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A vacuum automatic ellipsometer for principal angle of incidence measurement
Surface Science 0 North-Holland
96 (1980) 202-216 Publishing Company
A VACUUM AUTOMATIC
ELLIPSOMETER
FOR PRINCIPAL ANGLE OF
INCIDENCE MEASUREMENT
M. YAMAMOTO Physics Department, Received
20 August
* and OS. HEAVENS University of York, Heslington,
York YOI 5DD, UK
1979
A high-precision ellipsometer is described capable of following the thickness and refractive index of condensing films and of revealing small departures from homogeneity. Applied to a growing film of magnesium fluoride, the instrument reveals variations in the refractive index of the order 0.005 -0.01.
1. Introduction In order to establish the sources of inhomogeneities in multilayer dielectric films, it is essential to be able to monitor both thickness and refractive index during film deposition. This requires an in situ method of measurement capable of very high accuracy and suitable for films of a wide range of thicknesses. In previous studies of the inhomogeneities of zinc sulphide/cryolite multilayers, Netterfield [ 1] has made measurements of the transmittance at turning values during deposition. However this method cannot detect the effects of structural irregularities on the optical behaviour of the films during growth. Under the best conditions the departures from homogeneity of dielectric films are very small, although such departures may have significant effects on, e.g., the scattering behaviour of film systems. Thus the method of measurement adopted needs to have a very high sensitivity and a short response time, in order to be able to monitor continuously the optical properties during the deposition period. It is also advantageous if the wavelength of the probing radiation can be easily changed. These considerations point to an ellipsometer, housed completely within the vacuum system (to avoid the difficulties of changing birefringence of windows to the vacuum system) and of the type which does not employ a quarter-wave plate. The type adopted is that in which the two measured parameters are (a) the angle of incidence and (b) the azimuth of the polarizer in the incident beam [2]. The usual arrangment is where the two parameters are adjusted so that the light reflected
* Present
address:
Department
of Physics,
Gakushuin 202
University,
Mejiro, Tokyo,
Japan.
M. Yamamoto,
O.S. Heavens / Vacuum automatic eNipsometer
203
from the sample is circular’ly polarized. This may be detected by the use of a rotating analyzer, as in the ellipsometer by Archard, Clegg and Taylor [3] or by reflecting the beam back along its own path (O’Bryan [4], Yamamoto [5]). No compensator is used, so the wavelength can be changed to ensure the maximum sensitivity for any particular system under test. In the instrument described here, simultaneous operation in both of the above modes, i.e. as an automatic rotating analyzer ellipsometer (ARAE) and as a return path type ellipsometer (RPTE), is possible.
2. Principles of operation 2. I. Automatic rotating analyzer ellipsometer (ARA E) Fig. 1 shows the optical configuration of the ARAE. The polarizer azimuth x and the angle of incidence $ are adjusted until the light reflected by the specimen is circularly polarized. Under this condition, the angle of incidence is the principal angle #t, and the polarizer setting is the principal azimuth xp. If R,, R, are the amplitude coefficients for reflection of p and s components at the surface and A is the relative phase change, then the irradiance of the beam leaving the analyzer, for any (4, x), is given by
+ R,R,
sin 2x cos A sin 20) ,
(1)
REFLECTOR
D ; TO DETECTOR
Fig. 1. Principle of operation of ellipsometer.
204
M. Yamamoto, O.S. Heavens / Vacuum automatic ellipsometer
where 0 = wt, w being the angular rotation speed of the analyzer, sine components of the signal will therefore be zero if
The cosine and
RE cos’x =Rz sinZX,
(2)
cos A = 0 .
(3)
These two conditions value @p, such that
are achieved by adjustment
of the angle of incidence
and of the polarizer azimuth x to one of the principal azimuth positions,
$ to the
given by
x = xg = *tan-‘@n/R&=+ Under this condition, the irradiance of the beam emerging from the analyzer is constant and the beam reflected from the sample is circularly polarized. In this instrument, light from the rotating analyzer falls on a photomultiplier whose output is fed to two phase-sensitive detectors (PSD) tuned to the cosine and sine components of the incoming signal. The PSD outputs provide out-of-balance signals to drive motors adjusting the values of x and 4, as described in section 4.3. The signs of the PSD outputs depend on whether setting & or xp is chosen [eq. (S)] and are adjusted to ensure convergence. 2.2. Return path type ellipsometer (RPTE) In this configuration the rotating analyzer in fig. 1 is inoperative and use is made of the light reflected by the plane mirror mounted perpendicular to the beam reflected from the sample. For @=&I and x = x”p or xp [eq. (5)] circularly polarized light falls on the mirror which reverses both the beam direction and the handedness of the circular polarization. After reflection at the specimen surface, the returning beam is plane-polarized, with vibration direction perpendicular to that of the incident beam. The principal incidence/azimuth condition can therefore be detected as a null in the transmission of the return beam by the polarizer. For a small error Axp in the polarizer azimuth of the incident beam from the true balance position, the polarization azimuth of the returning beam is rotated by -AQ. Thus the sensitivity in detecting the null position is double that of the normal unidirectional system. Similar considerations apply to the sensitivity of detection of the principal angle of incidence @p. 2.3, Note on sensitivity and precision The sensitivity of a rotating analyzer ellipsometer (RAE) reaches a maximum when the beam reflected from the sample surface is circularly polarized (Aspnes [6]), which is the case with the ARAE. With a RAE, however, a compensator, an
M. Yamamoto,
0.X Heavens / Vacuum automatic ellipsometer
205
impediment for spectroscopic ellipsometry, is essential to achieve high precision on dielectric surfaces (Aspnes [6]). The use of a compensator may deteriorate the accuracy (Yamamoto [5]). Thus best results can be accomplished only by combining the RF’TE, which unfortunately is difficult to automate but excellent in accuracy and precision (Yamamoto [ 5]), with the ARAE.
3. Computer
analysis
In order to establish the design parameters of the ellipsometer it was necessary to establish the rates at which it would be necessary to change @,x during the evap oration, in order to attain a suitable target accuracy of the parameters being measured. The maximum deposition rate for the condensing materials was assumed to be a physical thickness of 6 rim/s.. A computer programme was written to enable the evolution of xp and $p with time to be calculated for a multilayer. The specific case considered was that of thirteen layers, each of quarter-wave optical thickness and with alternating refractive indices of 2.30 (ZnS) and 1.38 (MgF*). Although different results would be obtained for other materials, the differences will not be great and the results obtained will serve as an excellent general guide. It was established that it is possible to cover all multilayer requirements by arranging that the angle of incidence be in the range 50” < I#J< 90”. Calculations were made to establish the sensitivity required in order to detect an inhomogeneity in refractive index of 0.0005. It was established that the highest instrumental accuracy would be needed for the first low-index layer. It was established that the minimum change of xp necessary was practically independent of film thickness. Detection of an inhomogeneity in refractive index of 0.0005 required an accuracy of 21 seconds of arc in xr. The sensitivity to inhomogeneity of &, was found to be dependent on film thickness. Detection limits corresponding to inhomogeneity of 0.0005 are shown in table 1. In order to follow changes in film thickness for a deposition rate of 6 rim/s,, the following rates of change of xp and Gp.are required:
Table 1 Sensitivity thickness
in @p,
Film thickness 10 20 50 70
A@p,
(nm)
required
for detecting
an index change
A@p (set of arc) 0.6 3.0 12 35
of 0.0005
over the film
206
M. Yamamoto,
O.S. Heavens / Vacuum automatic ellipsometer
(1) The rate of change of gp with film thickness lies in the range 0 < Id@p/dZl< 0.2 deg/nm (Id&=/dtl < 1.2 deg/s) for low (1.38) index films, 0 < Idq+/dZl < 14 deg/nm (84 deg/s) for high (2.30) index fdms. (2) The maximum value of d’#,/dp is = 40 deg/(nm)’ (1400 deg/s’). (3) All possible polarizer orientations must be accessible. (4) The rate of change of xp with film thickness I lies in the range 0 < ldxp/dZl < 0.8 deg/nm (4.8 deg/s) for low index films, 0 < Idxp/dZl < 300 deg/nm (1800 deg/s) for high index films. (5) The maximum value of d2xr/dZ2 is = 1800 deg/(nm)2 (65000 deg/s’). Thus the dominant problems of response time arise in connection with the high index material.
4. The system 4.1. Deposition chamber Fig. 2 shows a schematic view of the main part of the system with the door of the chamber removed. The 56 cm diameter X 23 cm chamber is mounted on a 98 cm high, 56 cm long, and 112 cm wide framework accommodating a 600 l/s oil diffusion pump and a 11 m3/h rotary pump, which allow experiments to be carried out at a pressure below 10m4 Pa. Apart from the flanges seen in fig. 2, the chamber has four 6.3 cm diameter flanges on the back face and three windows on the door which is hinged at the left side of the chamber. Fig. 3 shows the polarizer and analyzer arms, sample holder and shielded evaporation sources. 4.2. Mechanics of the eZZipsometer 4.2.1. General description The ellipsometer is attached on to the top flange through a 10 cm diameter cylinder and a plate fixed horizontally at the bottom part of it. The polarizer arm (PA: left) and the analyzer arm (AA: right) swing symmetrically about each axis A and B as the sample holder block moves up and down since two sliders running along the slots of PA and AA are pinnted together onto the block. The block and the evaporation source assembly ES are fixed onto the silver steel shaft which is driven in a vertical direction by the stepping motor assembly SM. A light beam directed exactly along the axis A horizontally is folded through 90” by the reflector R onto the optical axis of the PA so as to pass through the polarizer and to strike the sample stage S at a fixed point. After being reflected at the sample surface, a part of the beam is reflected back by the mirror M to serve for the RPTE; the other part of the beam passes through a hole in M and goes through rotating analyzer assembly RA. The beam from RA is fed into a fixed fibre bundle which leads it to a small window on the side flange on which a lens system with an
M. Yamamoto,
O.S. Heavens / Vacuum automatic ellipsometer
A.B; Swing motion axes AA; Analyser Arm ES i Evaporation Sources M LMirror PA ;PolarlzerArm RA TRotatinganalyser Assembly S Sample Stage SM;3eote;g SM
Assembly VP. to Vacuum Pump R: Reflector
Fig. 2. Schematic layout of ellipsometer.
207
208
M Yamamoto, 0.S. Heavens / Vacuum automatic ellipsometer
iris is mounted to project the collected B photomultiplier.
flux on to the photo-cathode
of EMI 6094
4.2.2. Stepping motor assembly Adjustment of the angle of incidence requires accurately controlled vertical movement of the sample block holder. This is effected by an externally-mounted system in which a five-pole stepping motor drives a precision screw carrying a recirculating ball nut. The nut is coupled to a silver steel shaft passing through a sliding vacuum seal at the top of the chamber. From considerations of accuracy required and rate of following changes in film thickness, it was established that the setting accuracy for @Jneeded to be +12 set and the maximum time rate of change 84 deg/s. This was achieved by a stepping motor (1000 steps per revolution which will operate at up to 60 revolutions per second), a ball-bearing screw with 4 mm pitch of which the pitch accuracy is 5 pm and a double recirculating ball nut which is preloaded to reduce backlash to a negligible level. The screw is mounted in doubled angular contact bearings. The position of the sample holder is determined by counting the pulses to the stepping motor. A single step corresponds to a vertical displacement of 4 pm and the maximum linear drive speed is 240 mm/s. These correspond to a change in @ of 5.5 seconds of arc and to a maximum value of d@p/dtof 91 deg/s for @I- 90”. The straight-through position (4 = 90”) is established by setting the position of the specimen block by the use of a dial gauge with a resolution of l/l0000 inch. To protect the system from overrun two microswitches are arranged to disconnect the stepping motor supply at the ends of the traverse. Overall accuracy of the angle of incidence readout is found to be better than +16 seconds of arc after measuring the actual angle of rotation of the polarizing prism mounted on A by adjusting another polarizing prism mounted in a graduated circle to the extinction position. 4.2.3. Polarizer assembly In order to achieve the desired speed of operation? the inertia of the polarizer and analyzer arms must be kept as low as possible. This was achieved by the use of a harmonic drive component unit (HDCU) with a small dc servomotor and an optical shaft encoder, full details of which will be published elsewhere. A slimmeddown version of the HDCU (supplied by Harmonic Drive Systems Inc, Nishikamata, Japan) has a backlash of less than one minute of arc. The optical shaft encoder, with 512 lines on the glass disk on the input shaft, gives two outputs with a 90” phase difference which may be used to generate 2048 pulses per revolution. The overall accuracy of the azimuth reading of the polarizer was found to be +24 seconds of arc. The input shaft is driven by a dc servomotor either by a pair of wormgears and wormwheels or with a combination of 2.5 mm pitch sprockets and cogged belts.
M. Yamamoto, O.S. Heavens / Vacuum automatic ellipsometer
209
4.2.4. Sample holder assembly The main features of the sample holder designs are firstly that samples of various shapes can be set precisely in position without auxiliary adjustments and secondly that the height of the sample surface can be adjusted without a change of orientation; this is particularly important when operating at a large angle of incidence. In fig. 3 the plate D is fixed to the sample block holder on the shaft driven by the precision screw. The specimen is mounted on three bearing balls set in countersunk holes in plate E. Coupling between plates D and E is effected by the bracket F. The precise distance and orientation of the specimen surface with respect to the plate D can readily be adjusted. A separate polished rod running through a vacuum seal operates a shutter in front of the specimen surface: the shutter moves with the sample holder as the value of @Jis changed. 4.2.5. Mirror holder A silvered polished glass plate with a 2 mm diameter hole sits in a counterbored plate whose position can be adjusted with respect to the reflected beam axis. The inclination of the base plate is adjusted by three screws and secured by three clamping screws on an L-shaped bracket fixed on the analyzer arm.
Fig. 3. Polarizer/analyzer
arms, sample
holder
and evaporation
sources.
M. Yamamoto, O.S. Heavens / Vacuum automatic ellipsometer
210
4.2.6. Rotating analyzer assembly The Clan-Thompson prism is mounted in a cylinder attached to flanges on the hollow shaft of the synchronous motor, the prism and rotation axes being adjusted for exact parallelism. The effects of residual movement of the emerging beams were reduced by the use of a lens and an optical fibre rod. The position of the lens is adjusted so that the beam covers a large number of fibres, thus ensuring good transmission averaging and randomizing any residual polarization effects. Light from the end of the fibre rod is coupled to a flexible fibre, mounted adjustably close to the fibre rod of the analyzer. On the end of the shaft a chopper blade is fixed to obtain synchronized reference signals by chopping the light beams from a pair of bulbs exciting a pair of silicon phototransistors. The reference signal assemblies are mounted on the shell of the motor in such a way that the phase of the reference signals can be adjusted. The assembly is fixed to the analyzer arm by a bracket with slots for height adjustment and a bearing ball at the base for inclination adjustment. The whole passage of the beam, from the hole on the mirror to the photocathode of the photomultiplier, is enclosed to minimize the effect of stray light. 4.3. Electronics of the ellipsometer 4.3.1. Circuit description The servo electronics consists of two servo loops, one for the angle of incidence drive (AL), the other for the azimuth of polarization drive (PL). Fig. 4 shows the
DC
SERVO-
MOTOR
I
I ING
, POWER
MOTOR
D-A
-
CONVERTER l~~~llJ---DIGITAL
AMP.
2-CHANNEL T-Y RECORDER
DISPLAYS
Fig. 4. Block diagram of electronic
system.
M.
Yamamoto, O.S. Heavens / Vacuum automatic ellipsometer
211
loops with the system outline. The output from end-window type photomultiplier is fed to a Brookdeal low noise amplifier, but since the internal filter shows some phase shift with change of gain, a twin T narrow band amplifier tuned to 5.6 HZ (twice the speed of rotation of the motor) is used instead. The appropriately amplified signal goes into PSD 1 and 2 for AL and PL, respectively, where the amplitude of the sin f3 and cos 0 components [see eq. (l)] are picked up and fed to each PID (proportional/integral/differential) control circuit. The PID control circuit has three inverting operational amplifiers governing proportional, integral and differential signals of which the parameters can be adjusted so as to give best servo-response. The signals are then added through preset potentiometers to an inverting operational amplifier to be mixed in a ratio determined by the potentiometer settings. Each output is fed to’ another operational amplifier to provide gain control: then to an invertor to provide a choice of polarity which enables the appropriate polarity, x’p or xp (see section 2.1) to be chosen. Two different features are introduced. Operation of AL needs full wave rectification before voltage to frequency conversion together with a digitial signal generator to indicate the direction of rotation. For the polarizer drive, it is necessary to maintain oscillation of the drive shaft in order to overcome starting friction. This is effected by an applied signal of triangular waveform. The stepping motor and the dc servo motor driven by error signals converge the system to the null position giving negative feedback to the photomultiplier which closes each loop. The azimuth of polarizer x is digitized by the incremental optical shaft encoder, processed by up/down counter interface and displayed as a 6 digit decimal number. The information needed to determine the angle of incidence @is picked up digitally from the stepping motor driving circuit and after digital-to-analogue conversion produces permanent records on a two-channel or an X-Y recorder chart. The pulse repetition frequency of the error signal must lie below the maximum frequency at which the stepping motor can be swtiched on and off. This was measured and found to be 4 kHz, corresponding to an acceleration of 16 Hz/(ms)‘. For the maximum required value of d’@p/dt* (section 3) the corresponding acceleration is -1 Hz/(ms)‘. Thus the response of the drive system for the angle of incidence is more than sufficient to meet the design requirements. 4.3.2. Transient response of the system The performance of the servo system depends on (a) the steepness of the signal amplitude/angle of incidence and signal amplitude/polarizer azimuth curves, and (b) the PID parameters chosen. These parameters were examined for a ZnS coated glass plate. The system was set manually, and errors of 2.9 and 0.9 deg were introduced in @ and x settings, which produced an out-of-balance of 1 .O V to each PID circuit. The system was then switched to the automatic mode. The values of $I and x were found to vary linearly with time with little sign of overshoot. The times required to return to the balance condition were found to be 500 ms for the polarizer and 950 ms for the angle of incidence settings.
212
iE1. Yamamoto,
O.S. Heavens / Vacuum automatic ellipsometer
4.4. Problems of the vacuum system The particular problems associated with the vacuum system fall under three headings, viz., (a) vacuum seal for rapid linear motion of angle of incidence drive, (b) outgassing and lubrication problems of motors and (c)protection of optical components from evaporation material. 4.4.1. Vacuum seal for rapid linear motion The upper plate of the system carries two sliding seals. The main shaft (MS), 12.7 mm in diameter, runs through a pair of spring-loaded lipseals and provides the angle of incidence drive. The smaller shaft (SS), 6.35 mm in diameter, is sealed by a pair of 0 rings and provides the specimen shutter control. A pressure rise is observed for rapid (200 mm/s) sliding motion, the extent of the rise being proportional to the distance traveled and not to the speed, as observed by Dawton [7]. This’effect is minimized by giving the shafts a high polish and by pumping the space between the 0 ring seals. For a 200 mm traverse, the associated leak is less than 6 X 10m4 Torr 1. In actual operation, the maximum traverse in one second is of the order 25 mm. Thus for the 56 1 chamber the maximum pressure rise due to the sliding seals is -1.3 X 10m6 Torr, and acceptably low. 4.4.2. Performance of motors under vacuum Tests on the polarizer drive motor (Portescap 34L21-219P) with 20 V applied reveal a temperature rise in the sintered bronze bearing of only 14°C after one hour’s operation. From the change in drive current, a similar temperature rise in the coils is deduced. Since under normal operation maximum power is only very rarely needed, these tests indicate that no difficulties arise with this component. The rotating analyzer is driven by a SLO-SYN synchronous motor (TS 50E), which runs continuously during operation of the ellipsometer. The most suitable bearing lubricant is found to be a mixture of silicone vacuum grease and MO& powder. Although a starting voltage of 120 is required for this motor, 25 V suffices for running, under which condition the temperature rise after 2 h operation is found to be only 3°C. Under running conditions the motor speed, measured by an optical shaft encoder, was found to be 2.7859 f 0.0008 c/s. 4.4.3. Protection of optical components In view of the very high sensitivity of the instrument it is vital to ensure that no deposition occurs on any of the surfaces of the optical components. Shields over the evaporation source confine deposition to the sample area and minimize any temperature change in the ellipsometer components. No detectable change in the extinction ratio of the Glan-Thompson prisms is observed after many cycles of operation. The observed extinction ratio lies in the range 1O-8-1O-6, depending on the precise position of the beam through the prisms.
M. Yamamoto, O.S. Heavens / Vacuum automatic ellipsometer
213
5. Alignment of the ellipsometer 5. I. Optical alignment The alignment needs to be carried out with reference to the axis of linear motion of the main shaft (fig. 5). Axes A and B of fig. 3 need to be set symmetrically at precise points on a line through the centre of the axis of the sample holder block and accurately perpendicular to the drive axis. This is effected by the horizontal bar H (fig. 5). The T-shaped tool T, fitted into a reference block L which carries accurately parallel holes, serves to ensure that H is perpendicular to the drive axes. A mirror with a pinhole is mounted concentrically in the hollow shaft along axis A and serves to align the incident beam accurately along the axis. The upper surfaces of the slots on the polarizer and analyzer arms are accurately located with respect to the upper surface of H in the I$ = 90” position. These surfaces are maintained in sliding contact with precision ground blocks mounted concentrically on the sample block so as to maintain the angle of incidence accurately over the whole range. In the $J= 90” position, and after removal of H, the orientation of the reflector on A is adjusted to direct the beam accurately along the axes of the polarizer and analyzer arms. The rotating analyzer, mirror and polarizer are then mounted on the appropriate arms. An optically flat table is then mounted on the reference block L, with its normal accurately parallel to the drive shaft. The specimen orientation can now be accurately set by adjusting its orientation until interference fringes are seen.
Fig. 5. Auxiliary
alignment equipment.
M. Yamamoto,
214
5.2. Alignment
O.S. Heavens / Vacuum automatic ellipsometer
ofelectronics
The required 90” relative phase difference between the reference signals from the lamp/phototransistor assemblies on the rotating analyzer is achieved by ensuring that the output from one of the PSD’s is zero when the reference signal from the other PSD is examined. The angle of incidence of the beam on the sample is then set at the Brewster angle, so that the reflected beam is entirely the s component. The analyzer azimuth is adjusted in relation to the chopper blades thus ensuring the correct absolute phases of the error signals.
6. Window birefringence Since the ellipsometer is mounted inside the vacuum system, birefringence of the windows is of no consequence. However, the ellipsometer does in fact provide a methad for the determination of the window birefringence. Full details will be published elsewhere, but results are given to indicate the extent of birefringence likely to be encountered in suitably annealed Pyrex windows. Windows 40 mm diameter and of two thicknesses (5.2 mm and 3.0 mm) were examined. Measurements are made with the light beam at 2” to the window normal passing 2 mm from the centre of the window. Table 2 shows the results obtained for the 5.2 mm disc before and after annealing at a temperature of 570°C for 1.5 h followed by cooling to room temperature in 5-6 h. The sensitivity of the observed birefringence is illustrated by the fact that contact with a finger for a fraction of a second at a point -20 mm from the region being examined produced a rapid rise in birefringence, to X.1”, which decayed to -zero after a minute or so. For the 5.2 mm disc, the large change in birefringence on evacuating the chamber should be noted; and that contrary to expectations the performance of the thinner disc was superior to that of the 5.2 mm window.
Table 2 Residual birefringence Disc thickness (mm) 5.2 5.2 3.0
of vacuum
window
Residual birefringence (under vacuum)
Residual birefringence (in air) (deg) 1.166 0.039
~0.01~ 4
a) Barely detectable:
expected
glass
(deg)
1.196 (before annealing) 0.070 (after annealing) -CO.031 (after annealing) accuracy
-O.Ol”.
M. Yamamoto, 0.X Heavens / Vacuum automatic ellipsometer
21.5
7. Accuracy In operation, the angle of incidence $I is obtained from the position of the sample block, as deduced from the number of pulses supplied to the stepping motor. Separate angle measurements on the system established that the reproducibility of the angle of incidence setting was approximately +_0.004 deg. Attempts to use Polaroid as the polarizing element failed on account of beam wander coupled with a variable sensitivity of the photomultiplier cathode. This problem was much reduced by the use of a Clan-Thompson prism and further improvement resulted from the use of the fibre optic rod in the analyzer arm. The signal-to-noise ratio (maximum signal to that at balance) without the fibre rod was 33: use of the fibre rod improved this value to 300. The accuracies of measurements of c#+and xp in the RPTE and ARAE configurations were compared by examining an aluminium film specimen. The results are shown in table 3. The slight differences observed in $P in the two configurations are accounted for by a small ellipticity in the light transmitted by the Clan-Thompson prism. The above measurements were made in the manual mode since in the automatic mode, there is invariably a slight hunting. For the 4~ drive, the average amplitude of the hunting is 10 steps (0.010 deg at @= 56”). For the xp drive, about 8 steps of hunting (0 .O16 deg) are observed. The performance of the system is illustrated in fig. 6, which shows the variation of @r, xp during the deposition of a MgFz film up to a (metrical) thickness of 20 nm. The evaporation started at X and continued for 3 min stopping at Y. The sensitivity of the system to small inhomogeneities is apparent when the curve of fig. 6 is compared with the result appropriate to a film of constant refractive index, increasing in thickness with time, as shown by the dotted curve. The fluctuations in the recorded curve indicate inhomogeneity of refractive index of the film of the order 0.005-0.01.
Table 3 A set of ellipsometer Position
Mode
measurements
for an Al film Corresponding
Direct readouts
values
@p display
XP display
@p(k)
XP (ded
X’p
RPTE ARAE
6505 6503
925654 925609
80.213 80.216
+49.44 +49.54
XP
RPTE ARAE
6486 6433
970656 970700
80.241 80.320
-49.44 -49.54
M. Yamamoto, OS. Heavens / C‘acuum automatic ellipsometer
216
1.20 I
1.60 I @P (deg.)
XP
1.00 (deg.) I
Y
56.90
I
I
I
.80
.88
l’ig. 6. Results
for 20 nm MgF,
I
.40
xp film deposited
I
(deg.)
.20
on glass in 3 min.
Acknowledgements The authors are especially grateful to Robert Easton and Steve Lawson for their contributions to the mechanical and electronic design of the system: and to K. Itoh, of Harmonic Drive Systems Ltd, for collaboration in the design of the polarizer drive. The instrument was developed with support from the Science Research Council.
References [I ] [2] [3] [4] [5] [6] [7]
R.P. Netterficld, Appl. Opt. 15 (1976) 1969. K. Kinosita and M. Yamamoto, Surface Sci. 56 J.1:. Archard, P.J. Clegg and A.M. Taylor, Proc. H.M. O’Bryan, J. Opt. Sot. Am. 26 (1936) 122. M. Yamamoto, Japan. J. Appl. Phys. 14 (1975) D.E. Aspnes, J. Opt. Sot. Am. 64 (1974) 639. R.H.V.M. Dawton, Brit. J. Appl. Phys. 8 (1957)
(1976) 64. Phys. Sot. (London) Suppl.
B65 (1952)
758.
14-1,413.
414.
Discussion D.E. Aspnes (Bell Laboratories): What is the base pressure attainable, and did you have any difficulties with mechanical movements, such as rotations, in vacuum? Pa. For the second questions, yes. We had many diffiM. Yamamoto: The pressure is lo4 culties. For example, sealing of the main shaft since it moves quite rapidly in some cases. But eventually, if you polish the shaft carefully you can overcome the problem. For other problems such as outgassing from the motors, I put some details in the paper. T.H. Allen (Optical Coating Laboratory): You indicated that DC-704 diffusion-pump oil was used to lubricate the mechanical components of the ellipsometer. Since DC-704 has a free evaporation rate under vacuum of several angstroms per second, did you observe any indications that this molecular contamination affected the accuracy of your measurements? M. Yamamoto: I don’t think that will be a problem because only polarizing prisms could show a problem if the surfaces are contaminated badly. And I found only occasional cleaning of the surfaces is sufficient to avoid the problem.
96 (1980) 202-216 Publishing Company
A VACUUM AUTOMATIC
ELLIPSOMETER
FOR PRINCIPAL ANGLE OF
INCIDENCE MEASUREMENT
M. YAMAMOTO Physics Department, Received
20 August
* and OS. HEAVENS University of York, Heslington,
York YOI 5DD, UK
1979
A high-precision ellipsometer is described capable of following the thickness and refractive index of condensing films and of revealing small departures from homogeneity. Applied to a growing film of magnesium fluoride, the instrument reveals variations in the refractive index of the order 0.005 -0.01.
1. Introduction In order to establish the sources of inhomogeneities in multilayer dielectric films, it is essential to be able to monitor both thickness and refractive index during film deposition. This requires an in situ method of measurement capable of very high accuracy and suitable for films of a wide range of thicknesses. In previous studies of the inhomogeneities of zinc sulphide/cryolite multilayers, Netterfield [ 1] has made measurements of the transmittance at turning values during deposition. However this method cannot detect the effects of structural irregularities on the optical behaviour of the films during growth. Under the best conditions the departures from homogeneity of dielectric films are very small, although such departures may have significant effects on, e.g., the scattering behaviour of film systems. Thus the method of measurement adopted needs to have a very high sensitivity and a short response time, in order to be able to monitor continuously the optical properties during the deposition period. It is also advantageous if the wavelength of the probing radiation can be easily changed. These considerations point to an ellipsometer, housed completely within the vacuum system (to avoid the difficulties of changing birefringence of windows to the vacuum system) and of the type which does not employ a quarter-wave plate. The type adopted is that in which the two measured parameters are (a) the angle of incidence and (b) the azimuth of the polarizer in the incident beam [2]. The usual arrangment is where the two parameters are adjusted so that the light reflected
* Present
address:
Department
of Physics,
Gakushuin 202
University,
Mejiro, Tokyo,
Japan.
M. Yamamoto,
O.S. Heavens / Vacuum automatic eNipsometer
203
from the sample is circular’ly polarized. This may be detected by the use of a rotating analyzer, as in the ellipsometer by Archard, Clegg and Taylor [3] or by reflecting the beam back along its own path (O’Bryan [4], Yamamoto [5]). No compensator is used, so the wavelength can be changed to ensure the maximum sensitivity for any particular system under test. In the instrument described here, simultaneous operation in both of the above modes, i.e. as an automatic rotating analyzer ellipsometer (ARAE) and as a return path type ellipsometer (RPTE), is possible.
2. Principles of operation 2. I. Automatic rotating analyzer ellipsometer (ARA E) Fig. 1 shows the optical configuration of the ARAE. The polarizer azimuth x and the angle of incidence $ are adjusted until the light reflected by the specimen is circularly polarized. Under this condition, the angle of incidence is the principal angle #t, and the polarizer setting is the principal azimuth xp. If R,, R, are the amplitude coefficients for reflection of p and s components at the surface and A is the relative phase change, then the irradiance of the beam leaving the analyzer, for any (4, x), is given by
+ R,R,
sin 2x cos A sin 20) ,
(1)
REFLECTOR
D ; TO DETECTOR
Fig. 1. Principle of operation of ellipsometer.
204
M. Yamamoto, O.S. Heavens / Vacuum automatic ellipsometer
where 0 = wt, w being the angular rotation speed of the analyzer, sine components of the signal will therefore be zero if
The cosine and
RE cos’x =Rz sinZX,
(2)
cos A = 0 .
(3)
These two conditions value @p, such that
are achieved by adjustment
of the angle of incidence
and of the polarizer azimuth x to one of the principal azimuth positions,
$ to the
given by
x = xg = *tan-‘@n/R&=+ Under this condition, the irradiance of the beam emerging from the analyzer is constant and the beam reflected from the sample is circularly polarized. In this instrument, light from the rotating analyzer falls on a photomultiplier whose output is fed to two phase-sensitive detectors (PSD) tuned to the cosine and sine components of the incoming signal. The PSD outputs provide out-of-balance signals to drive motors adjusting the values of x and 4, as described in section 4.3. The signs of the PSD outputs depend on whether setting & or xp is chosen [eq. (S)] and are adjusted to ensure convergence. 2.2. Return path type ellipsometer (RPTE) In this configuration the rotating analyzer in fig. 1 is inoperative and use is made of the light reflected by the plane mirror mounted perpendicular to the beam reflected from the sample. For @=&I and x = x”p or xp [eq. (5)] circularly polarized light falls on the mirror which reverses both the beam direction and the handedness of the circular polarization. After reflection at the specimen surface, the returning beam is plane-polarized, with vibration direction perpendicular to that of the incident beam. The principal incidence/azimuth condition can therefore be detected as a null in the transmission of the return beam by the polarizer. For a small error Axp in the polarizer azimuth of the incident beam from the true balance position, the polarization azimuth of the returning beam is rotated by -AQ. Thus the sensitivity in detecting the null position is double that of the normal unidirectional system. Similar considerations apply to the sensitivity of detection of the principal angle of incidence @p. 2.3, Note on sensitivity and precision The sensitivity of a rotating analyzer ellipsometer (RAE) reaches a maximum when the beam reflected from the sample surface is circularly polarized (Aspnes [6]), which is the case with the ARAE. With a RAE, however, a compensator, an
M. Yamamoto,
0.X Heavens / Vacuum automatic ellipsometer
205
impediment for spectroscopic ellipsometry, is essential to achieve high precision on dielectric surfaces (Aspnes [6]). The use of a compensator may deteriorate the accuracy (Yamamoto [5]). Thus best results can be accomplished only by combining the RF’TE, which unfortunately is difficult to automate but excellent in accuracy and precision (Yamamoto [ 5]), with the ARAE.
3. Computer
analysis
In order to establish the design parameters of the ellipsometer it was necessary to establish the rates at which it would be necessary to change @,x during the evap oration, in order to attain a suitable target accuracy of the parameters being measured. The maximum deposition rate for the condensing materials was assumed to be a physical thickness of 6 rim/s.. A computer programme was written to enable the evolution of xp and $p with time to be calculated for a multilayer. The specific case considered was that of thirteen layers, each of quarter-wave optical thickness and with alternating refractive indices of 2.30 (ZnS) and 1.38 (MgF*). Although different results would be obtained for other materials, the differences will not be great and the results obtained will serve as an excellent general guide. It was established that it is possible to cover all multilayer requirements by arranging that the angle of incidence be in the range 50” < I#J< 90”. Calculations were made to establish the sensitivity required in order to detect an inhomogeneity in refractive index of 0.0005. It was established that the highest instrumental accuracy would be needed for the first low-index layer. It was established that the minimum change of xp necessary was practically independent of film thickness. Detection of an inhomogeneity in refractive index of 0.0005 required an accuracy of 21 seconds of arc in xr. The sensitivity to inhomogeneity of &, was found to be dependent on film thickness. Detection limits corresponding to inhomogeneity of 0.0005 are shown in table 1. In order to follow changes in film thickness for a deposition rate of 6 rim/s,, the following rates of change of xp and Gp.are required:
Table 1 Sensitivity thickness
in @p,
Film thickness 10 20 50 70
A@p,
(nm)
required
for detecting
an index change
A@p (set of arc) 0.6 3.0 12 35
of 0.0005
over the film
206
M. Yamamoto,
O.S. Heavens / Vacuum automatic ellipsometer
(1) The rate of change of gp with film thickness lies in the range 0 < Id@p/dZl< 0.2 deg/nm (Id&=/dtl < 1.2 deg/s) for low (1.38) index films, 0 < Idq+/dZl < 14 deg/nm (84 deg/s) for high (2.30) index fdms. (2) The maximum value of d’#,/dp is = 40 deg/(nm)’ (1400 deg/s’). (3) All possible polarizer orientations must be accessible. (4) The rate of change of xp with film thickness I lies in the range 0 < ldxp/dZl < 0.8 deg/nm (4.8 deg/s) for low index films, 0 < Idxp/dZl < 300 deg/nm (1800 deg/s) for high index films. (5) The maximum value of d2xr/dZ2 is = 1800 deg/(nm)2 (65000 deg/s’). Thus the dominant problems of response time arise in connection with the high index material.
4. The system 4.1. Deposition chamber Fig. 2 shows a schematic view of the main part of the system with the door of the chamber removed. The 56 cm diameter X 23 cm chamber is mounted on a 98 cm high, 56 cm long, and 112 cm wide framework accommodating a 600 l/s oil diffusion pump and a 11 m3/h rotary pump, which allow experiments to be carried out at a pressure below 10m4 Pa. Apart from the flanges seen in fig. 2, the chamber has four 6.3 cm diameter flanges on the back face and three windows on the door which is hinged at the left side of the chamber. Fig. 3 shows the polarizer and analyzer arms, sample holder and shielded evaporation sources. 4.2. Mechanics of the eZZipsometer 4.2.1. General description The ellipsometer is attached on to the top flange through a 10 cm diameter cylinder and a plate fixed horizontally at the bottom part of it. The polarizer arm (PA: left) and the analyzer arm (AA: right) swing symmetrically about each axis A and B as the sample holder block moves up and down since two sliders running along the slots of PA and AA are pinnted together onto the block. The block and the evaporation source assembly ES are fixed onto the silver steel shaft which is driven in a vertical direction by the stepping motor assembly SM. A light beam directed exactly along the axis A horizontally is folded through 90” by the reflector R onto the optical axis of the PA so as to pass through the polarizer and to strike the sample stage S at a fixed point. After being reflected at the sample surface, a part of the beam is reflected back by the mirror M to serve for the RPTE; the other part of the beam passes through a hole in M and goes through rotating analyzer assembly RA. The beam from RA is fed into a fixed fibre bundle which leads it to a small window on the side flange on which a lens system with an
M. Yamamoto,
O.S. Heavens / Vacuum automatic ellipsometer
A.B; Swing motion axes AA; Analyser Arm ES i Evaporation Sources M LMirror PA ;PolarlzerArm RA TRotatinganalyser Assembly S Sample Stage SM;3eote;g SM
Assembly VP. to Vacuum Pump R: Reflector
Fig. 2. Schematic layout of ellipsometer.
207
208
M Yamamoto, 0.S. Heavens / Vacuum automatic ellipsometer
iris is mounted to project the collected B photomultiplier.
flux on to the photo-cathode
of EMI 6094
4.2.2. Stepping motor assembly Adjustment of the angle of incidence requires accurately controlled vertical movement of the sample block holder. This is effected by an externally-mounted system in which a five-pole stepping motor drives a precision screw carrying a recirculating ball nut. The nut is coupled to a silver steel shaft passing through a sliding vacuum seal at the top of the chamber. From considerations of accuracy required and rate of following changes in film thickness, it was established that the setting accuracy for @Jneeded to be +12 set and the maximum time rate of change 84 deg/s. This was achieved by a stepping motor (1000 steps per revolution which will operate at up to 60 revolutions per second), a ball-bearing screw with 4 mm pitch of which the pitch accuracy is 5 pm and a double recirculating ball nut which is preloaded to reduce backlash to a negligible level. The screw is mounted in doubled angular contact bearings. The position of the sample holder is determined by counting the pulses to the stepping motor. A single step corresponds to a vertical displacement of 4 pm and the maximum linear drive speed is 240 mm/s. These correspond to a change in @ of 5.5 seconds of arc and to a maximum value of d@p/dtof 91 deg/s for @I- 90”. The straight-through position (4 = 90”) is established by setting the position of the specimen block by the use of a dial gauge with a resolution of l/l0000 inch. To protect the system from overrun two microswitches are arranged to disconnect the stepping motor supply at the ends of the traverse. Overall accuracy of the angle of incidence readout is found to be better than +16 seconds of arc after measuring the actual angle of rotation of the polarizing prism mounted on A by adjusting another polarizing prism mounted in a graduated circle to the extinction position. 4.2.3. Polarizer assembly In order to achieve the desired speed of operation? the inertia of the polarizer and analyzer arms must be kept as low as possible. This was achieved by the use of a harmonic drive component unit (HDCU) with a small dc servomotor and an optical shaft encoder, full details of which will be published elsewhere. A slimmeddown version of the HDCU (supplied by Harmonic Drive Systems Inc, Nishikamata, Japan) has a backlash of less than one minute of arc. The optical shaft encoder, with 512 lines on the glass disk on the input shaft, gives two outputs with a 90” phase difference which may be used to generate 2048 pulses per revolution. The overall accuracy of the azimuth reading of the polarizer was found to be +24 seconds of arc. The input shaft is driven by a dc servomotor either by a pair of wormgears and wormwheels or with a combination of 2.5 mm pitch sprockets and cogged belts.
M. Yamamoto, O.S. Heavens / Vacuum automatic ellipsometer
209
4.2.4. Sample holder assembly The main features of the sample holder designs are firstly that samples of various shapes can be set precisely in position without auxiliary adjustments and secondly that the height of the sample surface can be adjusted without a change of orientation; this is particularly important when operating at a large angle of incidence. In fig. 3 the plate D is fixed to the sample block holder on the shaft driven by the precision screw. The specimen is mounted on three bearing balls set in countersunk holes in plate E. Coupling between plates D and E is effected by the bracket F. The precise distance and orientation of the specimen surface with respect to the plate D can readily be adjusted. A separate polished rod running through a vacuum seal operates a shutter in front of the specimen surface: the shutter moves with the sample holder as the value of @Jis changed. 4.2.5. Mirror holder A silvered polished glass plate with a 2 mm diameter hole sits in a counterbored plate whose position can be adjusted with respect to the reflected beam axis. The inclination of the base plate is adjusted by three screws and secured by three clamping screws on an L-shaped bracket fixed on the analyzer arm.
Fig. 3. Polarizer/analyzer
arms, sample
holder
and evaporation
sources.
M. Yamamoto, O.S. Heavens / Vacuum automatic ellipsometer
210
4.2.6. Rotating analyzer assembly The Clan-Thompson prism is mounted in a cylinder attached to flanges on the hollow shaft of the synchronous motor, the prism and rotation axes being adjusted for exact parallelism. The effects of residual movement of the emerging beams were reduced by the use of a lens and an optical fibre rod. The position of the lens is adjusted so that the beam covers a large number of fibres, thus ensuring good transmission averaging and randomizing any residual polarization effects. Light from the end of the fibre rod is coupled to a flexible fibre, mounted adjustably close to the fibre rod of the analyzer. On the end of the shaft a chopper blade is fixed to obtain synchronized reference signals by chopping the light beams from a pair of bulbs exciting a pair of silicon phototransistors. The reference signal assemblies are mounted on the shell of the motor in such a way that the phase of the reference signals can be adjusted. The assembly is fixed to the analyzer arm by a bracket with slots for height adjustment and a bearing ball at the base for inclination adjustment. The whole passage of the beam, from the hole on the mirror to the photocathode of the photomultiplier, is enclosed to minimize the effect of stray light. 4.3. Electronics of the ellipsometer 4.3.1. Circuit description The servo electronics consists of two servo loops, one for the angle of incidence drive (AL), the other for the azimuth of polarization drive (PL). Fig. 4 shows the
DC
SERVO-
MOTOR
I
I ING
, POWER
MOTOR
D-A
-
CONVERTER l~~~llJ---DIGITAL
AMP.
2-CHANNEL T-Y RECORDER
DISPLAYS
Fig. 4. Block diagram of electronic
system.
M.
Yamamoto, O.S. Heavens / Vacuum automatic ellipsometer
211
loops with the system outline. The output from end-window type photomultiplier is fed to a Brookdeal low noise amplifier, but since the internal filter shows some phase shift with change of gain, a twin T narrow band amplifier tuned to 5.6 HZ (twice the speed of rotation of the motor) is used instead. The appropriately amplified signal goes into PSD 1 and 2 for AL and PL, respectively, where the amplitude of the sin f3 and cos 0 components [see eq. (l)] are picked up and fed to each PID (proportional/integral/differential) control circuit. The PID control circuit has three inverting operational amplifiers governing proportional, integral and differential signals of which the parameters can be adjusted so as to give best servo-response. The signals are then added through preset potentiometers to an inverting operational amplifier to be mixed in a ratio determined by the potentiometer settings. Each output is fed to’ another operational amplifier to provide gain control: then to an invertor to provide a choice of polarity which enables the appropriate polarity, x’p or xp (see section 2.1) to be chosen. Two different features are introduced. Operation of AL needs full wave rectification before voltage to frequency conversion together with a digitial signal generator to indicate the direction of rotation. For the polarizer drive, it is necessary to maintain oscillation of the drive shaft in order to overcome starting friction. This is effected by an applied signal of triangular waveform. The stepping motor and the dc servo motor driven by error signals converge the system to the null position giving negative feedback to the photomultiplier which closes each loop. The azimuth of polarizer x is digitized by the incremental optical shaft encoder, processed by up/down counter interface and displayed as a 6 digit decimal number. The information needed to determine the angle of incidence @is picked up digitally from the stepping motor driving circuit and after digital-to-analogue conversion produces permanent records on a two-channel or an X-Y recorder chart. The pulse repetition frequency of the error signal must lie below the maximum frequency at which the stepping motor can be swtiched on and off. This was measured and found to be 4 kHz, corresponding to an acceleration of 16 Hz/(ms)‘. For the maximum required value of d’@p/dt* (section 3) the corresponding acceleration is -1 Hz/(ms)‘. Thus the response of the drive system for the angle of incidence is more than sufficient to meet the design requirements. 4.3.2. Transient response of the system The performance of the servo system depends on (a) the steepness of the signal amplitude/angle of incidence and signal amplitude/polarizer azimuth curves, and (b) the PID parameters chosen. These parameters were examined for a ZnS coated glass plate. The system was set manually, and errors of 2.9 and 0.9 deg were introduced in @ and x settings, which produced an out-of-balance of 1 .O V to each PID circuit. The system was then switched to the automatic mode. The values of $I and x were found to vary linearly with time with little sign of overshoot. The times required to return to the balance condition were found to be 500 ms for the polarizer and 950 ms for the angle of incidence settings.
212
iE1. Yamamoto,
O.S. Heavens / Vacuum automatic ellipsometer
4.4. Problems of the vacuum system The particular problems associated with the vacuum system fall under three headings, viz., (a) vacuum seal for rapid linear motion of angle of incidence drive, (b) outgassing and lubrication problems of motors and (c)protection of optical components from evaporation material. 4.4.1. Vacuum seal for rapid linear motion The upper plate of the system carries two sliding seals. The main shaft (MS), 12.7 mm in diameter, runs through a pair of spring-loaded lipseals and provides the angle of incidence drive. The smaller shaft (SS), 6.35 mm in diameter, is sealed by a pair of 0 rings and provides the specimen shutter control. A pressure rise is observed for rapid (200 mm/s) sliding motion, the extent of the rise being proportional to the distance traveled and not to the speed, as observed by Dawton [7]. This’effect is minimized by giving the shafts a high polish and by pumping the space between the 0 ring seals. For a 200 mm traverse, the associated leak is less than 6 X 10m4 Torr 1. In actual operation, the maximum traverse in one second is of the order 25 mm. Thus for the 56 1 chamber the maximum pressure rise due to the sliding seals is -1.3 X 10m6 Torr, and acceptably low. 4.4.2. Performance of motors under vacuum Tests on the polarizer drive motor (Portescap 34L21-219P) with 20 V applied reveal a temperature rise in the sintered bronze bearing of only 14°C after one hour’s operation. From the change in drive current, a similar temperature rise in the coils is deduced. Since under normal operation maximum power is only very rarely needed, these tests indicate that no difficulties arise with this component. The rotating analyzer is driven by a SLO-SYN synchronous motor (TS 50E), which runs continuously during operation of the ellipsometer. The most suitable bearing lubricant is found to be a mixture of silicone vacuum grease and MO& powder. Although a starting voltage of 120 is required for this motor, 25 V suffices for running, under which condition the temperature rise after 2 h operation is found to be only 3°C. Under running conditions the motor speed, measured by an optical shaft encoder, was found to be 2.7859 f 0.0008 c/s. 4.4.3. Protection of optical components In view of the very high sensitivity of the instrument it is vital to ensure that no deposition occurs on any of the surfaces of the optical components. Shields over the evaporation source confine deposition to the sample area and minimize any temperature change in the ellipsometer components. No detectable change in the extinction ratio of the Glan-Thompson prisms is observed after many cycles of operation. The observed extinction ratio lies in the range 1O-8-1O-6, depending on the precise position of the beam through the prisms.
M. Yamamoto, O.S. Heavens / Vacuum automatic ellipsometer
213
5. Alignment of the ellipsometer 5. I. Optical alignment The alignment needs to be carried out with reference to the axis of linear motion of the main shaft (fig. 5). Axes A and B of fig. 3 need to be set symmetrically at precise points on a line through the centre of the axis of the sample holder block and accurately perpendicular to the drive axis. This is effected by the horizontal bar H (fig. 5). The T-shaped tool T, fitted into a reference block L which carries accurately parallel holes, serves to ensure that H is perpendicular to the drive axes. A mirror with a pinhole is mounted concentrically in the hollow shaft along axis A and serves to align the incident beam accurately along the axis. The upper surfaces of the slots on the polarizer and analyzer arms are accurately located with respect to the upper surface of H in the I$ = 90” position. These surfaces are maintained in sliding contact with precision ground blocks mounted concentrically on the sample block so as to maintain the angle of incidence accurately over the whole range. In the $J= 90” position, and after removal of H, the orientation of the reflector on A is adjusted to direct the beam accurately along the axes of the polarizer and analyzer arms. The rotating analyzer, mirror and polarizer are then mounted on the appropriate arms. An optically flat table is then mounted on the reference block L, with its normal accurately parallel to the drive shaft. The specimen orientation can now be accurately set by adjusting its orientation until interference fringes are seen.
Fig. 5. Auxiliary
alignment equipment.
M. Yamamoto,
214
5.2. Alignment
O.S. Heavens / Vacuum automatic ellipsometer
ofelectronics
The required 90” relative phase difference between the reference signals from the lamp/phototransistor assemblies on the rotating analyzer is achieved by ensuring that the output from one of the PSD’s is zero when the reference signal from the other PSD is examined. The angle of incidence of the beam on the sample is then set at the Brewster angle, so that the reflected beam is entirely the s component. The analyzer azimuth is adjusted in relation to the chopper blades thus ensuring the correct absolute phases of the error signals.
6. Window birefringence Since the ellipsometer is mounted inside the vacuum system, birefringence of the windows is of no consequence. However, the ellipsometer does in fact provide a methad for the determination of the window birefringence. Full details will be published elsewhere, but results are given to indicate the extent of birefringence likely to be encountered in suitably annealed Pyrex windows. Windows 40 mm diameter and of two thicknesses (5.2 mm and 3.0 mm) were examined. Measurements are made with the light beam at 2” to the window normal passing 2 mm from the centre of the window. Table 2 shows the results obtained for the 5.2 mm disc before and after annealing at a temperature of 570°C for 1.5 h followed by cooling to room temperature in 5-6 h. The sensitivity of the observed birefringence is illustrated by the fact that contact with a finger for a fraction of a second at a point -20 mm from the region being examined produced a rapid rise in birefringence, to X.1”, which decayed to -zero after a minute or so. For the 5.2 mm disc, the large change in birefringence on evacuating the chamber should be noted; and that contrary to expectations the performance of the thinner disc was superior to that of the 5.2 mm window.
Table 2 Residual birefringence Disc thickness (mm) 5.2 5.2 3.0
of vacuum
window
Residual birefringence (under vacuum)
Residual birefringence (in air) (deg) 1.166 0.039
~0.01~ 4
a) Barely detectable:
expected
glass
(deg)
1.196 (before annealing) 0.070 (after annealing) -CO.031 (after annealing) accuracy
-O.Ol”.
M. Yamamoto, 0.X Heavens / Vacuum automatic ellipsometer
21.5
7. Accuracy In operation, the angle of incidence $I is obtained from the position of the sample block, as deduced from the number of pulses supplied to the stepping motor. Separate angle measurements on the system established that the reproducibility of the angle of incidence setting was approximately +_0.004 deg. Attempts to use Polaroid as the polarizing element failed on account of beam wander coupled with a variable sensitivity of the photomultiplier cathode. This problem was much reduced by the use of a Clan-Thompson prism and further improvement resulted from the use of the fibre optic rod in the analyzer arm. The signal-to-noise ratio (maximum signal to that at balance) without the fibre rod was 33: use of the fibre rod improved this value to 300. The accuracies of measurements of c#+and xp in the RPTE and ARAE configurations were compared by examining an aluminium film specimen. The results are shown in table 3. The slight differences observed in $P in the two configurations are accounted for by a small ellipticity in the light transmitted by the Clan-Thompson prism. The above measurements were made in the manual mode since in the automatic mode, there is invariably a slight hunting. For the 4~ drive, the average amplitude of the hunting is 10 steps (0.010 deg at @= 56”). For the xp drive, about 8 steps of hunting (0 .O16 deg) are observed. The performance of the system is illustrated in fig. 6, which shows the variation of @r, xp during the deposition of a MgFz film up to a (metrical) thickness of 20 nm. The evaporation started at X and continued for 3 min stopping at Y. The sensitivity of the system to small inhomogeneities is apparent when the curve of fig. 6 is compared with the result appropriate to a film of constant refractive index, increasing in thickness with time, as shown by the dotted curve. The fluctuations in the recorded curve indicate inhomogeneity of refractive index of the film of the order 0.005-0.01.
Table 3 A set of ellipsometer Position
Mode
measurements
for an Al film Corresponding
Direct readouts
values
@p display
XP display
@p(k)
XP (ded
X’p
RPTE ARAE
6505 6503
925654 925609
80.213 80.216
+49.44 +49.54
XP
RPTE ARAE
6486 6433
970656 970700
80.241 80.320
-49.44 -49.54
M. Yamamoto, OS. Heavens / C‘acuum automatic ellipsometer
216
1.20 I
1.60 I @P (deg.)
XP
1.00 (deg.) I
Y
56.90
I
I
I
.80
.88
l’ig. 6. Results
for 20 nm MgF,
I
.40
xp film deposited
I
(deg.)
.20
on glass in 3 min.
Acknowledgements The authors are especially grateful to Robert Easton and Steve Lawson for their contributions to the mechanical and electronic design of the system: and to K. Itoh, of Harmonic Drive Systems Ltd, for collaboration in the design of the polarizer drive. The instrument was developed with support from the Science Research Council.
References [I ] [2] [3] [4] [5] [6] [7]
R.P. Netterficld, Appl. Opt. 15 (1976) 1969. K. Kinosita and M. Yamamoto, Surface Sci. 56 J.1:. Archard, P.J. Clegg and A.M. Taylor, Proc. H.M. O’Bryan, J. Opt. Sot. Am. 26 (1936) 122. M. Yamamoto, Japan. J. Appl. Phys. 14 (1975) D.E. Aspnes, J. Opt. Sot. Am. 64 (1974) 639. R.H.V.M. Dawton, Brit. J. Appl. Phys. 8 (1957)
(1976) 64. Phys. Sot. (London) Suppl.
B65 (1952)
758.
14-1,413.
414.
Discussion D.E. Aspnes (Bell Laboratories): What is the base pressure attainable, and did you have any difficulties with mechanical movements, such as rotations, in vacuum? Pa. For the second questions, yes. We had many diffiM. Yamamoto: The pressure is lo4 culties. For example, sealing of the main shaft since it moves quite rapidly in some cases. But eventually, if you polish the shaft carefully you can overcome the problem. For other problems such as outgassing from the motors, I put some details in the paper. T.H. Allen (Optical Coating Laboratory): You indicated that DC-704 diffusion-pump oil was used to lubricate the mechanical components of the ellipsometer. Since DC-704 has a free evaporation rate under vacuum of several angstroms per second, did you observe any indications that this molecular contamination affected the accuracy of your measurements? M. Yamamoto: I don’t think that will be a problem because only polarizing prisms could show a problem if the surfaces are contaminated badly. And I found only occasional cleaning of the surfaces is sufficient to avoid the problem.