Scale-up of pulsed laser deposition (PLD) for 4″-wafer coating

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applied

surface science ELSEVIER

Applied Surface Science 96-98

( 1996) 643-648

Scale-up of pulsed laser deposition (PLD) for 4”-wafer coating M. Panzner

*,

R. Dietsch, Th. Holz, H. Mai, S. Vdlmar Received 22 May 1995

Abstract PLD

of uniform

MBE-system. plumes excited

thin films

Thickness

on 4”.wafers

homogeneity

simultaneously

has been realized

over the total substrate

by the integration area is obtained

of a PLD-source

by precise

at two or more adjacent target locations. Computer

controlled

mto a commercial

spatial control

of the plasma

motion of a cylindrical

target

with respect to the stationary focal spots of the laser beams, has to provide (a) a suitable deflection

of the plume axis and (b)

a uniform target erosion. Process control was investigated

of the two techniques for

large-area

characterization.

1.

by computer

coating described in this paper is illustrated by the preparation Special film thickness

gradients were realized

Most of the industrial process lines for the synthesis of electronic devices are capable of handling substrates of diameters 2 4”. However, excellent properties of thin films synthesized by PLD were mainly achieved on small areas only, typically I cm’. Consequently, an upscaling of PLD was necessary to provide uniform coatings at large area substrates. Earlier work on large-area PLD was published by Greer [l] in 1989. His arrangement involves a rotating substrate, a large-diameter counterrotating target, and a scan of the laser beam at the target surface. Substrates of I” diameter were homogeneously coated by the so called off-axis PLD of Erington and Ianno [z] in 1990. This technology utilizes an offset of the plume to the rotating substrate without laser beam scanning used by Greer [I].

3582 349: fax: +49-351

Ol69-4332/96/$15.00 C 1996 Elsevier SSDI 0169.4332(95)00536~6

Science

B.V.

of DLC

by a particular

Introduction

Corresponding author. Tel.: +49-X51 1.592 230: c-mail: [email protected].

simulations.

Ail rights reserved

The efficiency

thin film specimens and their ellipsometric regime of target and

substrate

motion.

Nonuniformities could be kept < +5% over a substrate area of 2” by this method [6]. An improvement of this procedure was suggested by Eddy [3] and Smith [4]. Additional to the rotation a translation of the substrate holder was introduced. They succeeded in the deposition of thin films having maximum thickness deviations of +_6% over 5” substrates. Greer achieved a homogeneous deposition on 3”-diameter substrates (nonuniformities < + 6%) by a refinement of his method 151. These developments were initiated by the demands for perfect films of high-temperature superconducting YBa,Cu,O,_ ,. Substrate motion is necessary for all the technologies considered above. This could be a disadvantage for in-situ thin film characterization like ellipsometry, RHEED or X-ray diffraction as applied in the preparation of semiconductor films or X-ray multilayers. For one of the PLD-technologies developed by the present authors substrate motion is optional. Film uniformity is achieved only by control of the ablation plume of exactly adjusted PLD-sources. A fur-

hf. Panuzer et al./Applied

644

Sur@ce Science 96-98 (1996) 643-648

ther development of that technology using a linear substrate motion enables the synthesis of either homogeneous thickness profiles or precisely determined thickness gradients along one substrate direction. Knowledge about the particle distribution in the plume is necessary to optimize the deposition process for every large-area PLD technology. The angular distribution of the particle flux Z((Y) is suitably described by a cosine power function: I( a) = Ccos”( a),

ent focal spot diameters and power densities were realized by of the variation the lens-target distance. The thickness distribution of the deposited films on 4”-wafers was measured by ellipsometry. Fig. 1 shows results acquired across the total wafer diameter. The film thickness depending on substrate location d(r) can be derived from Eq. (1): d(r)

(2)

where: r, radial coordinate; L, distance between target and substrate; @a,,.,,,film mass density. Shape exponents n were determined by fitting the theoretical curve of Eq. (2) and the measured data. It has been found for equivalent pulse energies that: * smaller n-values are correlated to smaller focal spot diameters and reduced deposition rates; whereas . large n-values are found for increased focal spots and consequently higher deposition rates. Values between n = 3 and n = 9 were determined for FeSi,-ablation.

(1)

where C and n contain the information on intensity and width of the plume, respectively. IZ sensitively depends on laser beam parameters (like diameter and intensity distribution of the spot) and morphology of the target surface.

2. Experimental

=(C/e,,,)L-‘cos”+3(arctan(r/L)),

and results

2. I. Plume characteristic 2.2. Large area PLD

To investigate the flux distribution of the particle beam, experiments were carried out by conventional PLD in a simple high vacuum chamber utilizing the frequency doubled beam of a Nd:YAG laser. Differ-

An alternative technology to the conventional ones explained above has been proposed for an improve-

18-

60-

40-

20 I 04 . -60 -40 I

. A

--.----

1

I

I

-20 0 20 radius r [mm]

I

I

40

60

1900 pulses, nal = 8,97 +/- 0,51

.

920 pulses, net= 8,91 +/- 0.52

.

~~~~~ 1830 pulses, nttt= 3,37+0,09 910 pulses, “at = 3,33+/-0.08

Fig. 1. Thickness profiles of Fe%,-films synthesized by conventional PLD (line scans) parameters: pulse energy E, = 860 ml, pulse duration T= 8 ns, L = 65 mm, wavelength A = 532 nm, focal length I, = 545 mm, substrate temperature T, = 25°C.

M. Panzner et nl./Applied Surface Science 96-9X t/9961643-648

b

a

Fig. 2. Basic princtples of 4”-substrate coating technology with plume steermg. (a) Superposition motion.

of adjacent PLD-sources.

(b) Substrate

I.0

09 0.8 41

0.7 0.6 OS

0.980 -- 0.990

.I\

ma.970

-- 0.980

-0.960 -0.950

-- 0.970 -- 0.960

d

ma.970

-. 0.9x0

,0.960

_. 0.970

0.900 -- 0.950 -0.850

-- 0.900

-0.800

-- 0.850

Fig. 3. Calculated film thickness profiles for typical parameters of the PLD-MBE-system (a) Plume deflection. (b) Superposition of four adjacent plumes and simultaneous plume steering. (c) Substrate motion with steady velocity across a PLD source of plume steering. (d) Case (c) with substrate velocity depending on z-position.

646

M. Pmrner

et cd. /Applied

Sutj~~cr Scirnce 96-98

ment of the uniformity of PLD-films [7]. It is now in use for more than 4 years for X-ray mirror preparation and has been upscaled to realize the uniform coating of 4”-wafers [g]. Fig. 2(a) illustrates the principle of that technology. The 4”-substrate is located above a cylindrical target. The beams of two simultaneously operating lasers are scanned across the curved surface of the target so that a spatial control of the particle flux of the laser plume and a uniform ablation are realized [7]. The superposition of the particle fluxes originating from two or more target locations guarantee a homogeneous deposition across the total substrate area. A modified version of this basic principle of large area coating involves a controlled linear motion of the substrate (Fig. 2(b)). Two laser beams are focused at closely adjacent target locations to simulate a single but very intense particle source. Plume steering is achieved by the same procedure as explained above. Now the total length of the target can be continuously rasterscanned. Then, a uniform coating can be realized by a steady substrate motion in :-direction. Moreover the preparation of thin films having e.g. linear thick-

(1996)

643-648

ness gradients was realized by the appropriate linear :-velocity profile. A prerequisite for the precise deposition of a nominal thickness profile is a stationary average ablation rate throughout the entire coating process. The dependence of film thickness distributions deposited by the proposed technologies on the corresponding process parameters was investigated by computer simulations. The developed software system involves profile calculations for periodic target motion and one-dimensional substrate motion. Fig. 3 shows optimized thickness profiles calculated for typical parameters of Nd:YAG laser ablation (shape exponent of the particle flux n = 9). 2.3. PLD-MBE-deposition

system

The described PLD-source was integrated into a commercially available MBE-system thus providing a real alternative to conventional coating techniques. The experimental setup shown in Fig. 4 consists of two synchronized solid state lasers (E = 2 J, r = 8 ns), the UHV-system involving the stepper motor driven PLD-source, additional K-cells, thickness

Deposition method

I

PC-control

Fig. 4. PLD-MBE-deposition

system.

Ceeette

entrylock

M. Prm~wr

et al. /Applied

Surface

Science Y6-48

647

f IYYbj 643-648

a

2x5 -- 3ul

36.5 -. 38.0

-240

.- 255

-31.9

-. 33.4

-225

-- 240

-30.4

-- 31.9

Fig. 5. Thickness profiles of DLC-films on #-wafers synthesized by large-area PLD. (a) Superposition of adjacent PLD-sources ot steering plume. (b) Substrate motion across a PLD-source of steering plume

control units and the computer control system. Preparation of multilayers is realized by alternate ablation from targets of different composition. Therefore the target module provides 4 individual targets of 20 mm in diameter and 115 mm in length. A distance of 120 k 20 mm between substrate and target surface can be selected. The target can be rotated for ablation across its total surface. The beam supply system enables for each laser beam to he focused on two alternative positions of the corresponding halve of target. One can switch from one focal spot location to the other by an additional mirror that is moved into the beam. Quartz plates mounted in the laser ports in a slightly tilted orientation protect the laser windows from deposition. The film growth can be monitored by quartz oscillators, in-situ ellipsometer or RHEED-system. For the optimization of the large-area PLD-technique the frequency doubled beam of the lasers (A =532 nm, El, = 800 mJ) was used to deposit model films of DLC and Ni on silicon substrates of 4” diameter. At reduced laser power (P = 7 W. E, = 700 mJ, fd = IO Hz, A = 532 nm) a deposition rate of 1.6 A/s has been achieved for homogeneous films of carbon. It can be raised to values of typically 5 i/s. Spectroscopic ellipsometry was chosen to measure the thickness profiles of the synthesized films. Film thickness profiles deposited on 4” Si-wafers by very

80

,

70 -

,

,

.

,

,

.

,

,

o-o-&+.__o-o-o-o-o-o-o_o_o

-

60-

F 5

.50-

:,

z

40-

t = g

“= TO-

2.55 (632,8 nm)

k= - 0.78 d= 68.6 nm

.= *“_

0

IO

20

a)

30

40

50

60

1 [nm]

Ni/ClSiO,/Si

z-

LRim

thickness gradient independent of model:

2.0 , ,5 _

l.Ob)

0

I .6 nm SO,

(fixed) I4.8 nm C (fitted)

0

3.1 nm SO,

(fitted) 13.7 nm C (fined)

* 4

e

-2

1

1

c

0

2

4

1 /[ml

Fig. 6. Thickness profiles of films prepared by large-area PLD of well optimized parameters. (a) Uniform carbon film. (b) Gradient Ni-film on carbon underlay.

648

M. Pawner et al. /Applied Surface Science 96-98 (1996) 643-648

first large area coating experiments are shown in Fig. 5. Here already standard deviations of f7.4% and 7.3%, respectively are obtained. Further optimization of the target substrate motion regime has to be done to achieve thickness deviations of the calculated distributions (Fig. 3). After optimization of the steered plume technology for particular applications (substrate sizes: 60 mm, 80 mm), the thickness profiles show deviations from nominal shapes as represented in Fig. 6.

3. Conclusion New technologies for the deposition of homogeneous thin films on 4”-wafers by PLD were developed. The appropriate PLD-source was integrated into a commercially available MBE-system thus enabling a hybrid deposition together with alternative, conventional coating techniques. As a prerequisite of an intended industrial utilization of PLD film thickness uniformity across substrate sizes of typically 4” could be achieved with DLC-coatings synthesized for particular applications.

Acknowledgements The authors thanks Prof. Dr. G. Dijrfel and his coworkers for the technical realization of parts of the

projected experimental setup. The work was supported by the Federal Ministry of Education and Science under Grant No. 13 N 6487 and 0329395.

References [l] I.A. Greer. in: Proc. SPIE, Vol. 1835 (The International Society for Optical Engineering, Washington, 1993) p. 21. [2] K.B. Eringon and N.J. Ianno, in: Mater. Res. Sot. Symp. Prcc., Vol. 191 (Materials Research Society, Pittsburgh, PA, 1990) p. 115. [3] M. Eddy, Fall Meeting of the Materials Research Society. Boston (199 I). [4] E.J. Smith, Fall Meeting of the Materials Research Society, Boston (1992). [5] J.A. Grccr, J. Vat. Sci. Technol. A 10 (1992) 1821. [6] S.R. Foltyn et al., Appl. Phys. Lctt. 59 (1991) 1374. [7] R. Dietsch. H. Mai, W. Pompe and S. VGllmar, Adv. Mater. Opt. Electron. 2 (1993) 19. [S] H. Mai, R. Dietsch, Th. Holz, S, VGllmar, S. Hopfe, R. Scholz, P. WciObrodt, R. Krawietz, B. Wehner, H. Eichler and H. Wendrock, in: Optical Interference Coatings, Ed F. Abele, Proc. SPIE, Vol. 2253 (SPIE - The International Society for Optical Engineering, Washington, 1994) p. 268.