UHV RHEED system for in-situ studies of sputtered films

ELSEVIER

Thin Solid

Films 270 ( 1995) 314-319

UHV RHEED system for in-situ studies of sputtered films S.R. Das a, L. LeBrun a, P.B. Sewell b, T. Tyrie ’ a Institute for Microstructural Sciences, National Research Council, Ottawa, Ont. KlA OR6, Canada b LAB-6 Box 259, Woodlawn. Ont. KOA 3M0, Canada ’ Kimball Physics Inc., Wilton, NH 030869742, USA

Accepted22 May 1995

Abstract An all-metal, bakeable, ultra-high vacuum reflection high-energy electron diffraction (UHV RHEED) system has been developed for insitu studies of films during deposition on surfaces by processes such as r.f. sputtering, where pressures in the region of the specimen may be as high as 3-4 Pa. The design separates the 50 kV gun vacuum from that of the specimen chamber with a single differential pumping aperture, which also serves as the beam-defining aperture. The electron optics enable a focussed spot of about 50 pm to be produced on the detector at a distance of 50 cm from the end of the magnetic lens housing, with adequate current for RHEED studies. The RHEED system has been installed in an UHV r.f. magnetron sputter-deposition chamber and has been successfully applied to (i) monitor, in the presence of the sputter discharge, the surface of a chemically cleaned n-type Si( 100) wafer and (ii) study the subsequent growth of Pt films on the silicon surface under different conditions of sputtering pressure and r.f. power. Keywords:

High energy electron diffraction;

Platinum; Silicides; Sputtering

1. Introduction Reflection high-energy electron diffraction (RHEED) has proven to be a very useful technique for monitoring epitaxial growth of thin films in high vacuum. Since RHEED is sensitive to surface rather than bulk structure, the technique can reveal, almost instantaneously, changes in the coverage of the sample surface by adsorbates as well as the crystallographic orientation and morphology of a thin film during growth [ 11. Indeed, RHEED has been extensively applied during molecular beam epitaxy [ 21 growth of device-quality epitaxial semiconductor thin films and multilayer structures. In recent years, several workers [3-71 have reported the growth of epitaxial semiconductor thin films by sputter-deposition. The epitaxial growth of high-purity copper [ 81 and aluminum [ 93 films for ultra-large scale integrated circuits and in-situ substrate surface cleaning for high-quality thin film formation [ IO,1 1] using low-energy ion bombardment in a magnetron sputtering system have also been reported. Although RHEED has been employed as an in-situ characterization tool in several of these studies [7-l 11, typically the RHEED patterns either have been recorded for the substrate surface before deposition or for the film surface after deposition, but not during the film growth or surface cleaning process. This is presumably due to the difficulty of operating conventional RHEED systems at the high pressures and in Elsevier Science S.A. SSDIOO40-6090(95)06750-7

the presence of r.f. fields and magnetic fields present during a sputter discharge. We should point out that RHEED observations at high pressures have been reported previously [ 121, but not in a sputtering environment. In this paper, we describe an all-metal, bakeable, ultra-high vacuum (UHV) RHEED system [ 131 that enables in-situ studies of specimens at pressures as high as 3 Pa (22.5 mTorr). The design separates the vacuum of the 50 kV gun chamber from that of the specimen chamber with a single 250 p,rn diameter differential pumping aperture, which also acts as the beam defining aperture. The RHEED system has been installed in an UHV system and has been utilized to study the growth of platinum (Pt) films on Si ( 100) substrates by r.f. magnetron sputtering. We present selected results from this study to demonstrate the successful operation of the RHEED system at various pressures and r.f. power levels used during growth of the Pt films.

2. Experimental 2.1. General The RHEED system, shown as a schematic in Fig. 1 (a), consists of (i) a 50 kV electron gun (A) with a LaB, cathode, (ii) a post-anode set of magnetic beam alignment coils (H),

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A LaB, source (Kimball Physics Inc., ES-423E style 9015) has been selected to produce adequate beam current through the 250 p.m limiting aperture, even at low accelerating voltages of 10 kV. At 10 kV, the brightness for a LaB, source operating at about 1850 K is about 2 X lo5 A cm-* sr ~ I [ 141 and the beam current through a 250 pm aperture located 34 cm from the source is about 1.5 X lo-’ A, which is suitable for RHEED studies from an efficient fluorescent screen. This can be achieved with the Style 90- 15 LaB, source with a heater current of 2 A and a total emission of about 40 p_A. At 50 kV, the beam current at the same total emission will be about 7.5x.10-’ A, which is more than required for RHEED studies. In practice, at 35 kV, adequate intensity of RHEED patterns from silicon substrates and overgrowths was obtained with a total electron emission of 20-25 pA. 2.3. The magnetic lens

09 Fig. 1. Schematic drawing of the 50 kV differential RHEED system. (a) Basic column components with column mounted on hW63CF flange. (b) Details of magnetic lens and differential aperture assembly. See text for description of components.

(iii) a magnetic lens (M) with the differential aperture (G) in the bore of the pole piece and (iv) a set of post-lens magnetic deflection coils (I) to allow adjustment of the grazing angle of incidence of the beam with respect to the sample. The entire column is of an all-metal design to allowing baking to about 520 K. A feature of the system is the small magnetic lens and post-lens deflection coil assembly, which fits over a 1.25 cm diameter vacuum wall (J) and is shrouded by an outer vacuum wall (K) of 5.7 cm diameter such that the entire assembly can be inserted through a standard NW63 vacuum port on a NW63CF (11.35 cm diameter) conflat flange. This enables the lens assembly to be inserted into the deposition chamber. This has the advantage of bringing the final lens of the column close to the specimen. This can be a particular advantage when using other versions of this system which use two lenses and permit the formation of sub-micron beams at specimens located a few centimeters from the end of the column. For the present system it has the convenience that the differential aperture unit is readily accessible for service from the deposition chamber.

2.2. The electron gun

The electron gun chamber (A) contains several NW16 conflat ports for ancillary apparati, including a viewing window (B), and one larger NW35 port (C) with a vacuum conductance of 65 1 s- I. The gun firing unit (D) is mounted on a ceramic insulator and an NW35 conflat flange assembly with bellows adjustment (E), which enables movement of the firing unit with respect to the anode (F) for coarse alignment of the beam with the aperture (G) in the magnetic lens (M).

The small magnetic lens (M) (Fig. 1 (b) ) has been designed with a gap-to-bore ratio to minimize spherical aberration and to allow for a minimum focal length of about 5 cm at 50 kV. For RHEED applications, the focal length is typically about 20 cm. Under no conditions of operation has it been found necessary to water cool the lens assembly. The lens windings and housing can be baked to 520 K so that the system is compatible with UHV requirements. The lens bore is co-axial with the outer diameter of a 1.23 cm o.d. stainless steel tube (J) which is connected to an outer vacuum housing (K) mounted on an NW63 conflat flange, so that the lens and the post-lens alignment coils (I) can be inserted into the experimental chamber. 2.4. The differential aperture The aperture mounting is shown in more detail in Fig. 1 (b), where the tubular unit containing the aperture is mounted in the bore of the lens vacuum wall (J) on a NW16 conflat flange assembly (L) so that the only conductance from the specimen chamber through to the gun chamber is via a replaceable 250 km diameter aperture. The importance of this type of arrangement is seen from computations of gas leakage for a 20 p,rn clearance on the diameter between the mounting tube and the vacuum wall which show that the conductance through such fit is about 0.02 1 s- ‘. This is about five times the gas conductance through the 250 pm diameter differential aperture. Similarly the aperture mounting assembly assures a tight fit of the standard 0.3 cm o.d. aperture disc containing the 250 p_rn aperture. The vacuum conductance of this aperture is about 0.004 1 s-’ for argon. Hence, with a chamber pressure of 2.67 Pa (20 mTorr) , the gas load to the gunchamberwas1.06X10-*Pa1s~‘(8~10~’T0rr1s-’). With a 60 1s- ’ turbopump closely coupled to the gun, a speed of about 30 1 s- ’ could be achieved with a resulting operating pressure in the gun chamber of about 4 X lop4 Pa (4 X lop6 Torr). In the present application, with the pump located further from the gun, the speed was reduced to about 5 1 SC’,

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with a resulting gun chamber pressure of about 2.7 Pa (2 X 10e5 Torr). With argon as the major gas, this presented no serious problem with the operation of the LaB, source.

Main Chamber

1

2.5. RHEED operation

I

The electron source is located about 34 cm from the bore of the lens and the screen was about 50 cm from the lens. The source magnification at focus is about X 1.5, so that a 15 p,m diameter cross-over in the gun would produce a 23 pm diameter spot at focus, in the absence of aberrations. In practice, the focussed spot diameter was much smaller than the resolution of the fluorescent screen. The physical mounting of the aperture in the bore of the lens did not allow for precise alignment of the aperture with the electron optical axis of the lens. However, the off-axis effect, which manifests itself in the through focal image displacement, was very small and did not limit the system for RHEED studies. The astigmatism of the lens was not resolvable in examination of the through focal spot sequence on the fluorescent screen at X 10 magnification. For RHEED studies, the specimen was withdrawn from the beam and the direct beam, without post-lens deflection, was focussed on the screen. The specimen was brought forward to intercept the beam at almost grazing incidence and then withdrawn from the beam and this movement was followed with the post-lens deflector until a suitable angle of incidence was obtained for diffraction studies. RHEED patterns were recorded from the fluorescent screen by a camera with an image reduction of X 0.75. High-quality RHEED patterns could be recorded for pressures in the deposition chamber as high as 2.7 Pa in the absence of the r.f. field of the sputter gun. With the sputter gun in operation at a power level of 100 W, the electron beam was deflected slightly normal to the surface of the sample but following fine adjustment of the grazing angle of incidence, RHEED patterns of reasonable quality could be observed continuously during the deposition process. 2.6. Deposition chamber The general layout of the experimental system is shown in Fig. 2. The main chamber containing the sample manipulator and the r.f. sputter guns is a 30 cm diameter stainless steel chamber with its axis horizontal. Details of the sputter-deposition system have been published elsewhere [ 151. A 60 1 S - -)turbopump, backed by a molecular drag pump and diaphragm pump, served to evacuate the specimen loadlock chamber, rough the main chamber and pump the RHEED gun, using a suitable valve sequence. The main chamber was pumped with a cryopump with a pumping speed for water of 4000 1 s-‘. Following a mild bake out, the base pressure of the chamber was maintained at about 1 X 10e6 Pa. The RHEED system was mounted in the chamber at 45” to the horizontal via a NW63 conflat flange and its vacuum chamber separated from the main chamber by the differential aperture.

J -

Fig. 2. Schematic drawing of the r.f. magnetron sputter-deposition chamber with differential RHEED system and pumping configuration. IG( 1, 2, 3). ionization gauges.

RHEED patterns were recorded from a 10.16 cm diameter fluorescent screen mounted in line with the RHEED system. The screen was protected during prolonged r.f. sputter deposition experiments with a mechanical shutter. RHEED patterns were recorded photographically. 2.7. Experimental procedures With this system, a preliminary study was made of the deposition of platinum onto chemically cleaned [ 151, lightly doped, 4-7 R cm, n-type Si (100) substrates by r.f. magnetron sputtering. The substrates were held at room temperature. Research-grade argon gas (ppb impurity levels) was used as the sputter gas. Argon pressures of 0.67 Pa (5 mTorr) and 1.33 Pa (10 mTorr) were used at various r.f. power levels between 20 and 100 W. One sequence of observation was to observe the development of the film in stages by short periods of deposition, followed by examination of the specimen by RHEED in the absence of the r.f. field. This allowed time for careful RHEED observation of the sample from various azimuths and for varying accelerating voltages. Exposure times of up to 20 s were required to record diffraction patterns and with the times required to adjust the specimen, several minutes elapsed between the sputter deposition sequences. However, in this manner a sequence of observations of the outer surface of a film was recorded in detail during the total growth period. With the sample in place in the specimen manipulator, the pressure was increased to the required value. The r.f. magnetron sputter gun was started with a shutter in place close to the specimen surface. When the plasma was stable, the shutter was opened for a controlled period to allow the deposition of platinum. The plasma was then turned off and RHEED patterns were recorded. With the shutter again in place, the plasma was started again and then another fixed period of deposition was measured. In this way, the growth of the film

S.R. Dus et (11./Thin Solid Films 270 (1995) 314-319

was observed in stages and careful RHEED observations could be made at the end of each deposition period. Alternatively, it was found possible to record RHEED patterns during the sputter-deposition process. However, during such experiments, the RHEED patterns were being recorded during the growth process and were an average of the changing conditions during the camera exposure time of some l& 20 s. For example, at 0.67 Pa and 100 W r.f. power, the

317

deposition rate of Pt in the system was measured at 9.5 nm min-’ Hence, during a 10 s RHEED exposure the film grew by about 1.6 nm. During the early stages of growth of the film, significant changes in structure occurred under some conditions of deposition. RHEED patterns taken during continuous sputter growth therefore needed to be interpreted with caution. To understand this early period of growth, a combination of both intermittent and continuous growth obser-

Fig. 3. RHEED patterns taken during intermittent sputter-deposition in Ar at a pressure of 0.67 Pa and r.f. power of 20 W: (a) Si( 100)-H surface at base pressure; (b) Si( lOO)-H.surface with the chamber at 0.67 Pa; (c) 1 nm Pt: (d) 4 nm Pt; (e) 6.3 nm Pt; and (f) 20 nm Pt with fiber orientation and Pt(lll)~~Si(lOO).

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vations seemed desirable. Alternatively, recorder would overcome this limitation.

the use of a video

3. Results and discussion Fig. 3 shows RHEED patterns taken during an intermittent growth experiment. Fig. 3(a) is the Si( 100-H surface, as prepared, recorded at the base pressure of the system. The sharp pattern and the observation of Kikuchi lines [ 161 clearly demonstrated the efficacy of our substrate cleaning process in producing a high crystalline-quality, clean, hydrogen-passivated surface. Increasing the chamber pressure to 0.67 Pa by introducing Ar did not change the RHEED pattern (Fig. 3 (b) ), thereby demonstrating the successful operation of the RHEED system at high pressures. Following 30 s of deposition at an r.f. power of 20 W, about 1 nm of Pt was deposited (Fig. 3 (c) ) . This film suppressed nearly all reflections from the Si substrate, except the intense (400), and gave diffraction effects from an almost amorphous layer. Following a further exposure of 90 s, the total thickness of the Pt was about 4 nm and some ordering of the Pt was observed, as seen in Fig. 3 (d) . After a total deposition of 6.3

nm of Pt, both epitaxial and randomly oriented material were observed (Fig. 3 (e) ) . As the Pt film thickness was increased to 20 nm, the diffraction pattern showed diffuse reciprocal lattice rods (rel-rods) (Fig. 3(f)) which have been interpreted as fiber-oriented Pt with the ( 111) planes parallel to the substrate. A similar sequence of RHEED observations is shown in Fig. 4, taken during the growth of Pt sputter-deposited at an Ar pressure of 0.67 Pa and a r.f. power of 100 W, corresponding to a deposition rate of about 10.0 nm min- ‘. For the early stage of growth, the observations were similar to those of Figs. 3( a)-3( c). However, as the film thickness increased to 6.3 nm, after 40 s of deposition, the outer surface of the film showed diffraction effects from fiber-oriented material only (Fig. 4(a) ) Diffuse rel-rods indicated a fine particle size. After a total deposition time of 120 s the thickness increased to about 19 nm and the diffraction pattern (Fig. 4 (b) ) showed sharpening of the rel-rods, indicating an increase in the grain size in the film. During further observations on this growth process, the RHEED patterns were observed continuously during deposition and Fig. 4(c) was recorded using a 30 s camera exposure during the deposition time between 230 and 260 s. Fig. 4(d) shows the diffraction pattern from the same

Fig. 4. RHEED patterns taken during sputter-deposition in Ar at a pressure of 0.67 Pa and r.f. power of 100 W: (a) 6.3 nm Pt; (b) 19 nm Pf; (c) recorded during growth with r.f. power on; and (d) 41 nm Pt. with r.f. power off.

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Fig. 5. Effect of gas pressure in the deposition 0.67 Pa; and (c) Ar pressure,

Solid Films 270 (1995)

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chamber on the RHEED pattern from 88 nm of Pt on Si( 100): (a) pressure at 5 X 1O-5 Pa; (b) Ar pressure,

1.33Pa.

film at a total thickness of 41 nm, recorded with the r.f. power off. Further deposition in this experiment led to a total film thickness of about 88 nm of Pt. The effects of chamber pressure (or the lack thereof) on the RHEED pattern from this film, recorded at 35 kV, are shown in Fig. 5.

with X-ray diffraction and transmission electron microscopy measurements reported earlier [ 171 on similarly grown samples.

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4. Conclusions A differentially pumped UHV RHEED system has been successfully applied to the study of the growth of Pt films on Si (100) substrates by r.f. magnetron sputtering in argon. The pressure in the reaction chamber has been 0.67 Pa during the RHEED observations reported here and successful experiments have also been done with a pressure as high as 3 Pa. In the presence of the r.f. field, the electron beam is deflected normal to the surface of the sample and results in a slight elongation and displacement of the focussed spot at the screen. The beam can be adjusted with respect to the specimen so that RHEED patterns can be recorded during the deposition process. Preliminary studies have shown that during the sputter-deposition of Pt in Ar at 0.67 Pa and 20 W r.f. power, there is an initial formation of an amorphous layer, followed by the development of a fiber orientation of Pt, with the ( 111) planes parallel to the Si (100). At a thickness of about 6.0 nm, some polycrystallinity is observed; however, this is not seen as the film increases in thickness and at 20 nm the outer layer of the film is entirely of fibre orientation. During the growth of Pt at 100 W r.f. power, following the observation of amorphous material within the first 1 nm of deposition, the fiber orientation is observed throughout the entire growth process. The grain size in the film increases as the total thickness increases to about 40 nm, as shown by the increase in sharpness in the RHEED rel-rods. These results are consistent

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