LA INMUNIDAD O EL ARREPENTIMIENTO

Ground

RF POWER

GENERATOR

PRESSURE

MONITOR

C

_Z2M

TO PUMP

Figure (2). Schematic diagram of the plasma reactor used for preparing the surfaces. Rf power is applied to two copper bands around the outside of the chamber. The monomer gas is introduced into the chamber from the top.

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w hich is th e n evacuated. The gas is th e n in tro d u ce d an d the power tu rn e d on for a predeterm ined length of tim e.

M ica su rfa ce s p retrea ted w ith w ate r vapor p la sm a were silan ated by exposure to silane vapor in a preequilibrated vessel in sid e a d esicc ato r. (T ridecafluoro-1 ,1 ,2 ,2 -T e tra h y d ro o c ty l)-l- D im ethylchlorosilane (henceforth th is m olecule will be referred to as fluorocarbon silane) w as u se d for te stin g th e effect of the p la s m a p ro cess p a ra m e te rs. ESCA a n a ly sis of th e m odified su rfa ce s were carried o u t w ith a Leybold H eraeu s sp ectro m eter u sin g an A1 Ka X-ray source and a hem ispherical analyzer and the relative n u m b e r of th e different ato m s w as q u an tified by the p ro cedure described in ref. 6.

T he th ic k n e s s ch an g e in th e m ic a a fte r p la s m a a n d fluorocarbon silane tre a tm e n t w as d eterm in ed a s follows: Mica w hich h a d b een glued to a silica disc (silver side down) w as m a sk e d by p lacin g a n o th e r th in s h e e t of m ica over h a lf th e su rfa c e . T he disc w as th e n tre a te d a n d th e m a s k rem oved leaving a disc w ith one half treated an d th e o th er h alf u n trea ted . A nother silvered piece of mica w as th e placed over th e top of the disc (silver side up) an d the difference in th ic k n e ss betw een th e t r e a t e d a n d u n t r e a t e d p a r t s w a s d e t e r m i n e d in te rfero m e trica lly .

7 7 5.3 Surface Characterization.

Mica w hich h a s been exposed to w ater vapor p la sm a is. in c o n tra s t to n o rm al m ica, reactive tow ard sila n a tio n in th e gas p h a s e . T he ch an g e in reactiv ity is d u e to a c h a n g e in th e chem istry of th e surface. The modified surface w as characterized by co n tact angle, ESCA an d surface force m easu rem en ts.

T he th ic k n e ss of th e m ica w as found to be e sse n tia lly u n a lte re d after tre a tm e n t w ith H2O P lasm a (20W, 2.5 m in u te s e x p o s u re . 1.5 SCCM) w h e n in s p e c te d in te rfe ro m e tric a lly . E x p o su re of th e tre a te d m ica to th e fluorocarbon sila n e for th re e m in u te s re n d e r s th e s u r fa c e h y d ro p h o b ic w ith a n advancing co n tact angle of 93-96° a n d a receding angle of 70- 75°. The silane layer w as 1.0 (±0.5nm) nm th ic k as estim ated by in te rfero m e try . The large u n c e rta in ty is d u e to difficulty in focusing th e optical system .

T h e ESCA s p e c tr a for m ic a , a c tiv a te d m ic a a n d fluorocarbon silanated m ica are show n in Fig. 3. It is clear from th e s p e c tra th a t th e signal from p o ta ssiu m , alu m in iu m an d oxygen are red u ced after activation w h erea s th e silicon signal rem a in s n early th e sam e. The d ecrease in photoelectron signals is a r e s u lt of chem ical c h a n g e s in th e o u te rm o s t layer. The decrease in th e p o ta ssiu m an d alu m in iu m signals co rresp o n d s to a lo ss of 1.7x10*4 a n d 3 .7 x l 0 * 4 a to m s p e r cm *2, respectively. F or co m p ariso n , th e n u m b e r of p o ta s s iu m a n d alu m in iu m atom s in the surface layer of m ica is 2. lx 1 0*4 . The ESCA s p e c tru m a fte r tr e a tm e n t of a c tiv a te d m ica w ith th e fluorocarbon silane show s a large fluorine peak. The p ea k a re a

0 ( 1 s) _{Bare Mica} Si(2p) Al(2p) H2O Plasma Treated After Silanation 0 Kinetic Energy eV 1500

Figure (3). ESCA spectra recorded for untreated mica (top), H2 O Plasma treated (20W, 2.5 Minutes) mica before (middle) and after (lower) silanation with the fluorocarbon silane.

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indicates th a t 2.7 xlO 15 m olecules are b o u n d per cm2 and this gives a surface density of one silane molecule p er 45 Ä2.

The change in ESCA peak in te n sity for alum inium , oxygen a n d p o ta s s iu m (norm alized by th e se n sitiv ity fa c to r of th e p h o to electro n signal) as a fu n ctio n of th e w ate r v apor p la sm a exposure tim e is illu strate d in fig. 4. The v alu es were m easu red relative to th e silicon peak w hich rem ained n early co n stan t. It is clear th a t th e am o u n t of potassium , oxygen an d alum inium in the surface region d ecreases w ith th e th e p la sm a tre a tm e n t time.

T he e x a c t m e c h a n is m b y w h ic h th e m ic a (for th e s tru c tu re of m ica see p a rt I ch a p ter 2) surface is modified by the p la sm a tre a tm e n t is unclear. However, th e a m o u n t of potassium , alu m in iu m an d oxygen in th e surface region decreases relative to t h a t of silico n d u rin g tre a tm e n t. T he d e c re a se in su rfa c e alu m in iu m is so large th a t n o t only ato m s from th e te tra h e d ra l layer b u t also atom s from th e o ctah ed ral layer 0.5 nm below the original su rfa ce m u s t have been s p u tte re d away. R oughly 2.7 alu m in iu m atom s are lost for every p o ta ssiu m leaving a surface layer consisting largely of oxygen an d silicon. After only 0.5 min tre a tm e n t tim e th e n u m b e r of a lu m in iu m ato m s lo st is eq u iv alen t to th e n u m b e r alu m in iu m ato m s in th e te tra h e d ra l layer. Twice th e a m o u n t of oxygen p er alu m in iu m atom is lost at sh o rte r tre a tm e n t tim es and after 2.5 m in u te s the a m o u n ts lost becom e equivalent. Rem oval of a lu m in iu m m u s t lead to the rem oval of som e oxygen (two hydroxyl g roups are b o u n d to two a lu m in iu m s for every potassium ). A d ecrease in th e ratio of oxygen to alu m in iu m lost indicates th e in corporation of oxygen or hydroxyl gro u p s from th e p la sm a. The fact th a t th e trea ted

Time (minutes))