Investigation of surface amorphization of silicon wafers during ion-milling

Ill;h'gnlll~!

Ultramlcroscopy 41 (1992) 429-433 North-Holland

Short note

Investigation of surface amorphization of silicon wafers during ion-milling T Schuhrke, M Mhndl, J Z w e c k a n d H H o f f m a n n Institute of Apphed Physws, Unwerstty of Regensburg, Unwersttatsstrasse31, W-8400 Regensburg, Germany Received 9 January 1992 Artifacts in the preparation of samples for transmission electron microscopy (TEM) are a well known but often underestimated problem To investigate changes m the surface structure during the final preparation step of TEM samples, the ion-etching to electron transparency, (100)-oriented St wafers were ion-milled with Ar beams of varying energy and angle of incidence One effect of lon-mllhng is the generation of an amorphous surface layer whose thickness increases with Ion energy and angle of incidence A simple model Is used to compare the measured thickness of the layers with theoretical values These were calculated from tabulated data of damage functions deduced from transport theory

1. Introduction I n r e c e n t y e a r s l o n - m i l h n g has b e c o m e t h e m o s t i m p o r t a n t m e t h o d to p r o d u c e s u i t a b l e thin s a m p l e s , e s p e c i a l l y cross sections, for T E M d u e to t h e fact t h a t c h e m i c a l e t c h i n g o f t e n falls in t h e case o f m u l t i c o m p o n e n t systems T h o u g h Ionmilling is w i d e s p r e a d t o d a y , use o f this m e t h o d is n o t w i t h o u t p r o b l e m s b e c a u s e it c a u s e s a n u m b e r of artifacts Maybe the most unfortunate of them is t h e p r o d u c t i o n o f r a d i a t i o n d a m a g e in the s a m p l e s given by single l a t t i c e d e f e c t s o r even a m o r p h o u s s u r f a c e layers if t h e ion d o s e is l a r g e e n o u g h It lS well k n o w n f r o m c o m p u t e r simulations t h a t t h e s e a m o r p h o u s layers o n t o p o f o t h erwise crystalline m a t e r i a l s seriously d i m i n i s h t h e quality o f H R E M p i c t u r e s o f t h e crystal s t r u c t u r e [1] I n o r d e r to find an o p t i m u m m e t h o d o f p r e p a r a t i o n with m i n i m u m d a m a g e , SI s a m p l e s w e r e 1on-milled with A r p a r t i c l e s o f varying energy a n d a n g l e o f i n c i d e n c e , a n d t h e t h i c k n e s s o f t h e a m o r p h o u s layers was i n v e s t i g a t e d

2. Experiment A ( 1 0 0 ) - o r i e n t e d silicon w a f e r was cut a l o n g t h e {011} l a t t i c e p l a n e s into p i e c e s o f 2 5 m m

l e n g t h with a d i a m o n d - w i r e saw T h e size o f t h e p i e c e s is l i m i t e d by t h e s p e c i m e n h o l d e r s o f t h e u s e d ion mill, a G a t a n D u a l I o n Mill M o d e l 600 T h e s q u a r e s w e r e m e c h a n i c a l l y p o l i s h e d with a series o f g r i n d i n g p a p e r s with d e c r e a s i n g g r a n u larity a n d finally with Syton W30, an e m u l s i o n o f 125 n m small SIO 2 particles, as is usually d o n e for t h i n n i n g o f T E M s p e c i m e n s S u b s e q u e n t l y several of t h e s a m p l e s w e r e i o n - m i l l e d with varying p a r a m e t e r s T h e a c c e l e r a t i o n v o l t a g e at t h e ion gun was v a r i e d b e t w e e n 2 a n d 9 5 kV, t h e angle o f i n c i d e n c e b e t w e e n 5 ° a n d 35 ° T h e samples w e r e c o o l e d by liquid n i t r o g e n a n d w e r e r o t a t e d d u r i n g t h e milling p r o c e s s T h e m l l h n g t i m e was 1 h with a p r o b e c u r r e n t of 0 5 m A T h e ion d o s e was high e n o u g h to g u a r a n t e e t h e form a t l o n o f a c o n t i n u o u s a m o r p h o u s layer on the surface In o r d e r to m e a s u r e the thickness o f t h e s e layers, cross sections o f t h e s a m p l e s w e r e p r e p a r e d F o r this p u r p o s e t h e m i l l e d Si s q u a r e s w e r e g l u e d to u n p r o c e s s e d o n e s o f t h e s a m e size O u t o f this " s a n d w i c h " a slice o f 0 3 m m thickness was cut with t h e d i a m o n d - w i r e saw vertical to t h e i n t e r f a c e This slice was m e c h a n i c a l l y poli s h e d on o n e side, g l u e d to a small m e t a l ring a n d a g a i n g r o u n d a n d p o l i s h e d to a thickness o f a b o u t 20 /~m on the o t h e r side In a last s t e p the

0304-3991/92/$05 00 © 1992 - Elsevier Science Pubhshers B V All rights reserved

T Sehuhrke et a l / Surface amorphlzatLon of sthcon wafers dunng ton-mdlmg

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samples were thinned to electron transparency in the ion mill with 5 kV acceleration voltage and an Incidence angle of 14 ° The samples were investigated in a Phihps CM30 transmission electron microscope We made high-resolution images of the amorphous layers to illustrate the dependence of their thickness on the milling p a r a m e t e r s For this purpose, the samples were oriented in the microscope in (110) direction Therefore we could be sure that the electron b e a m transmits the sample in a plane parallel to the surface of the wafer in order to measure the exact thickness of the amorphous layer from the H R E M images The composition of the amorphous layer was determined by means of electron energy-loss spectroscopy (EELS)

3. Transport theory The principle of formation of the amorphous layer is Illustrated In fig 1 [2] The argon particles hit the surface, some of t h e m are reflected, others penetrate the surface and create small amorphous regions by knocking target atoms from their lattice positions If the density of the projectiles is great enough, the amorphous zones will form a continuous layer With the knowledge of the depth of the zone-center below the surface (a in fig 1) and the longitudinal and transversal

Ar

crystalline silicon

Ar, Si

amorphous region

Fig 1 Principle of surface amorphizatlon used in transport theory the Ar particles produce amorphous zones, which form a continuous layer if their density IS high enough The layer thickness d can be calculated with the knowledge of the three values a, b and c determining the position and width of the a m o r p h o u s regions

extensions (b and c in fig 1) of the zone, computation of the thickness of the amorphous layer is possible We obtained the width of the amorphous zones out of data of so-called " d a m a g e functions" tabulated by WInterbon [3] The damage function F IS an expression for the energy dissipated per unit volume by the projectile and the target atoms, which are set in motion by collisions The basic equation in transport theory, which determines the damage function F, is

F(r, v) = NlSR[ f[ F(r, v') + ff(r, v")] do.' + ( 1 - N [ S R l fdo-')F(r-Sg, v), (1) where F(r, v) is the energy dissipation of the projectile with velocity v at the position r, F(r, v') analogous, v ' the velocity of the projectile after colhsion, F ( r , v " ) the energy dissipation of a target atom which is set in motion by a colhsion, N the density of the atoms in the target, 8R the path element of the projectile and do-' the differential scattering cross section The projectile moves In the target from point r-~R to point r F(r, v) equals F(r, v')+ F ( r , v'), If the projectile and a target atom are scattered to the states v ' respectively v" with the probabihty N [ 8RI do-' along the way 8R Integrating over all possible scattering states gives the first expression on the right-hand side of eq (1) With a probability of 1 - N [ 8R [ f do-' the projectile is not scattered on its way 8R and R remains unchanged, that means F(r, v)= F(r~R, v) which is described by the second expression on the right-hand side of eq (1) Eq (1) takes into account only collisions between two particles due to the atom energies of only a few keV In this so-called "linear cascade" region the recoil atoms possess enough energy to generate new cascades, but the density of moving atoms is so low that collisions of higher order are extremely improbable [4] Developing the second term in eq (1) to first order In 8R and splitting off a fraction d o e = do.'

T Schuhrke et al / Surface amorphtzatton ofsthcon wafers durmg ton-mdlmg - do" o f e l e c t r o n i c excitation f r o m t h e total scatt e r i n g cross s e c t i o n d ~ ' o n e o b t a i n s

v OF(r, v ) v

431

s e l e c t e d p r o j e c t i l e / t a r g e t c o m b l n a n o n s with diff e r e n t m a s s rattos

=

fx"F(x, E, cos

O) d x ,

(3)

Or

=Nf[F(r,v)-F(r,v')-F(r,v")] OF + Se~-,

do.

(2)

with S e = N f T e dt~e, do. = d o ' ' - d o ' e , do.e is t h e e l e c t r o n i c p a r t o f t h e d i f f e r e n t i a l cross s e c t i o n a n d Te t h e e n e r g y t r a n s f e r to t h e e l e c t r o n s F o r t h e d i f f e r e n t i a l cross section do, W i n t e r b o n [3] uses L l n d h a r d ' s f o r m for s c r e e n e d C o u l o m b p o t e n t i a l s [5] in t h e T h o m a s - F e r m i approximation do e and therefore S e are taken prop o r t i o n a l to t h e velocity I n s t e a d o f t h e d a m a g e f u n c t i o n F its m o m e n t s ( x n) w e r e c a l c u l a t e d by W l n t e r b o n [3] for s o m e

w h e r e x is t h e d e p t h into t h e target, E t h e p a r t i c l e e n e r g y a n d 0 t h e i n c i d e n c e angle T h e s e values o f ( x n) a r e t a b u l a t e d for the case of n o r m a l i n c i d e n c e T h e first m o m e n t ( x ) , which is the m e a n d e p t h o f the f u n c t i o n F , m u l t i p l i e d with sm 0, c a n b e i d e n t i f i e d with a in fig 1 T h e l o n g i t u d i n a l s t r a g g h n g ~/(A x 2) = ~/( x 2) _ ( x )2 ts u s e d for b a n d t h e t r a n s v e r s e straggling f ~ y 2 ) for c in fig 1 W i t h t h e k n o w l e d g e o f t h e s e t h r e e values o n e can c a l c u l a t e t h e layer thickness

4. Results

F i g 2 shows a cross section of a s a m p l e e t c h e d with a r g o n ions a c c e l e r a t e d by a v o l t a g e of 9 k V

Fig 2 HREM pmture of a cross section of a Sl wafer etched at an incidence angle of 25° with argon pamcles accelerated by a voltage of 9 kV, (a) crystalhne slhcon, (b) amorphous layer, (c) glue, (©) probe diameter for EELS analysis

T Schuhrke et al / Surface amorphtzatton of sthcon wafers durmg ton-mdhng

432

70

7O

o E

6O

r

& o

E .¢. 6 0 ¢/) th e-

so

~ 4o ~

i

&

3o

~o

~

&

50 &

40

o

o

i

&

o

2O 20

10 2

i

i

i

4

6

8

o 10

Acceleratmon voltage [kV] Fig 3 Thickness of the amorphous layer as a function of acceleration voltage at the Ion guns The m o d e n c e angle is 14° for all values ( zx) Theoretical, (©) expenrnental values

and hitting the surface at an angle of incidence of 25 ° One can see an amorphous surface layer between the glue and the crystalline silicon The composition of this layer was analysed with EELS To prevent carbon contamination the sample was cooled with liquid nitrogen in a cryo-TEM holder and a small nanoprobe of 5 nm diameter was used for these m e a s u r e m e n t s Within the detection limits ( < 1 at%) no other elements but sillcon were found Therefore it can be excluded that the amorphous region is merely an oxide layer or that it contains incorporated Ar atoms Fig 3 shows the measured layer thickness as a function of the acceleration voltage of the ion guns The angle of Incidence is 14 ° throughout As has been expected the thickness of the layer increases with increasing energy The theoretical values were calculated from Winterbon's tabulated data assuming an energy of x keV for ions accelerated with a voltage of x kV They show the same tendency as the experimental values but are nearly always larger The best agreement exxsts at higher voltages To illustrate the reproducibility of the deviation between the theoretical and the experimental values as given in fig 3, a series of samples was ion-milled with a constant acceleration voltage of 5 kV, but varying angle of incidence Fig 4 shows the large difference between experimental and theoretical values for all angles with a maxim u m for the lowest angle of 5 °

[

i

J

10

20

30

10

0

,

40

Incidence angle [degree]

Fig 4 Thickness of the amorphous layer as a function of the angle of m o d e n c e The acceleration voltage at the ion guns is 5 kV for all values ( zx ) Theoretical, (©) experimental values

The error of the experimental values ( ~ 0 5 nm) which is indicated in fig 3 IS caused by the uncertainty of the determination of the layer thickness from electron-microscope negatives This is a consequence of the sometimes not dearly defined boundaries amorphous layer/crystalline silicon and amorphous l a y e r / g l u e The error of the theoretical values arises from two contributions First, the tabulated data include an error of about 1% for the first and about 2% for the second moments of the damage function F Second, we had to Interpolate between the data of two tables because no data for the special proJectile/target combination argon/sillcon are available But these errors are random and cannot explain the fact that the theoretical

m

==

o _= I

t

I

'

!

08

16

24

32

40

Particle energy [keV]

Fig 5 The energy distribution m the particle beam produced by the ion gun at an acceleration voltage of 4 kV (according to

Crockett [7])

T Schuhrke et al / Surface arnorphlzatlon ofsthcon wafers dunng zon-mdhng

values are, throughout, higher than the experimental values Using the data calculated with the more accurate H a r t r e e - F o c k atomic potentials and the assumption S eot v ° 7 [6] the theoretical values are shghtly shifted to even higher layer thicknesses A n explanation for the &screpancy between experimental and theoretical values can be given by the energy spectrum of the particles in the beam produced by the 1on guns This spectrum has been measured by Crockett [7] on a gun similar to that used in the G A T A N Ion mill There exists a relatively small peak at 4 keV which is mostly caused by doubly charged ions (fig 5) [7] As G A T A N states [8], their guns produce only a neghglble percentage of doubly or higher charged tons, so that most of the particles in the beam have a lower energy than assumed for the calculation of the theoretical values If one uses lower energies the theoretical values are shifted to lower layer thicknesses

5. Conclusions We investigated the amorphlzatlon of S~ surfaces by 1on-milling as a function of acceleration voltage and incidence angle The thickness of the amorphous layer increases w~th increasing voltage

433

and angle These thicknesses were compared with theoretical values calculated from data of damage functions tabulated by Wmterbon The discrepancy between theoretical and experimental values was explained by the fact that the particles of the etching gas possess considerably lower energies than originally assumed for the calculations Therefore, we conclude that the effect of amorphizatlon can be described in terms of transport theory Due to the fact that thick amorphous layers decrease the quality of samples for H R E M investigations, it proves advantageous to etch the specimens in a last step with low energy a n d / o r low incidence angle

References [1] G R Anstls, M J Gorlnge, J L Hutchlson and B J Muggridge, Inst Phys Conf Ser 68 (1983) 169 [2] D G Iveyand G R Piercy,Thin Sohd Films 149 (1987) 73 [3] K B Wlnterbon, Ion Implantation Range and Energy Deposition Tables, Vol 2, Low Incident Ion Energies (IFI/Plenum, New York, 1975) [4] R Behrlsch, Ed, Sputtering by Particle Bombardment I (Springer, Berlin, 1981) [5] J Llndhard, V Nielsen and M Scharff, Mat Fys Medd Dan Vid Selsk 36(10)(1968)1 [6] J F Zlegler, J P Baersack and U Llttmark, The Stopping and Range of Ions in Solids (Pergamon, Oxford, 1985) [7] C G Crockett, Vacuum 23 (1972) 11 [8] GATAN Instruction Manual Duo Mill Model 600DIF