8/9/2019 ARXPS (1)

1/4

Thickness of Spin-Cast Polymer Thin Films Determined byAngle-Resolved XPS and AFM Tip-Scratch Methods

C. Ton-That, A. G. Shard, and R. H. Bradley*

M ateri als Su rf aces and I nterfaces Gr oup, School of Appl ied Sciences, The Robert Gordon

U n i ver si ty, S t. A n d r ew S tr e et, A b er d een A B 2 5 1 H G , U .K .

Received May 18, 1999. I n F in al F orm: N ovember 5, 1999

Polystyrene (PS) and poly(methyl methacrylate) (PMMA) thin films (

8/9/2019 ARXPS (1)

2/4

c an t i l eve r s ( le s s s t i f f ) we r e u s e d f or i m ag i n g t h e r e s ul t i n gs cr at c h es . S c r at c h es we r e f or m e d on t h e p ol y m er f i lm s b ys can n i n g t h e t i p r e p e at e d ly b a c k an d f or t h ove r t h e s am e l i n ewi t h h i g h l oad i n g f or c es . S e ver al d i ff er e n t s cr at c h es we r emeasured for each f i lm to allow sta tistical an alysis of data . Thenormal spring consta nts of the cant ilevers were calculat ed usingtheir measur ed unloaded resona nt frequencies.11,12 Individuallymeasured spring consta nts of each cant ilever were used in theestima tion of loading forces an d the calibra tion of the instru ment.

The calibrated va lues for th e short cant ilevers were in the ra ngefrom 0.46 to 0.76 N/m. A loading force of approxima tely 260 nN(inclusive of the capillary force due to adsorbed water layer) ,w h i ch w a s e va l u a t e d f r om t h e c a li br a t e d v a l u es of s pr i n gc on s t an t s , w as u s ed f or s cr at c h i n g of a l l t h e f il m s . Th e s a m eforce was used to scratch the PS and PMMA films because thet wo p ol y m er s h ave s i m il ar p h y s ic al p r ope r t i es an d s u r f acestif fness. Scratch tests on bare mica substra tes using the samecantilevers and force ha ve shown no dam age on mica surfa ces.It w as a lsofound by subsequent ima ging in tapping mode, whichha s been demonstra ted to be less intrusive,13 that contact modei m ag i n g i n t h e l ow l oad i n g f or c e r e gi m e (

8/9/2019 ARXPS (1)

3/4

of the photoelectron intensit ies f rom the f i lm and sub-stra te, respectively. Atomic sensitivity factors of 0.25 (C1s) and 0.23 (Si 2s) f rom the Kratos Vision sof tware,version 1.4.0, w hich ta ke into a ccount t he t ra nsmissionfunction of the Kra tos instrument ha ve been used in thework presented here. The use of al ternative ASFs e.g.from ref 14 result ed in differences in Kva lues of less tha n3%and consequently differences in film depth well withinexperimenta l error . The S i 2s peak w a s used instea d ofthe ma in peak Si 2p beca use the Si 2s photoelectrons ha ve

their bindin g energy closer t o tha t of C 1s photoelectrons.The attenuation length of C 1s and Si 2s photoelectronsis ef fectively the same because of their similar kineticenergies (if using t he E1/2 law, i .e., E1/2 w h e r eEis theelectron energy in eV,15 t h e d i ff er e n ce i n a t t e n u a t i onlengt hs of C 1s an d Si 2s photoelectrons is less th a n 5%).

Ignoring al l hydrogen atoms w hich a re not detecta bleby XPS , stoichiometric molar fra ction of carbon in PMMAand silicon in mica (KAl2S i4O12H) is 5/7 a n d 4/19, respec-tively. The K v a l u e f or C 1 s/S i 2 s i n t e n s it y r a t i o i scalculated by

Figure 2 shows a plot of eq 8 for th e PMMA film ca st from0.02%w /v solution using th e calculat ed K va lue above.The linear regression line of th e da ta ha s a slope of 0.47.The film t hickness of the P MMA film d) 0.47 ) 0.47 3.0 nm ) 1.41 nm (ta king the a t tenua tion length of C 1sa n d S i 2 s p h o t o e l e c t r o n s i n P M M A a s 3 . 0 n m 16). Thelinearit y of the gra ph is an indicat ion of uniformit y in thefilm thickness.I ma ging with AFM of the spun-cast P MMAand PS f i lms also reveals f lat a nd uniform surfaces withRMS roughness mea sured on 5m 5m area less tha n0.3 nm.

The sa me a lgori thm is applied to th e thin P S f i lms onm i ca s u b st r a t e w i t h a KC 1s /Si 2s va lue of 5.16 and ) 3.1nm for C 1s photoelectrons in P S. 16 Figure 3 shows a plotof eq 8 a nd t he line of linea r regression for th e PS film ca stfrom 0.02%w/v, w hich h a ve a slope of 0.43. The th icknes sof th e PS film is d) 0.43) 1.33 nm. Results of mea suredd/ values from the slope of the plots of eq 8 and f i lmthicknesses for t he P S an d P MMA fi lms spun-cast f romdifferent solution concentrations are shown in Table 1.

3.2. AFM Tip-Scratch Method. For this method togive an accurate measure of f i lm thickness, three maincri ter ia must be fulf i l led. First , only the f i lm must bescratched by the AFM t ip with no dam age being causedto the substra te. A force of 260 nN w as found t o scratch

t h e p o l y m e r f i l m b u t t o c a u s e n o d a m a g e t o t h e m i c asurfa ce (tha t could be detected by AFM in either t a ppingmode or in contact mode with a low loading force (

8/9/2019 ARXPS (1)

4/4

The XPS method can only be used for very thin films ( d