u b- Nan o metre urface Texture and URF 2

Kevin Lindsey, National Physical Laboratory, Teddington, Middlesex / UK; Stuart T. Smith, University of Warwiclc / UI<; Colin J. Robbie, National Physical Laboratory, Teddington, Middlesex / UI<

Received on February 4,1988

Abstract

A surface texture and profile-measuring s ty lus instrument, developed a t NPL and ca l l ed 'NANOSURF 2', which has a continuous dynamic horizontal range of 50 nm t o 50 mm, is being used f o r sub-nanometre su r face t ex tu re and p r o f i l e measurement on a wide range of engineering, o p t i c a l and semi-conductor surfaces . The paper presents a se l ec t ion of r e s u l t s on such specimens.

Bu i l t almost e n t i r e l y of near-zero thermal expansivi ty ma te r i a l s , with a t i g h t measurement loop and minimum i n t e r f a c e s , t he instrument exh ib i t s very low s u s c e p t i b i l i t y t o thermal and mechanical disturbances.

Some fea tu res of t he instrument a r e incorporated i n a novel, ul t ra-high precis ion machine too l under development, which i s b r i e f l y mentioned i n the paper.

Key words: Sub-nanometre surface measurement. low environmental s u s c e p t i b i l i t y , l a rge dynamic range, u l t r a precis ion s l i d e .

Introduct ion

Nanometre and o f t en sub-nanometre measurement of the t ex tu re and form of su r faces a r e needed f o r a wide range of app l i ca t ions . For some time, x-ray op t i ca l components, eg mirror telescopes and d i f f r a c t i o n g ra t ings , provided some of t he most exact ing requirements [Franks, 19773. However, there have been many developments i n recent years t h a t require nanometre l eve l su r f ace metrology; f o r example, synchrotron op t i c s , op t i c s f o r low-scat ter and high power v i s i b l e app l i ca t ions , r i ng l a s e r gyroscopes, some spec ia l bearings and many microelectronic and e l ec t ro -op t i c devices.

No one instrument can cover a l l metrological requirements, but the combination of t he v e r t i c a l s e n s i t i v i t y (and low noise) of t he Taylor-Hobson Talystep t ransducerls tylus system with a prec i s ion l i n e a r t racking arrangement ( t h e Talystep has arcuate t racking) has proved very useful and adaptable. NPL's f i r s t such combination Nanosurf 1 [Lindsey 19861, has now been superseded with Nanosurf 2 which has a grea te r horizontal range, improved l i n e a r motion accuracy and lower s u s c e p t i b i l i t y t o thermal and mechanical upset . This i n s e n s i t i v i t y t o ex te rna l dis turbances has been achieved primarily by using a much t ightened, s impl i f i ed reference loop b u i l t almost e n t i r e l y of low thermal expansivi ty mater ia ls .

In t h i s paper w e present r e s u l t s on a diverse range of surfaces . Many surfaces display complex, superimposed short- and long-range s t r u c t u r e , a l l of which is o f t en of d i r e c t funct ional relevance. Thus the importance of using instruments with l a rge dynamic ranges, e g Nanosurf 2 , is demonstrated.

P r inc ip l e of operat ion

A l abe l l ed photograph of Nanosurf 2 is shown i n Figure 1. The reference (measurement) loop of t h i s instrument i s constructed almost e n t i r e l y from the near zero thermal expansivi ty mater ia l Zerodur [Lindig and Pannhorst, 19853, except f o r the Talystep s ty lus transducer assembly, ST. This has been r e b u i l t using fused s i l i c a , Invar and some Zerodur, i t being impract ical t o construct the more i n t r i c a t e p a r t s i n t h i s hard and b r i t t l e g l a s s ceramic. The specimen is mounted on a 3-point kinematic support on the l eve l l i ng t a b l e , T, which is i n tu rn mounted on the ca r r i age , C2. This ca r r i age is f i t t e d with dry polymeric bear ing pads t o form an in t e r f ace with the precis ion l i n e a r sl ideway, SW. The horizontal micrometer, M1, pushes the specimen ca r r i age along the slideway v i a the s l a v e ca r r i age , C 1 . This is p a r t of t he near zero inf luencing d r ive coupling arrangement. The micrometer i s dr iven by a remotely mounted DC motor which gives a continuously va r i ab le t r ave r se speed range of 0.05 t o 10 mmjmin. The s ty lus l t r ansduce r assembly is moved i n t o contact with the specimen v i a the v e r t i c a l micrometer, M2. During operat ion, t h i s micrometer and the specimen t i l t adjust ing screw, S, a r e de-coupled from the reference loop.

The output s igna l from the transducer is read i n t o a Hewlett Packard 9836 micro-computer v i a the Talystep amplif ier . The d i g i t i s e d s igna l is then analysed using algorithms developed f o r t h i s p ro jec t .

Character isat ion

The instrument has been i n f u l l use f o r about 18 months following the cha rac t e r i s a t ion phase. The cha rac t e r i s a t ion phase comprised the evaluat ion o f :

- i n t e r n a l o r s e l f generated noise sources , eg the transducer and e l ec t ron ic s , t he ca r r i age d r ive motor, micrometer act ion and the s t a b i l i t y of i n t e r f aces i n the instrument.

- ex te rna l inf luences such a s a i rborne and foundation v ib ra t ions and thermal va r i a t ions .

t he accuracy and r epea tab i l i t y of the reference s l i d e motion. -

(i) Noise

Noise was measured over a 100 sec period with the s t y l u s contact ing a s t a t iona ry specimen. An rms noise l e v e l of t yp ica l ly

50 pm (picometres) has been measured with no d i f f e rence being d i sce rn ib l e with the motor running but not connected t o the carr iages . The general noise l e v e l is found t o vary hour by hour by up t o 10 pm.

The f i r s t dynamic noise inves t iga t ion was ca r r i ed o u t using an NPL polished 25 mm diameter Spectosi l (pure s i l i c a g l a s s ) specimen, For comparison, outputs from a s t a t i c noise t e s t and a dynamic su r face measurement taken over a similar du ra t ion of t i m e a r e shown i n Figure 2. With the specimen t r ave r s ing a t 0.15 mmlmin. t he t o t a l R ( = r m s ) ( a l l noise sources + specimen surface t ex tu re ) was 84 p% on a sampling length of 25 I.rm. Over a similar per iod, a s t a t i c noise of 52 pm has been measured ( t h e lowest recording over a s imi l a r time period is 40 pm). Note t h a t i n the rest of t h i s paper quoted specimen roughness f igu res include instrumental noise , An op t i ca l interferometer [Downs et a l , 19851 surface t ex tu re measurement of t he same specimen gave an R of 35 t o 40 pm. The very small d i f f e rences between s t a t i c andA dynamic noise and the noisel t ime dependence make quan t i f i ca t ion of s l i d e noise and other sources d i f f i c u l t .

h

F l g 1 NPL Nanosurf 2 .

Annals of the ClRP Vol. 37/1/1988 51 9

(ii) Vibration

Air-borne v ib ra t ions were found t o be more se r ious than ground-borne ones. The s e n s i t i v i t i e s of Nanosurf 2 and a s tandard Taylor-Hobson Talystep instrument were compared i n the same room using an air-borne shock produced by dropping a 200 mm square piece of plywood through 100 mm onto a similar hand held piece 1 metre away from the instrument and a t the same height as the transducer. This was enough t o produce a 20 nm de f l ec t ion on the Talystep output , and a de f l ec t ion 30 times smaller on Nanosurf 2. Placing a hermetically sealed, 80 mm thick wood composite and foam polystyrene box around the Talystep reduces i t s response t o t h a t of t he open Nanosurf 2.

(iii) Thermal

The modified s t y l u s transducer assembly contains ma te r i a l s with var ious thermal expansion coe f f i c i en t s and having thicknesses down t o about 0.5 mm. The th in components have a f a s t e r thermal response t h a t those p a r t s of the s t r u c t u r e t h a t have w a l l th icknesses i n the region of 20 t o 50 mm. Thermal expaflsion c o e f f i c i e n t s of r e q v a n t materials a r e approximately ~ o - ~ K - f o r Zerodur, 5 x_lO-'K- f o r fused s i l i c a , 22 x 10 'K- f o r Invar 36 and 240 x 10 7K-1 f o r aluminium a l loy . Subs t i t u t ing fused s i l i c a f o r t he aluminium tube t h a t connects the s t y l u s t o the t ransducer i n the o r i g i n a l Ta-pstep w i l l reduce the r e ponse from (ca l cu la t ed ) 1000 nm K t o 48 nm K-l (and t o 4 nm K-' i f Zerodur could have been used i n the r eky i ld ) . Measured f a s t thermal response of approximately 50 nmK v e r i f i e s t h i s model ( see Figure 3). This f a s t response has been produced by placing a hand near t o the s t y l u s assembly; t he temperature being measured with a thermistor which was posi t ioned approximately 15 mm from the pick-up c o i l s . From t h i s f i gu re the s t rong co r re l a t ion between temperature and s t y l u s output is c l ea r ly demonstrated. The slow thermal response w i l l be a mixture of response r a t e s , magnitudes and d i r ec t ions from the Invar i n the transducer assembly t o the Zerodur comprising most of t he r e s t of the instrument. Response t o slow thermal inputs has been monitored with the operator s t ay ing a t l e a s t 1 metre away from the instrument (Figure 4). Over a 1 hour per iod, t he room temperature f luc tua t ion was approximately 0.1 K peak-valley. After a b r i e f s e t t l i n g period of a few minutes the re i s a l i n e a r d r i f t of 7 nmfhr. There i s no obvious co r re l a t ion between the thermistor and transducer output ; t h e transducer output d r i f t i s l i n e a r with time t o sub-nanometre l e v e l s . Possibly t h i s 7 nmfhr a l s o includes some mechanicalfinterface non-thermal d r i f t . No attempt has been made t o inves t iga t e t h i s phenomenon.

( i v ) S l i d e c h a r a c t e r i s t i c s

(a ) S t a r t up. For t h i s work, a f l a t , polished s i l i c o n carbide specimen has been used r a the r than g l a s s o r Zerodur. The comparatively low e l a s t i c modulus and hardness of t he l a t t e r materials would have produced an indentat ion of t he specimen during the test s e t t l i n g time of a depth s i g n i f i c a n t to t h i s i nves t iga t ion . Even with s i l i c o n carbide, zero indentat ion may not be assumed. although its near zero magnitude can be deduced from the r e s u l t s . Tests were performed with s e t t l i n g times ranging from t ens of seconds t o an hour. A number of representat ive t r a c e s a r e shown i n Figure 5. I n each case, the t r a n s i t i o n t o motion ind ica t ed a r i s e of the s t y l u s of approximately 1.5 nm and t h i s is inva r i an t over t he range of s e t t l i n g times. This 1.5 nm comprises a blend of unknown proportions of specimen indentat ion and t h e s l i d e bearing start up c h a r a c t e r i s i t i c .

( b ) Motion r ep roduc ib i l i t y . Reproducibil i ty has been determined by t racking repeatedly over similar port ions of 2 f l a t specimens. I n t h e f i r s t experiment, two t r aces have been taken of t h e c e n t r a l 40 mm port ion of a 50 mm diameter Spec t ros i l g l a s s o p t i c a l f l a t , made by I C Optical Systems Ltd. Two separate t r aces are shown f o r comparison i n Figure 6. In a second t r i a l , t he c e n t r a l 5 mm of a Zerodur mirror has been measured and two separate t r aces can be compared i n Figure 7. From these two f igu res i t can be seen t h a t t h e r ep roduc ib i l i t y over a t r ave r se length of 40 mm i s approximately 1.5 nm r m s . while t h a t over a t r ave r se of 5 mm is b e t t e r than 0.25 nm rms. This l eve l of r epea tab i l i t y obtained during the cha rac t e r i s a t ion phase, is s t i l l being r e a l i s e d 18 months l a t e r .

Applications

This s ec t ion w i l l ou t l i ne the use of Nanosurf 2 f o r t h e measurement of surfaces on a number of d i f f e r e n t materials. The components considered i n t h i s s ec t ion have been chosen from the f i e l d s of o p t i c s , semi-conductor e l ec t ron ic s , and p rec i s ion engineering.

Measurements of 3 o p t i c a l components have been presented i n the cha rac t e r i s a t ion sec t ion , namely a pure s i l i c a g l a s s (Spec t ros i l ) x-ray mirror (Figure 2). a pure s i l i c a g l a s s ( o p t i c a l l y f l a t ) Fabry Perot interferometer p l a t e (Figure 6) and a Zerodur l a s e r gyro mirror (Figure 7). It is i n t e r e s t i n g t o note t h a t t h e l a s e r gyro mirror had a sho r t range r m s roughness ( R ) of b e t t e r than 0.1 nm, i e c lose t o the highest qua l i t y of e x d i n e d o p t i c a l surfaces , but shows s ign i f i can t long-range deviat ions. Laser gyro mirrors from other sources have shown poorer short-range R but l e s s long-range s t ruc tu re ; an example of t he need f o r w i d & dynamic range cha rac t e r i s a t ion . S imi l a r ly , t he Spec t ros i l x-ray mirror (Figure 2) shows short-range roughness l i t t l e above instrumental noise l e v e l but has s t r u c t u r e of wavelength 0.1 t o 0.2 mm of height approximately 0.5 nm.

I n microelectronics there is a requirement f o r smooth s u b s t r a t e s upon which t o deposi t t h in f i lms. Assessment of both

subs t r a t e s and overstructures a r e required a t the sub-nanometre l e v e l . Figures 8. 9 and 10 show t h e surface f ea tu res of an extremely t h i n oxide l aye r on a s i l i c o n subs t r a t e over sampling lengths of 0.05, 0.5 and 5 mm respect ively. These t r a c e s reveal superimposed s t r u c t u r e of wavelengths from less than 1 pm up to some 4 mu. The amplitude of these wavelengths increase from 1 nm t o 20 nm. Figures 11 and 12 show su r face f ea tu res on 2 (uncoated) indium phosphide wafers. The wafer i n Figure 11, polished by the conventional alcohol/bromine system, is dominated by undulations ("orange pee l " ) of wavelength 0.1 mm and amplitude 10 nm. The wafer shown i n Figure 12, polished by a new I C I Wafer Technology process, shows the lowest l eve l of medium-range s t r u c t u r e ever

Q recorded on any specimen with the Nanosurf, and a short-range R of 0.14 nm.

Figure 13 shows the short-range s t r u c t u r e of a convex moulded p l a s t i c l e n s ( t h e primary specimen curvature has been computer sub t r ac t ed ) . R was 0.99 nm f o r sampling lengths from 8 t o 50 pm ind ica t ing litke medium range s t ruc tu re .

Figure 14 shows the surface s t r u c t u r e of a s i l i c o n carbide synchrotron mirror made by Fe r ran t i Astron and shows a short-range R of 0.21 nm. Figures 15 and 16 a r e of a Japanese f e r r i t e d i s c rgading head. The head is shaped t o generate an aerodynamic l i f t - o f f , i e t o " f ly" over the d i s c s , t he gap being some 150 nm. [Ikawa e t a l , 1987 and Kambayashi e t a2 19821. I n the f i g u r e s , it can be seen t h a t t he shape comprises 2 f l a t f a c e t s , both being shown a t low magnification i n Figure 15. The surface f i n i s h of the main f a c e t , and the t r a n s i t i o n t o the second ( c lose t o the right-hand end of the t r a c e ) a r e shown i n Figure 16.

Discussion

Nanosurf cha rac t e r i s a t ion and use shows t h a t t he dry polymer-based reference slideway has a noise l e v e l of l e s s than 20 pm and nanometre long-range r ep roduc ib i l i t y . One other app l i ca t ion of t h i s slideway technology under development i s i n the construct ion of t he NPL novel, patented u l t r a precis ion machine too l ca l l ed Tetraform 1 shown i n Figure 17.

Conclusion

In t h i s paper we have presented a number of s t y l u s measurements of surface f ea tu res of wavelengths from 50 nm to 40 mm. The complexity of r e a l su r f aces , be they o p t i c a l , semiconductor o r r e a l engineering components, has been demonstrated.

It has a l s o been demonstrated t h a t s t y l u s instruments can be b u i l t with wide dynamic range and low environmental ( thermal and v ib ra t ion ) s e n s i t i v i t y .

Acknowledgements

The authors acknowledge the support of D r A Franks and the many use fu l discussions with other members of t he Division of I-lechanical and Optical Metrology, NPL, We a l s o acknowledge the development of Dr M Stedman, NPL and M r P Moore, Universi ty of Warwick. of t he software t h a t has been used t o generate many of t he r e s u l t s presented i n t h i s paper. STS i s the r e c i p i e n t of an SERC CASE award i n associat ion with the National Physical Laboratory. Rank Taylor Hobson L t d have provided valuable a s s i s t ance and continue t o he associated with t h i s p ro jec t .

References

DOWNS M J, McGIVERN W H and FERGUSON H J . 1985, "An o p t i c a l system f o r measuring the p r o f i l e s of supersmooth surfaces" , Precis ion Engineering, Vol.7, No 4 , 211-215.

FRANKS, A, 1977. "X-ray opt ics" , Sc i . Prog. Oxf., Vol. 64, 371-422.

IKAWA N . SHIMADA S and MOROOKA H, 1987. "Nanoperformance of machine too l elements". Proc 6th 1nt.Conf. on Prod.Eng. 64-69.

KAMBAYASHI A. NAKATA K , and MAKINO M . 1982, "Precis ion machining i n t h e manufacture of magnetic heads". Bul l . Japan. Sac of Prec Eng. Vol 16, NO 1. 37-42.

L I N D I G 0 and PANNHORST W, 1985, "Thermal expansion and length s t a b i l i t y of Zerodur i n dependence on time and temperatures". Applied Optics, Vol. 24 (20). 3330-3334.

LINDSEY K. 1986, "The assessment of u l t r a smooth s u b s t r a t e s and overcoatings", Vacuum, Vol. 25 (10-11). 499-502.

520

l5

I I I I

i L I I I I 0 01 1 022 033 0 4 4 0 5 5

Fig 2 Trace ( A ) is combmed instrument dynamic noise t surface texture of an NPL-polished pure sil ica glass specimen Trace

( m m )

( 8 ) is instrument static nolse, i e sty lus static on g lass specimen

30 n- '1 2o

119 7

1 1 9 2 I I I I 19 0

0 12 24 36 48 60 Time (mins)

Fig 4 Response to slow thermal input Trace (A) is temperoture.(B)is tronsducer displacement

15 I I I I I 011 022 033 0 4 4 055

F lg 5 Slide s ta r t -up cha rac te r l s t l cs ,~ e transition From static s la te lo t raversing silicon carbide speclmen at O15mm rnin-1 Trace(A) 1s startlng up af ter the sty lus has been stat ic on the specimen for 6 mins,(8) 10 mins, (C) 15 mms

0 (mm)

60 I I I I I 0 7 8 15 7 23 5 31 4 39 2

( m m ) Fig 6 Repeat 4Omm long traverses on a pure silica glass optical flat

15

I I I I

(mm)

0.98 1.96 2.94 3.92 4.9

F l g 7 ReDeat 5 mn Ions traverses on a Zerodur laser gyro m l r r o r .

15 ~p----pr-----,---~

,020 .U31 ,041 I 051 0 ,010 (nm)

F lgs 8, 9 and 10 Very t h l n oxlde growth layer on a s l l l c o n wafer. The t races are over sampllng lengths of 0.05 mn, 0.5 nn and 5 n e resPect lve ly .

7.5 1 1

521

l . ~ ~ -. 30 !~- ~ 1,962 2 .943 3.942 4 I 904 0 ,981 (mn)

F1g 10 (Sampling length 5 lirn)

1 5 0 ,081 ,163 ,244 ,325 ,401

(mnl

An lndlm Phosphlde wafer pollshed by the COnVentlOnal F1g 11 Alcohol/ Bromlne system,

15

7.5

(ml

0

7.5

15

-i -+--+--

-492 ,098 I 197 ,295 ,394

F1g 12 An lndlm Phosphlde wafer pollshed by the new I C I Wafer Technology process.

30

(rm)

0

I , - - - I 60 ___- 7 - 7 __- r-- - 0 ,010 ,020 ,031 ,041 ,051

(mn)

1 Flg 13 Short-range structure on a Japanese moulded convex plastlc lens

(the prlmary speclmen curvature has been computer subtracted) I

-- _- -r ___-- ,010 ,020 ,031 ,041

15

(nml

51

F1g 14 A Slllcon Carblde synchrotron mlrror ~ollshed by Ferrantl Astron.

1200

(nrn)

600

,145 ,210 ,290 (mn) ,073 563

Figs 15 and 16 Overall t~o-facet shape (Flg 15), and surface flnlsh (Flg 161 of one facet of a Japanese ferrlte dlsc readlng head I

30

15

(nm) 0

15

30 II ,177 ,354 ,531 ,700 I 885

(mn)

Fig 16 (showlng surface flnlsh, and the facet-to-facet transltlon at the rlght hand end of trace).

F1g 17 Tetraform 1, a novel ultrapreclslon machlne tool now under development at NPL. It Incorporates Nanosurf 2 preclslon sllde technology

522