prevent substrate cracking. The temperature of the substrate was monitored by four thermocouples attached to the four corners of the substrate.
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A water-cooled 1/4 in thick copper block with proper opening for the substrates was used for efficient heat transfer and to carry excess heat to the outside of the chamber. Two 500 W, 6 in, quartz lamps enclosed in a stainless steel housing were placed at the back of the substrate. Fiberfrax insulation was used wherever necessary to prevent heat loss. An O m e g a Engineering automatic ten-channel signal scanner together with a 2168A Omega digital thermometer were used to measure the temperature from the thermocouples. The system was pumped down to a base pressure of 2.66x10 5 Pa (2 x 1 0 - 7 Torr) and during deposition the pressure was kept below 5 x 10 -4 Pa. It has been observed that during the deposition of silicon, for chamber pressures above 5 x 10 4 Pa, the silicon spattered out of the crucible. This required maintaining the evaporation rate within 0.36 nm s-1, controlling the heat dissipation within the chamber and developing a certain pattern for turning on the power to the heater and to electron beam gun. Even though the evaporation rate is a crucial parameter in determining the growth characteristics, because of system limitations a rate higher than 0.36 nm s-1 could not be used. Hence it was decided to use constant electron beam gun emission power to build a 1 p,m thick film in 1 h and to postpone a rate variation until a later date when we have carried out system modification. During deposition, temperature increase of the substrate was less than 10 °C. 99.9999% pure silicon was used as source material. The substrates were cleaned with Alconox, washed and rinsed with deionized water several times and dried with dry nitrogen. At times a 1.5 lam layer of gold was deposited at the back of the substrates to enhance uniform heat transfer and distribution during deposition and was then lifted off using Scotch tape. The substrates were mounted in the chamber and the system was pumped down. The shutter was kept closed, and the electron beam emission current was adjusted for a constant rate of evaporation. Deposition of silicon onto a hot substrate was achieved by opening the shutter. At the completion of deposition, the shutter was closed and the beam emission current was reduced and turned off. Cooling of the substrate was very critical to prevent microcracks on the film. After removing the sample from the system, it was placed in another system for electrode deposition.
2.2. Structural properties Structural properties of the film studied include film thickness measurement, X-ray diffraction analysis, secondary ion mass spectrometry (SIMS) and scanning electron microscopy (SEM) studies. The film thickness was measured using a Sloan interferometer, model M 100. X-ray diffraction analysis was performed on the film to determine its crystal structure, using a Diano Corporation microprocessorcontrolled diffractometer. A P e r k i n - E l m e r model 600 SIMS system was used to study the impurities in the deposited films. SIMS analysis was carried out on the source material for comparison. An SEM machine made by International Scientific Instruments, model ISI 40, was used to look at the film under a high magnification and to estimate the grain size.
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N. ANNAMALAI, N. MEYYAPPAN, A. KHONDKER
2.3. Electrical properties The determination of conductivity type (n or p), the resistivity, the carrier concentration and the Hall mobility etc., was carried out on the polysilicon films grown on various insulating substrates. Conductivity type was determined by a hot probe technique. A clover-leaf Van der Pauw structure was adopted for resistivity measurement, and a Keithley Instruments model 614 electrometer with an input impedance greater than 1012~ was used for the voltage measurement. Current was measured using a Keithley Instruments model 410A picoammeter. A special copper chamber was built for the Hall effect measurements. The chamber was fixed permanently between the pole faces of the electromagnet. The Berkeley Scientific Laboratory Inc. electromagnet and Harvey Wells power supply provided fields up to 1 0 0 0 0 G at 50A and 100V d.c. A Walker Scientific digital gaussmeter, model M G 3 D , was used to measure the magnetic field accurately. 3. RESULTS
3.1. Structural To develop a low cost large area display panel, a suitable substrate is glass. Silicon deposition on flint glass substrates was attempted at around 500-600 °C. At low temperatures, below 550 °C, the film was amorphous. The material was found to be p type by the hot probe technique. SIMS analysis showed all elements and compounds seen in silicon plus the compounds of glass substrates such as MgCO3 and NazCO3. This confirms the fact that M g C O 3, NazCO3 etc. out-diffused from glass into the silicon film during deposition when glass was used as substrate. Therefore, it was decided to use a substrate such as oxidized silicon or fused quartz. The impurities, if any, in these substrates do not out-diffuse at 600 700 °C and hence these substrates are suitable for the present purpose of learning more about the polysilicon growth properties on insulating substrates. Next, silicon dioxide was thermally grown on silicon wafers at GE, Syracuse, and this was used as the substrate. A substrate temperature of 600-700 °C was used. The pressure during deposition was less than 5 x 10 4 Pa (4 x 1 0 - 6 Torr). The use of a hot probe technique indicated the sample to be p type. On X-ray analysis, no peaks were seen, thus indicating no crystal structure. Aluminum doping is observed to be the cause of films' becoming p type. It was decided to try to use fused quartz as substrate for the next set of experiments because of the difficulties in performing X-ray diffraction studies on silicon films grown on SiO2 substrates. Silicon films were grown on high purity fused quartz at various substrate temperatures. The X-ray analysis result of a film grown on a fused quartz at 600 °C is shown in Fig. 1. The (111) and (220) crystal directions were seen. Peak heights, even though small compared with the standard, occurred at the expected angles. An SEM photograph of the above film on an unpolished quartz substrate is shown in Fig. 2. For this study, the surface of the film was prepared by etching it with a Dash etch to delineate the grain boundaries. The grainy structure of the polysilicon within the pits of the unpolished quartz substrate can be seen in Fig. 2.
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P O L Y - S i G R O W N ON I N S U L A T O R S BY EB G U N E V A P O R A T I O N
co
SAMPLE
Z
NUMBER
71
0
)-
z z
I
x 20
I
I
I
I
30
40
50
60
20 A N G L E
(DEGREES)
Fig. 1. X-ray intensity plot of polysilicon on fused quartz substrate.
Fig. 2. SEM photograph ofpolysilicon on an unpolished fused quartz substrate. (Magnification, 2800 x .) X-ray diffraction analysis was routinely performed on all the films. The results of the X-ray studies showed quite a scatter, indicating the non-uniformity of the films. This led us to believe in a non-uniform temperature distribution across the substrate surface. Hence X-ray analysis was performed on various spots of the same substrate. The variation in growth of the film across the substrate surface was proved, thereby confirming a variation in the substrate temperature distribution across the substrate surface. Thus, to ensure uniform substrate temperature, the temperature at various locations on the substrate was monitored. The correction of the non-uniform heating across the substrate surface was not easy, because the substrate was an insulator and heating was to be provided from the back. Several heater materials and designs were tried, to arrive at the final
102
N. ANNAMALAI,N. MEYYAPPAN, A. KHONDKER
a r r a n g e m e n t discussed in Section 2 to produce a uniform film. The rate of cooling of the substrate was critical in avoiding microcracks. In the earlier runs, after deposition was complete, the heater power was t u r n e d off a n d the sample was allowed to cool in the system. O n observing these films under SEM, microcracks were seen o n the surface. S E M p h o t o g r a p h s of the microcracks are shown in Fig. 3 at lower magnification and in Fig. 4 at higher magnification. The microcracks
Fig. 3. SEM photograph of microcracks ofa polysiliconfilmon a fused quartz substrate. (Magnification, l12x.I
Fig. 4. SEM photograph ofmicrocracks ofa polysiliconfilm on a fused quartz substrate. (Magnification, 5600 × .)
POLY-Si GROWN ON INSULATORS BY EB GUN EVAPORATION
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developed are due to the mismatch in the coefficients of thermal expansion between silicon (2.6 x 10 -6 °C 1) and fused quartz (5.5 z 10 7 oC-1), in order to eliminate the microcracks, two procedures were adopted: (1) instead of completely turning off the power to the heater at the end, it was reduced slowly and thus the cooling rate was adjusted; (2) a heat sink was used, consisting of a copper block or a thin layer of deposited copper on the back of the substrate. Final heater design using two quartz lamps and copper-deposited quartz substrates helped us to produce microcrack-free uniform silicon films. Microcracks on films grown on insulating substrates have been observed in other work too 12. A microcrack-free silicon film on a quartz substrate seen under SEM is shown in Fig. 5, and the same film is shown in Fig. 6 at higher magnification.
Fig. 5. SEM photograph ofpolysilicon film with microcracks almost eliminated. (Magnification, 112 x .)
G r o w t h characteristics of the polysilicon film were studied as a function of substrate temperature while all the other parameters were kept constant. The results of this study are presented here.
3.1.1. X-ray diffraction X-ray diffraction studies on films grown on a fused quartz substrate for various substrate temperatures are presented in Table I. Film thicknesses are also presented here. The evaporation rate was around 0.36 nm s 1 and in one or two cases it was 0.26 n m s - 1. From this table, the following observations are made. For substrate temperatures below 585 °C, the (220) crystal direction is dominant over the (111) crystal direction. At temperatures above 585 °C, the (111) crystal direction is preferred to the (220) crystal direction. Saraswat 13 observed that, using LPCVD, films grown above a 580 °C substrate temperature were polycrystalline. The films grown below 580 °C were found to be
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N. ANNAMALAI, N. MEYYAPPAN, A. KHONDKER
f
Fig. 6. SEM photograph of polysilicon film with microcracks almost eliminated. (Magnification, 5600 x .) TABLEI R E L A T I V E X - R A Y I N T E N S I T Y A N D T H I C K N E S S OF V A R I O U S SAMPLES
Substrate temperature
Sample number
(C) 525 540 550 555 560 585 600 625 640
Film thickness a
Relative intensities b
(~m) 73 74 75 72 78 76 77 70 71
0.8834 0.8025 0.8837 1.0171 1.3011 0.9629 1.4230 1.3235 1.3315
(11I)
(220)
0.0 0.0 0.0 0.0 0.0 0.80 0.73 0.98 1.00
I).71 0.62 0.98 0.88 0.80 c 0.49 0.49 0.44 0.40
Thickness was determined using an interferometer. b X-ray machine setting were kept exactly the same for every run. Glass substrate, all others are fused quartz substrates.
amorphous. However, the present authors, using the electron beam deposition technique, have shown that polysilicon films could be grown when the substrate temperature is above 525 °C. Films grown by this technique are amorphous when the substrate temperature is kept below 525 °C. Hence this method is a definite improvement over the L P C V D technique for growing films on low cost large area substrates such as glass. Saraswat 13 observed the (220) crystal direction to be dominant over the (111) direction for the temperature range 580 640 °C. He also observed that the intensity of the (220) direction increased as the temperature increased from 580 to 620 °C
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POLY-Si GROWN ON INSULATORS BY EB GUN EVAPORATION
where the intensity reached a m a x i m u m and then started to decrease. In the same temperature range, using the electron beam method, it was observed that the (111) direction was dominant over the (220) direction. Saraswat has observed the (311) crystal direction in the temperature range 580-620 °C, which has been observed using the electron beam method at a substrate temperature of 700 °C according to a recent study in this laboratory ~4. This shows that the crystal growth mechanisms for silicon using the L P C V D and the electron beam techniques may be very different in nature. LPCVD, being a gaseous reaction on the substrate, requires a higher temperature than the electron beam deposition to form a polysilicon film on an insulating substrate. The substrate so far discussed is fused quartz. It was possible to grow polysilicon film on a glass substrate and an X-ray study of such a film is shown in Fig. 7.
SAMPLE
NUMBER
78
Z
0 0
pZ ILl I.--
t~ tN
Z
I X
20
I
I
I
•
30
40
50
60
29
ANGLE
(DEGREES)
Fig. 7. X-ray intensity plot of polysilicon on glass substrate.
3.1.2. Scanning electron microscopy studies SEM studies were performed on the films grown at various substrate temperatures to view the surface structure and to estimate the grain size. The surface of the films was etched using a Sirtl etch (50 g of chromium trioxide dissolved in 100 ml of water, mixed with 100 ml of hydrofluoric acid) to delineate the grain boundaries. An SEM photograph of a film grown with a substrate temperature of 585 °C is shown in Fig. 8. The film was found to be polycrystalline on X-ray diffraction analysis and, as can be seen from the photograph, it is a smooth fine-grain film. Similar SEM photographs for films grown at substrate temperatures of 625 °C and 640°C are shown in Figs. 9 and 10 respectively. Magnification of all photographs were taken at 10000x to make grain comparisons possible. On comparing Figs. 8-10, the grain size increases remarkably when the substrate temperature is increased. The grain size is very small and hard to determine when the substrate temperature is 585 °C. The grain size is 90 nm at the 625 °C substrate temperature and 100 nm at the 640 °C substrate temperature. The estimation was performed by counting the number of grains in an area of size 1.2 crn x 1.2 cm (1 pm x 1 lam on the film) on the photograph. The grain size estimation was carried out at various spots on the film. The film was seen to be very uniform and proves that the substrate temperature was kept uniform over the surface during deposition.
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N. ANNAMALAI,
N. MEYYAPPAN,
Fig. S. SEM photograph 8000 x .)
of polysilicon
on polished
fused quartz
substrate
Fig. 9. SEM photograph
of polysiiicon
on polished
fused quartz. substrate
A. KHONDKER
at 585 C. (Magmficatlon,
at 62S’.C. (Magnilicatlon,
8000 k .) SaraswatL3 has observed that at 625 ‘C, with a film thickness of 1 pm, the average grain size was 300 nm. This shows that the grain size of as-deposited films formed by the LPCVD technique is higher than that of the electron beam deposition technique at these substrate temperatures. We were successful in growing a polysilicon film even with a substrate temperature of 525 “C, but Saraswat mentioned his inability to grow as-deposited polysilicon films below 580,‘C. It has
POLY-Si G R O W N ON INSULATORS BY EB GUN EVAPORATION
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Fig. 10. SEM photograph of polysilicon on polished fused quartz substrate at 640 cC. (Magnification, 8000 x .)
also been observed by Saraswat that as the substrate temperature was increased the grain size also increased. This is in agreement with our findings. 3.1.3. Secondary ion mass spectrometry studies SIMS studies were carried out on the films deposited at various substrate temperatures to study the impurities in the film. A SIMS study was also carried out on the silicon source material in order that the impurities picked up by the film during deposition can be understood. The + S I M S profile of the silicon source material is shown in Fig. 1 l(a) for 0-100 ainu and Fig. 1 l(b) for 100-200 ainu. It can be seen that the silicon source material contains impurities as follows: lithium, sodium, calcium, magnesium, potassium, phosphorus, copper, 02, chlorine, MoO, molybdenum and tin. The - S I M S profile of the silicon source material used is shown in Fig. 12 where impurities such as magnesium, O2, carbon and chlorine can be seen. - S I M S is more sensitive to chlorine than + S I M S . A + S I M S profile of a film deposit on a fused quartz substrate at a temperature of 600 °C is shown in Fig. 13 and reveals the incorporation of the same type of impurities. The film is far purer than the silicon source material. This may be due to preferential sticking of silicon compared with m a n y impurities. The film incorporates some of these impurities such as sodium, calcium, phosphorus, 02, magnesium, potassium, copper, molybdenum, MoO, tin and chlorine. All these impurities except molybdenum arise as a result of their presence in the source material. Molybdenum may have been introduced from the molybdenum shutter in the chamber. Copper impurities in these films have doped them to be an n-type polysilicon. The copper is introduced from the source material, the copper block used for uniform heat distribution of the substrate and the copper crucible. A - SIMS profile for the film deposited at a substrate temperature of 600 °C is shown in Fig. 14. Impurities such as oxygen, magnesium, chlorine, and carbon are seen clearly on the
N. ANNAMALAI,
108 7
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,
,
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profiles of silicon source material used for (a) O-100 amu and(b)
100-200
amu
film. Even though the deposition of silicon was carried out at pressures well 5~10~~ Pa (4x lO_‘Torr), incorporation of oxygen into the deposited inevitable. In addition the source silicon material also had oxygen in it. analysis showed an oxygen presence in the source silicon material and also deposited film as evidenced by Figs. 11-14.
below film is SIMS in the
3.2. Electrical The electrical films grown on fused quartz substrates at various substrate temperatures were subjected to a number of electrical measurements such as conductivity type, resistivity and Hall mobility, and the results of the study are presented here.
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Fig. 12. - SIMS profile of silicon sourcematerial used for 0-100 ainu.
3.2.1. Conductivity type The conductivity type of the films deposited as tested using the hot-probe technique and the results are listed in Table II. The source material was intrinsic silicon and hence intrinsic or slightly p-type films were expected. The first film deposited was intrinsic, as expected. All the other films were n type as a result of subsequent copper contamination. The SIMS analysis shows the copper presence in both the source material and the film. 3.2.2. Resistivity measurement It was necessary to have good ohmic contact with the films when we deposited metallic contact pads on the silicon film. The I - V characteristic between the pads was checked using a transistor curve tracer. The polarity of the pads was changed and the I - V characteristics were checked. Straight-line I - V characteristics resulted, indicating that the contact was ohmic. The resistivity of the films deposited on fused quartz substrates at various temperatures was measured and are listed in Table III. The resistivity of the film deposited at 535 °C (sample number 66) was 1.3 × 107 f2 cm (see Table II). The film was found to be intrinsic. It was impossible to determine the Hall voltage on this film. Hence the Hall mobility and carrier concentration of this film could not be calculated. The doping concentration will definitely be higher than the measured carrier concentration owing to carrier trapping at the grain boundaries. The doping concentrations of the films estimated using the work of Seto 15 were all found to be in the 10 is cm - 3 range, which is the critical doping concentration for polysilicon films 15. One can observe a large variation in the resistivity and mobility values for this film. The reason for this large variation can be explained by noting that the doping concentration for the polysilicon film is near a critical concentration 15 where the resistivity and mobility vary over several orders of magnitude.
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N. ANNAMALAI,
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N. MEYYAPPAN.
WITH OXYGEN
A. KHONDKER
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10
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+ SIMS
3.2.3.
150 MASS
160
I
I
170
180
190
200
UNITS
profiles of silicon on fused quartz substrate
for (a) O-100 amu and (b) 100-200 amu
Hull mohilit~~ meusurement The Hall mobility of the films deposited on fused quartz substrates at various temperatures was measured and the results are listed in Table III. The Hall mobility values are low, the highest value being about 10 cm2 V1 s- I. The low mobility values are expected for these polysilicon films, considering the grain boundaries. Furthermore, differences in the grain size distributions of these films may have contributed to the observed variation in resistivities and mobilities. In addition, grain size is also a function of film thickness.
111
POLY-Si GROWN ON INSULATORS BY EB GUN EVAPORATION
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50
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l.
I,L 80
90
100
Fig, 14. - SIMS profile of silicon on fused quartz substrate for 0-100 ainu.
TABLE lI CONDUCTIVITY TYPE OF VARIOUS DEPOSITED FILMS (FUSED QUARTZ SUBSTRATES)
Substrate temperature (c)
Sample number
Conductivity type
525 535 540 550 555 560 570 580 585 600 600 625 640
73 66(first film deposited) 74 75 72 78(only glasssubstrate) 67 68 76 69 77 70 71
n Intrinsic n n n n n n n n n n n
4. CONCLUSION P o l y c r y s t a l l i n e s i l i c o n films h a v e b e e n s u c c e s s f u l l y g r o w n o n g l a s s a n d f u s e d q u a r t z s u b s t r a t e s u s i n g a n e l e c t r o n b e a m g u n e v a p o r a t i o n t e c h n i q u e in a h i g h v a c u u m ( b e t t e r t h a n 5 × 10 - 4 Pa). F i l m s w e r e s e e n t o b e a m o r p h o u s in s t r u c t u r e w h e n g r o w n a t s u b s t r a t e t e m p e r a t u r e s b e l o w 525 °C. T h e films w e r e p o l y c r y s t a l l i n e w h e n g r o w n a t s u b s t r a t e t e m p e r a t u r e s a b o v e 525 °C.
ll2
N. A N N A M A L A I , N. M E Y Y A P P A N , A. K H O N D K E R
T A B L E II1 RESISTIVITY, HALL MOBILITY AND CARRIER CONCENTRATION AS A FUNCTION OF SUBSTRATE TEMPERATURE
Substrate temperature
Sample number
Resistit, ity (f~ cm)
(C)
Hall mobility (cm2V
525 535 540 555 570 580 600
73 66 74 72 67 68 77
3.16 1.3 x 107 6.97 88.70 456.00 93.00 200.00
Carrier concentration 1 s t)
2.40 8.78 0.35 1.18 1.37 10.13
(cm 3) 8,84 -1.37 2.01 1.18 5.10 3.30
x 10 t 7 x × × × ×
1017 10 Iv 1016 1016 101 s
The as-deposited films on the glass substrate are polycrystalline in nature. The impurities from the glass have been found to out-diffuse into the silicon film. In order to produce a device-quality film on the glass substrate, a transparent diffusion barrier layer between the glass and silicon layer has to be grown. The grain size of the films increased as the substrate temperature was increased. The average grain sizes of the films deposited at 625 °C and 640 "C were 90 nm and 100 nm respectively. The grain size could not be determined for films grown at lower substrate temperatures because the grains were very fine, These grain sizes are for asgrown films. Annealing techniques can be adopted to increase the grain sizes of the films. Laser annealing will be suitable for this purpose. It has been found that device-quality polysilicon films can be grown using the electron beam evaporation technique in a high vacuum (better than 5 x 10 . 4 Pa) on fused quartz substrates at low temperatures. It has been shown that the electron beam evaporation can be used to grow polysilicon films on low cost substrates such as glass. It has been found that copper added to the silicon source material used during evaporation yields n-type films and aluminum added to the silicon source material yields p-type films. Both n-type and p-type films could be grown, and hence it is possible to fabricate complementary metal oxide semiconductor devices on glass substrates, but the stability and the performance of these films need to be investigated, ACKNOWLEDGMEN3-
Part of this work was funded by NSF Grant ECS-8403880. REFERENCES 1
P . K . W e i m e r , A n evaporated thin film triode, IRE A1CE Development Research ConiC, Stan[brd
2
A . F . M . Anwer and A. N. K h o n d k e r , A n analytical model for the threshold voltage o f a polysilicon M O S F E T , Materials Research Society Symposia Proceedings, Vol. 47, Materials Research Society, Pittsburgh, P A , 1985. H . S . Lee, T h e field effect electron mobility of laser annealed polycrystalline silicon M O S F E T , Solid State Electron., 26 ( 11 ) (1981 ) 1059 1066.
University, Stanford, CA, June 1961.
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4 5 6 7 8 9
10
11 12
13 14 15
113
C . A . T . Salama and L. Young, Evaporated silicon thin-film transistors, Solid State Electron., 10 (1967) 473-484. G. Kramer, Thin-film transistor stability investigation, Rep. 74-0265/AESC 5014, January 1975 (Aerojet Electrosystems Company, Azusa, CA 91702). J.C. Erskine and A. Cserhati, Cadmium selenide thin film transistors, J. Vac. Sci. Technol., 15 (6) (1978) 1823 1835. H. Baudrand, E. Hamadto and J. L. Amalric, An experimental and theoretical study of polycrystalline thin film transistor, SolidState Electron., 24 (12) (1981) 1093 1098. F.C. Luo, I. Chen and F. C. Genovese, A thin-film transistor for flat panel displays, IEEE Trans. Electron Devices, 28 (6) (1981) 740-743. S.W. Depp, B. G. Huth, A. Juliana and R. W. Koepcke, Theory of MOSFET operation in smallgrain polysilicon, Materials Research Societ) Symposia Proceedings, Vol. 5, Materials Research Society, Pittsburgh, PA, 1982. F . C . Luo, Applications of CdSe thin-film transistors in flat panel displays, Materi,~ls Research Society Symposia Proceedings, Vol. 33, Materials Research Society, Pittsburgh, PA, 1984, pp. 239 245. T.O. Brody, Large scale integration for display screens, 1EEE Trans. Consum. Electron., 21 (3) (1975) 260 289. B.A. Lombos and M. Pietrantonio, Deep level analysis of polysilicon for solar cells. In J. P. Dismukes, E. Sirtl, P. RaiChondhury and L. P. Hunt (eds.), Proc. 3rd Symp. on Materials and New Proceeding Technologies,for Photovoltaics, Vol. 82-8, Electrochemical Society, Pennington, N J, 1982, pp. 343 355. K.C. Saraswat, Physical and electrical properties of polycrystalline silicon thin films, Materials Research Society Symposia Proceedings, Vol. 5, Materials Research Society, Pittsburgh, PA, 1982. R. Morrisey, personalcommunication, 1984. J.Y.W. Seto, The electrical properties ofpolycrystalline silicon films, J. Appl. Phys., 46 (12) (1975) 5247 5254.
97
PREPARATION AND CHARACTERIZATION
P R O P E R T I E S OF P O L Y C R Y S T A L L I N E S I L I C O N G R O W N O N I N S U L A T I N G S U B S T R A T E S BY E L E C T R O N BEAM G U N EVAPORATION N. K. ANNAMALAI
RADC-ESR, Hanscom Air Force Base, MA 01731 (U.S.A.) NARA MEYYAPPAN AND A. N. KHONDKER
C/arkson University, Potsdam, N Y 13676 (U.S.A.) (Received September ! 7, 1986; accepted June 4, 1987)
Growth of polycrystalline silicon on insulating substrates such as glass, silicon dioxide and fused quartz was studied using an electron beam gun evaporation technique. Growth characteristics were studied as a function of substrate temperature. The results of scanning electron microscopy, secondary ion mass spectrometry and X-ray diffraction studies on various films are presented. Hall mobility, resistivity and carrier concentration measurements are also presented. Growth of polycrystalline films (as determined by X-ray diffraction studies) on glass substrates at as low a temperature as 525 °C were observed. Below this substrate temperature, films became amorphous. The grain size increased with the increase in the substrate temperature. The highest value of the Hall mobility measured was about 10 cm 2 V - 1 s 1. Both n-type and p-type films were obtained.
1. INTRODUCTION
The CdS thin film transistor (TFT) was developed by Weimer ~ in 1961. A T F T is a field effect transistor made up of a polycrystalline semiconductor deposited on an insulating substrate. The current is modulated on the same principle as that of a metal oxide semiconductor transistor (MOST). A T F T has an insulating substrate and an M O S T has a semiconducting substrate. Hence, a T F T has a lower capacitance than an MOST, and it is possible to construct high speed circuits with TFTs. This polycrystalline layer is of high resistivity owing to grain boundaries. This in effect increases the threshold voltage VT required to turn on the transistor in the case of the T F T as opposed to the M O S T 2'3. A number of researchers 4 9 have fabricated TFTs using semiconductors such as silicon, PbS, PbSe, CdS, InSb and CdSe. Salama and Young 4 studied evaporated p-type silicon on a sapphire substrate. Kramer's investigation 5 revealed that PbS and PbSe TFTs were stable with A120 3 as the gate insulator and that PbTe TFTs were unstable. Erskine and Cserhati 6 developed vacuum-deposited CdS TFTs with sputtered SiO2 as the gate insulator. Baudrand e t al. 7 produced InSb TFTs and developed a theory of currentvoltage characteristics. Luo e t al. 8 reported on the characteristics ofCdSe TFTs they fabricated. The polycrystalline silicon (polysilicon) was deposited by low pressure 0040-6090/87/$3.50
© ElsevierSequoia/Printed in The Netherlands
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N. ANNAMALAI, N. MEYYAPPAN, A. KHONDKER
chemical vapor deposition (LPCVD) at 625 °C on fused quartz or alumina or oxidized single-crystal silicon 9. A 120 nm gate oxide was grown at 1050 °C and both n-channel and p-channel FETs were fabricated. Luo I o has reported on CdSe TFTs being successfully utilized in large-area TFT-addressed liquid crystal and electroluminescent display panels. Flat panel displays up to 0.1 m x 0.1 m with 250 x 250 elements have been demonstrated 1°. CdSe TFTs have exhibited acceptable electrical characteristics, stability and reproducibility for display applications. The main purpose in developing a T F T on an insulating substrate such as glass is to produce an active matrix flat panel display, especially a television screen. The important application of T F T s is as a switching element for active matrix fiat panel displays. An active matrix, according to Brody 11, is one which contains gainproducing, switching and/or memory elements at every mesh point, as opposed to a passive matrix, which is normally composed of two sets of parallel conductors, oriented at right-angles to each other, with the display medium usually sandwiched between them. To produce large area display panels (1 m x I m), it is necessary that the semiconductor can be deposited on a glass or a transparent substrate. The device layer being on top of an insulator, the device is radiation hardened and is suitable for high speed applications. The material selected for T F T fabrication is silicon. It is possible to grow silicon by electron beam gun evaporation. Silicon, being an elemental semiconductor, poses no stoichiometric problems as opposed to compound semiconductors. Silicon is cheap and pure compared with other T F T materials such as CdSe and CdS. Even though silicon could be deposited by L P C V D on an insulating substrate, it is not possible to deposit it on a large area substrate. Vacuum deposition of silicon on a large area substrate is merely an engineering problem. Vacuum deposition of materials on large area panels or substrates has been carried out previously for solar cell applications and for defense purposes 1°'11. It is possible to deposit various layers and materials through proper masks mounted on a rotatable carousel to fabricate a coplanar device in a single pump-down. Therefore, in this work we studied the growth properties of polysilicon on an insulating substrate, and the goal is to fabricate TFTs which are useful to produce flat panel displays, especially flat panel televisions. 2. EXPERIMENT
2.1. Polysilicon deposition A Veeco VE 747 high vacuum evaporator system equipped with a CTI Cryogenics Cryo-Torr 10 p u m p was used for polysilicon deposition. The system has a 32 in stainless steel water-cooled bell jar and 14 kW electron beam gun, model VB-14, with a power supply. In this study, three types of insulating substrates were used: (1) a 3 in x l in Corning microslide known as flint glass; (a) a thermally oxidized silicon wafer; (3) 1 in x I in Dynasil 6000 fused quartz, 1 m m thick, which is equivalent to Corning code 7940. A specially designed and built heater assembly was used for uniform heating of the substrate and to prevent substrate cracking. The temperature of the substrate was monitored by four thermocouples attached to the four corners of the substrate.
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A water-cooled 1/4 in thick copper block with proper opening for the substrates was used for efficient heat transfer and to carry excess heat to the outside of the chamber. Two 500 W, 6 in, quartz lamps enclosed in a stainless steel housing were placed at the back of the substrate. Fiberfrax insulation was used wherever necessary to prevent heat loss. An O m e g a Engineering automatic ten-channel signal scanner together with a 2168A Omega digital thermometer were used to measure the temperature from the thermocouples. The system was pumped down to a base pressure of 2.66x10 5 Pa (2 x 1 0 - 7 Torr) and during deposition the pressure was kept below 5 x 10 -4 Pa. It has been observed that during the deposition of silicon, for chamber pressures above 5 x 10 4 Pa, the silicon spattered out of the crucible. This required maintaining the evaporation rate within 0.36 nm s-1, controlling the heat dissipation within the chamber and developing a certain pattern for turning on the power to the heater and to electron beam gun. Even though the evaporation rate is a crucial parameter in determining the growth characteristics, because of system limitations a rate higher than 0.36 nm s-1 could not be used. Hence it was decided to use constant electron beam gun emission power to build a 1 p,m thick film in 1 h and to postpone a rate variation until a later date when we have carried out system modification. During deposition, temperature increase of the substrate was less than 10 °C. 99.9999% pure silicon was used as source material. The substrates were cleaned with Alconox, washed and rinsed with deionized water several times and dried with dry nitrogen. At times a 1.5 lam layer of gold was deposited at the back of the substrates to enhance uniform heat transfer and distribution during deposition and was then lifted off using Scotch tape. The substrates were mounted in the chamber and the system was pumped down. The shutter was kept closed, and the electron beam emission current was adjusted for a constant rate of evaporation. Deposition of silicon onto a hot substrate was achieved by opening the shutter. At the completion of deposition, the shutter was closed and the beam emission current was reduced and turned off. Cooling of the substrate was very critical to prevent microcracks on the film. After removing the sample from the system, it was placed in another system for electrode deposition.
2.2. Structural properties Structural properties of the film studied include film thickness measurement, X-ray diffraction analysis, secondary ion mass spectrometry (SIMS) and scanning electron microscopy (SEM) studies. The film thickness was measured using a Sloan interferometer, model M 100. X-ray diffraction analysis was performed on the film to determine its crystal structure, using a Diano Corporation microprocessorcontrolled diffractometer. A P e r k i n - E l m e r model 600 SIMS system was used to study the impurities in the deposited films. SIMS analysis was carried out on the source material for comparison. An SEM machine made by International Scientific Instruments, model ISI 40, was used to look at the film under a high magnification and to estimate the grain size.
100
N. ANNAMALAI, N. MEYYAPPAN, A. KHONDKER
2.3. Electrical properties The determination of conductivity type (n or p), the resistivity, the carrier concentration and the Hall mobility etc., was carried out on the polysilicon films grown on various insulating substrates. Conductivity type was determined by a hot probe technique. A clover-leaf Van der Pauw structure was adopted for resistivity measurement, and a Keithley Instruments model 614 electrometer with an input impedance greater than 1012~ was used for the voltage measurement. Current was measured using a Keithley Instruments model 410A picoammeter. A special copper chamber was built for the Hall effect measurements. The chamber was fixed permanently between the pole faces of the electromagnet. The Berkeley Scientific Laboratory Inc. electromagnet and Harvey Wells power supply provided fields up to 1 0 0 0 0 G at 50A and 100V d.c. A Walker Scientific digital gaussmeter, model M G 3 D , was used to measure the magnetic field accurately. 3. RESULTS
3.1. Structural To develop a low cost large area display panel, a suitable substrate is glass. Silicon deposition on flint glass substrates was attempted at around 500-600 °C. At low temperatures, below 550 °C, the film was amorphous. The material was found to be p type by the hot probe technique. SIMS analysis showed all elements and compounds seen in silicon plus the compounds of glass substrates such as MgCO3 and NazCO3. This confirms the fact that M g C O 3, NazCO3 etc. out-diffused from glass into the silicon film during deposition when glass was used as substrate. Therefore, it was decided to use a substrate such as oxidized silicon or fused quartz. The impurities, if any, in these substrates do not out-diffuse at 600 700 °C and hence these substrates are suitable for the present purpose of learning more about the polysilicon growth properties on insulating substrates. Next, silicon dioxide was thermally grown on silicon wafers at GE, Syracuse, and this was used as the substrate. A substrate temperature of 600-700 °C was used. The pressure during deposition was less than 5 x 10 4 Pa (4 x 1 0 - 6 Torr). The use of a hot probe technique indicated the sample to be p type. On X-ray analysis, no peaks were seen, thus indicating no crystal structure. Aluminum doping is observed to be the cause of films' becoming p type. It was decided to try to use fused quartz as substrate for the next set of experiments because of the difficulties in performing X-ray diffraction studies on silicon films grown on SiO2 substrates. Silicon films were grown on high purity fused quartz at various substrate temperatures. The X-ray analysis result of a film grown on a fused quartz at 600 °C is shown in Fig. 1. The (111) and (220) crystal directions were seen. Peak heights, even though small compared with the standard, occurred at the expected angles. An SEM photograph of the above film on an unpolished quartz substrate is shown in Fig. 2. For this study, the surface of the film was prepared by etching it with a Dash etch to delineate the grain boundaries. The grainy structure of the polysilicon within the pits of the unpolished quartz substrate can be seen in Fig. 2.
101
P O L Y - S i G R O W N ON I N S U L A T O R S BY EB G U N E V A P O R A T I O N
co
SAMPLE
Z
NUMBER
71
0
)-
z z
I
x 20
I
I
I
I
30
40
50
60
20 A N G L E
(DEGREES)
Fig. 1. X-ray intensity plot of polysilicon on fused quartz substrate.
Fig. 2. SEM photograph ofpolysilicon on an unpolished fused quartz substrate. (Magnification, 2800 x .) X-ray diffraction analysis was routinely performed on all the films. The results of the X-ray studies showed quite a scatter, indicating the non-uniformity of the films. This led us to believe in a non-uniform temperature distribution across the substrate surface. Hence X-ray analysis was performed on various spots of the same substrate. The variation in growth of the film across the substrate surface was proved, thereby confirming a variation in the substrate temperature distribution across the substrate surface. Thus, to ensure uniform substrate temperature, the temperature at various locations on the substrate was monitored. The correction of the non-uniform heating across the substrate surface was not easy, because the substrate was an insulator and heating was to be provided from the back. Several heater materials and designs were tried, to arrive at the final
102
N. ANNAMALAI,N. MEYYAPPAN, A. KHONDKER
a r r a n g e m e n t discussed in Section 2 to produce a uniform film. The rate of cooling of the substrate was critical in avoiding microcracks. In the earlier runs, after deposition was complete, the heater power was t u r n e d off a n d the sample was allowed to cool in the system. O n observing these films under SEM, microcracks were seen o n the surface. S E M p h o t o g r a p h s of the microcracks are shown in Fig. 3 at lower magnification and in Fig. 4 at higher magnification. The microcracks
Fig. 3. SEM photograph of microcracks ofa polysiliconfilmon a fused quartz substrate. (Magnification, l12x.I
Fig. 4. SEM photograph ofmicrocracks ofa polysiliconfilm on a fused quartz substrate. (Magnification, 5600 × .)
POLY-Si GROWN ON INSULATORS BY EB GUN EVAPORATION
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developed are due to the mismatch in the coefficients of thermal expansion between silicon (2.6 x 10 -6 °C 1) and fused quartz (5.5 z 10 7 oC-1), in order to eliminate the microcracks, two procedures were adopted: (1) instead of completely turning off the power to the heater at the end, it was reduced slowly and thus the cooling rate was adjusted; (2) a heat sink was used, consisting of a copper block or a thin layer of deposited copper on the back of the substrate. Final heater design using two quartz lamps and copper-deposited quartz substrates helped us to produce microcrack-free uniform silicon films. Microcracks on films grown on insulating substrates have been observed in other work too 12. A microcrack-free silicon film on a quartz substrate seen under SEM is shown in Fig. 5, and the same film is shown in Fig. 6 at higher magnification.
Fig. 5. SEM photograph ofpolysilicon film with microcracks almost eliminated. (Magnification, 112 x .)
G r o w t h characteristics of the polysilicon film were studied as a function of substrate temperature while all the other parameters were kept constant. The results of this study are presented here.
3.1.1. X-ray diffraction X-ray diffraction studies on films grown on a fused quartz substrate for various substrate temperatures are presented in Table I. Film thicknesses are also presented here. The evaporation rate was around 0.36 nm s 1 and in one or two cases it was 0.26 n m s - 1. From this table, the following observations are made. For substrate temperatures below 585 °C, the (220) crystal direction is dominant over the (111) crystal direction. At temperatures above 585 °C, the (111) crystal direction is preferred to the (220) crystal direction. Saraswat 13 observed that, using LPCVD, films grown above a 580 °C substrate temperature were polycrystalline. The films grown below 580 °C were found to be
104
N. ANNAMALAI, N. MEYYAPPAN, A. KHONDKER
f
Fig. 6. SEM photograph of polysilicon film with microcracks almost eliminated. (Magnification, 5600 x .) TABLEI R E L A T I V E X - R A Y I N T E N S I T Y A N D T H I C K N E S S OF V A R I O U S SAMPLES
Substrate temperature
Sample number
(C) 525 540 550 555 560 585 600 625 640
Film thickness a
Relative intensities b
(~m) 73 74 75 72 78 76 77 70 71
0.8834 0.8025 0.8837 1.0171 1.3011 0.9629 1.4230 1.3235 1.3315
(11I)
(220)
0.0 0.0 0.0 0.0 0.0 0.80 0.73 0.98 1.00
I).71 0.62 0.98 0.88 0.80 c 0.49 0.49 0.44 0.40
Thickness was determined using an interferometer. b X-ray machine setting were kept exactly the same for every run. Glass substrate, all others are fused quartz substrates.
amorphous. However, the present authors, using the electron beam deposition technique, have shown that polysilicon films could be grown when the substrate temperature is above 525 °C. Films grown by this technique are amorphous when the substrate temperature is kept below 525 °C. Hence this method is a definite improvement over the L P C V D technique for growing films on low cost large area substrates such as glass. Saraswat 13 observed the (220) crystal direction to be dominant over the (111) direction for the temperature range 580 640 °C. He also observed that the intensity of the (220) direction increased as the temperature increased from 580 to 620 °C
105
POLY-Si GROWN ON INSULATORS BY EB GUN EVAPORATION
where the intensity reached a m a x i m u m and then started to decrease. In the same temperature range, using the electron beam method, it was observed that the (111) direction was dominant over the (220) direction. Saraswat has observed the (311) crystal direction in the temperature range 580-620 °C, which has been observed using the electron beam method at a substrate temperature of 700 °C according to a recent study in this laboratory ~4. This shows that the crystal growth mechanisms for silicon using the L P C V D and the electron beam techniques may be very different in nature. LPCVD, being a gaseous reaction on the substrate, requires a higher temperature than the electron beam deposition to form a polysilicon film on an insulating substrate. The substrate so far discussed is fused quartz. It was possible to grow polysilicon film on a glass substrate and an X-ray study of such a film is shown in Fig. 7.
SAMPLE
NUMBER
78
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t~ tN
Z
I X
20
I
I
I
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30
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50
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29
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Fig. 7. X-ray intensity plot of polysilicon on glass substrate.
3.1.2. Scanning electron microscopy studies SEM studies were performed on the films grown at various substrate temperatures to view the surface structure and to estimate the grain size. The surface of the films was etched using a Sirtl etch (50 g of chromium trioxide dissolved in 100 ml of water, mixed with 100 ml of hydrofluoric acid) to delineate the grain boundaries. An SEM photograph of a film grown with a substrate temperature of 585 °C is shown in Fig. 8. The film was found to be polycrystalline on X-ray diffraction analysis and, as can be seen from the photograph, it is a smooth fine-grain film. Similar SEM photographs for films grown at substrate temperatures of 625 °C and 640°C are shown in Figs. 9 and 10 respectively. Magnification of all photographs were taken at 10000x to make grain comparisons possible. On comparing Figs. 8-10, the grain size increases remarkably when the substrate temperature is increased. The grain size is very small and hard to determine when the substrate temperature is 585 °C. The grain size is 90 nm at the 625 °C substrate temperature and 100 nm at the 640 °C substrate temperature. The estimation was performed by counting the number of grains in an area of size 1.2 crn x 1.2 cm (1 pm x 1 lam on the film) on the photograph. The grain size estimation was carried out at various spots on the film. The film was seen to be very uniform and proves that the substrate temperature was kept uniform over the surface during deposition.
106
N. ANNAMALAI,
N. MEYYAPPAN,
Fig. S. SEM photograph 8000 x .)
of polysilicon
on polished
fused quartz
substrate
Fig. 9. SEM photograph
of polysiiicon
on polished
fused quartz. substrate
A. KHONDKER
at 585 C. (Magmficatlon,
at 62S’.C. (Magnilicatlon,
8000 k .) SaraswatL3 has observed that at 625 ‘C, with a film thickness of 1 pm, the average grain size was 300 nm. This shows that the grain size of as-deposited films formed by the LPCVD technique is higher than that of the electron beam deposition technique at these substrate temperatures. We were successful in growing a polysilicon film even with a substrate temperature of 525 “C, but Saraswat mentioned his inability to grow as-deposited polysilicon films below 580,‘C. It has
POLY-Si G R O W N ON INSULATORS BY EB GUN EVAPORATION
107
Fig. 10. SEM photograph of polysilicon on polished fused quartz substrate at 640 cC. (Magnification, 8000 x .)
also been observed by Saraswat that as the substrate temperature was increased the grain size also increased. This is in agreement with our findings. 3.1.3. Secondary ion mass spectrometry studies SIMS studies were carried out on the films deposited at various substrate temperatures to study the impurities in the film. A SIMS study was also carried out on the silicon source material in order that the impurities picked up by the film during deposition can be understood. The + S I M S profile of the silicon source material is shown in Fig. 1 l(a) for 0-100 ainu and Fig. 1 l(b) for 100-200 ainu. It can be seen that the silicon source material contains impurities as follows: lithium, sodium, calcium, magnesium, potassium, phosphorus, copper, 02, chlorine, MoO, molybdenum and tin. The - S I M S profile of the silicon source material used is shown in Fig. 12 where impurities such as magnesium, O2, carbon and chlorine can be seen. - S I M S is more sensitive to chlorine than + S I M S . A + S I M S profile of a film deposit on a fused quartz substrate at a temperature of 600 °C is shown in Fig. 13 and reveals the incorporation of the same type of impurities. The film is far purer than the silicon source material. This may be due to preferential sticking of silicon compared with m a n y impurities. The film incorporates some of these impurities such as sodium, calcium, phosphorus, 02, magnesium, potassium, copper, molybdenum, MoO, tin and chlorine. All these impurities except molybdenum arise as a result of their presence in the source material. Molybdenum may have been introduced from the molybdenum shutter in the chamber. Copper impurities in these films have doped them to be an n-type polysilicon. The copper is introduced from the source material, the copper block used for uniform heat distribution of the substrate and the copper crucible. A - SIMS profile for the film deposited at a substrate temperature of 600 °C is shown in Fig. 14. Impurities such as oxygen, magnesium, chlorine, and carbon are seen clearly on the
N. ANNAMALAI,
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film. Even though the deposition of silicon was carried out at pressures well 5~10~~ Pa (4x lO_‘Torr), incorporation of oxygen into the deposited inevitable. In addition the source silicon material also had oxygen in it. analysis showed an oxygen presence in the source silicon material and also deposited film as evidenced by Figs. 11-14.
below film is SIMS in the
3.2. Electrical The electrical films grown on fused quartz substrates at various substrate temperatures were subjected to a number of electrical measurements such as conductivity type, resistivity and Hall mobility, and the results of the study are presented here.
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3.2.1. Conductivity type The conductivity type of the films deposited as tested using the hot-probe technique and the results are listed in Table II. The source material was intrinsic silicon and hence intrinsic or slightly p-type films were expected. The first film deposited was intrinsic, as expected. All the other films were n type as a result of subsequent copper contamination. The SIMS analysis shows the copper presence in both the source material and the film. 3.2.2. Resistivity measurement It was necessary to have good ohmic contact with the films when we deposited metallic contact pads on the silicon film. The I - V characteristic between the pads was checked using a transistor curve tracer. The polarity of the pads was changed and the I - V characteristics were checked. Straight-line I - V characteristics resulted, indicating that the contact was ohmic. The resistivity of the films deposited on fused quartz substrates at various temperatures was measured and are listed in Table III. The resistivity of the film deposited at 535 °C (sample number 66) was 1.3 × 107 f2 cm (see Table II). The film was found to be intrinsic. It was impossible to determine the Hall voltage on this film. Hence the Hall mobility and carrier concentration of this film could not be calculated. The doping concentration will definitely be higher than the measured carrier concentration owing to carrier trapping at the grain boundaries. The doping concentrations of the films estimated using the work of Seto 15 were all found to be in the 10 is cm - 3 range, which is the critical doping concentration for polysilicon films 15. One can observe a large variation in the resistivity and mobility values for this film. The reason for this large variation can be explained by noting that the doping concentration for the polysilicon film is near a critical concentration 15 where the resistivity and mobility vary over several orders of magnitude.
110
N. ANNAMALAI,
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I
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170
180
190
200
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profiles of silicon on fused quartz substrate
for (a) O-100 amu and (b) 100-200 amu
Hull mohilit~~ meusurement The Hall mobility of the films deposited on fused quartz substrates at various temperatures was measured and the results are listed in Table III. The Hall mobility values are low, the highest value being about 10 cm2 V1 s- I. The low mobility values are expected for these polysilicon films, considering the grain boundaries. Furthermore, differences in the grain size distributions of these films may have contributed to the observed variation in resistivities and mobilities. In addition, grain size is also a function of film thickness.
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POLY-Si GROWN ON INSULATORS BY EB GUN EVAPORATION
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2
t.O
u)
,~ 10
20
30
40 ATOMIC
I1 I
50
60
MASS
UNITS
70
l.
I,L 80
90
100
Fig, 14. - SIMS profile of silicon on fused quartz substrate for 0-100 ainu.
TABLE lI CONDUCTIVITY TYPE OF VARIOUS DEPOSITED FILMS (FUSED QUARTZ SUBSTRATES)
Substrate temperature (c)
Sample number
Conductivity type
525 535 540 550 555 560 570 580 585 600 600 625 640
73 66(first film deposited) 74 75 72 78(only glasssubstrate) 67 68 76 69 77 70 71
n Intrinsic n n n n n n n n n n n
4. CONCLUSION P o l y c r y s t a l l i n e s i l i c o n films h a v e b e e n s u c c e s s f u l l y g r o w n o n g l a s s a n d f u s e d q u a r t z s u b s t r a t e s u s i n g a n e l e c t r o n b e a m g u n e v a p o r a t i o n t e c h n i q u e in a h i g h v a c u u m ( b e t t e r t h a n 5 × 10 - 4 Pa). F i l m s w e r e s e e n t o b e a m o r p h o u s in s t r u c t u r e w h e n g r o w n a t s u b s t r a t e t e m p e r a t u r e s b e l o w 525 °C. T h e films w e r e p o l y c r y s t a l l i n e w h e n g r o w n a t s u b s t r a t e t e m p e r a t u r e s a b o v e 525 °C.
ll2
N. A N N A M A L A I , N. M E Y Y A P P A N , A. K H O N D K E R
T A B L E II1 RESISTIVITY, HALL MOBILITY AND CARRIER CONCENTRATION AS A FUNCTION OF SUBSTRATE TEMPERATURE
Substrate temperature
Sample number
Resistit, ity (f~ cm)
(C)
Hall mobility (cm2V
525 535 540 555 570 580 600
73 66 74 72 67 68 77
3.16 1.3 x 107 6.97 88.70 456.00 93.00 200.00
Carrier concentration 1 s t)
2.40 8.78 0.35 1.18 1.37 10.13
(cm 3) 8,84 -1.37 2.01 1.18 5.10 3.30
x 10 t 7 x × × × ×
1017 10 Iv 1016 1016 101 s
The as-deposited films on the glass substrate are polycrystalline in nature. The impurities from the glass have been found to out-diffuse into the silicon film. In order to produce a device-quality film on the glass substrate, a transparent diffusion barrier layer between the glass and silicon layer has to be grown. The grain size of the films increased as the substrate temperature was increased. The average grain sizes of the films deposited at 625 °C and 640 "C were 90 nm and 100 nm respectively. The grain size could not be determined for films grown at lower substrate temperatures because the grains were very fine, These grain sizes are for asgrown films. Annealing techniques can be adopted to increase the grain sizes of the films. Laser annealing will be suitable for this purpose. It has been found that device-quality polysilicon films can be grown using the electron beam evaporation technique in a high vacuum (better than 5 x 10 . 4 Pa) on fused quartz substrates at low temperatures. It has been shown that the electron beam evaporation can be used to grow polysilicon films on low cost substrates such as glass. It has been found that copper added to the silicon source material used during evaporation yields n-type films and aluminum added to the silicon source material yields p-type films. Both n-type and p-type films could be grown, and hence it is possible to fabricate complementary metal oxide semiconductor devices on glass substrates, but the stability and the performance of these films need to be investigated, ACKNOWLEDGMEN3-
Part of this work was funded by NSF Grant ECS-8403880. REFERENCES 1
P . K . W e i m e r , A n evaporated thin film triode, IRE A1CE Development Research ConiC, Stan[brd
2
A . F . M . Anwer and A. N. K h o n d k e r , A n analytical model for the threshold voltage o f a polysilicon M O S F E T , Materials Research Society Symposia Proceedings, Vol. 47, Materials Research Society, Pittsburgh, P A , 1985. H . S . Lee, T h e field effect electron mobility of laser annealed polycrystalline silicon M O S F E T , Solid State Electron., 26 ( 11 ) (1981 ) 1059 1066.
University, Stanford, CA, June 1961.
3
POLY-Si GROWN ON INSULATORS BY EB GUN EVAPORATION
4 5 6 7 8 9
10
11 12
13 14 15
113
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