偏振调制红外反射吸收光谱法（PM-IRRAS）测量薄膜

本文介绍了偏振调制红外反射吸收光谱（PM-IRRAS）的概况、系统以及薄膜测量的样品数据。利用薄膜的先进功能，如电、光学和/或机械性能的几种工业产品现在变得越来越流行，薄膜的制造和评估技术也得到了稳步改进。众所周知，红外光谱是对这些类型的薄膜最有用的评估方法之一，它提供了关于分子结构和取向以及光学厚度和电学性质的信息。红外反射吸收光谱（IRRAS）可用于厚度在几十埃范围内的金属衬底上的非常薄的薄膜的分子结构分析。然而，由于一些需要更高性能和更高功能的新型器件的开发，人们越来越需要更高灵敏度的测量和薄膜生成的过程监控。为了满足这一要求，JASCO开发了一种使用偏振调制红外反射吸收光谱（PM-IRRAS）的真空室内薄膜生成监测系统，其灵敏度高于普通IRRAS。Application Note260-PR-0231 Measurement of Thin Film by Polarization-ModulationInfrared Reflection-Absorption Spectroscopy (PM-IRRAS)2/3Application Note Measurement of Thin Film by Polarization-Modulation Infrared Reflection-Absorption Spectroscopy (PM-IRRAS) Introductio n This report exp l a i ns the outline of the Pola r iza t i on- Modu l at i o n In f rared Reflect i on-Absorption Spectroscopy (PM-I RRAS)，system as well as sample data from measurement of thin films. Several i n dust r ial products utilizing the advanced functional i ties of th i n f il ms such as electric， optical and/or mecha n ical proper t ies are now becoming more popular and tech ni ques f or man u facture and evaluation of f il m have steadily improved . Infra r ed spectroscopy i s well k nown as one of the most useful evaluat i on met h ods for these types of thin f ilm providing information on molecular st r uct u re and orientat i on along with optical t h ickness and elect r i c properties. I nfrared Reflec t ion-Absorptio n Spect r oscopy (IRRAS) i s usef u l for molec u la r st r uct u re a n alysis of a very th in fi l ms on metal substrates with a t h ickness in the region o f tens of angst r oms. However ， th ere i s a growing n eed for h igher se n sit i vity measurement and also p rocess m o n itoring of f il m gene ra tion because of some the development of newer devices t h at require highe r p e r formance a nd h i gher func t ionalit y .To meet t h is r eq u i r eme n t ， JASCO h as deve l oped a moni t ori n g system for f ilm generation in v a c u um chamber using Polar iz ation-Modu l at i on I n frared Ref l ec t ion -Absor p t i o n Spect r oscopy (PM-IRRAS) wi t h greater sensitivity t han o r dinary IRRAS. Princ ip le of P M-I R RAS IRRAS provid e s IR spectra measureme n t of thin f ilms o n metal su b strates with high s e nsit i vity usi n g p-p olarized l ight p a r al l el to t h e i ncid e nc e plane . Th e p -p olar i zed l ig h t genera t es an e lec t ri c f i e ld wi th sta ti onary vi b ration which i ncreases s e nsitivity (S e e Fig.1)； t h is allows f ilm t h ick n ess m e asur e m e nt at th e angs t rom l e ve l . Howev e r， a b sorpt i on peaks obta i ned by I RRAS ar e usually v e ry small， and may of t e n re quir e a l ong t ime accum ul at i o n t ime . In addit i o n ， both a r e fer e nce a n d a sample subs t rate must be m easured . For t hese r easons the spectra can be sig ni f i ca n tly affected by a b sor p tion of H，0and CO， in atmosphere. Figure 1. Diag r am of IRRAS method On the other h and， PM-IRRAS is a met h od of f i ndi n g intensity difference of s - and p-p olar i zed l i ght (Al = Ip -I s) vert i cal and paral l el to the i n cidence plane respect i vely usi n g a Photoelastic Modu l a t or (PEM). As s-polarized l i ght does not gene r ate an electric f i eld of stationary vibration， t h e absorpt i on i s m uch smal l er than that of p-polar i zed l i ght . In addition ， in PM-IRRAS， t he s u m of s- and p- polar i zation signal (Z 1 = lp + I s) i s used as r eference and so t h ere is no need to measure a reference substrate. Hence， the effec t due to the absorption of atmosp h eric H，O and CO，； can be greatly decreased.As the m easurement of r eference substrate i s not required， the results are free from t h e effect of differences between substrates and t h e overall measu r ement t i me can be s h ortened. PM-IRRAS measu r ement al l ows h i ghe r sensitivity by detecting small Al signal using direct l oc k -i n detection by adopting dual modu l at i on spectroscopy o f the FTIR interferometer and PEM. Fig. 2 shows the results of a PMAA thin film on a n Al mirror measured by IRRAS and PM-I RRAS with all othe r condi t ions remaining the same. The S /N of spec tr a obtained using PM-IRRAS are several t i mes better than IRRAS Figure 2. Co m par i son b etwee n I R RAS s p ect r um and PM-I RRAS spectr u m (n o r m a lized， offset p l otting) Ins t rument Fig. 3 PM-IRRAS measuremen t system and system layo u t of the polarizatio n modulation unit . While t h e optimal i ncidence a n gle i s generally conside r ed to be in t he range 80 ~ 89°depe n ding on the kinds of metal a n d wave n umber of inc i dence light， th e i n cide n ce angle of the polari z ed light i n this syste m i s 85°. I n add iti on， the en ti re system c a n b e subject to a vacuum. In p r i n ciple PM-IRRAS ca n reduce t h e n oise contribu ti on fr om H20 a nd CO2 but vacuum opt i cs can en h ance the sensitivity st il l fa rt h er. As standard t h e syste m ca n fl ow gas on t o the sample or heated sam p le， enabli n g it to be used for monitor i ng a f i lm du r ing t h e generat i on p rocess， o r e va lu ati n g st r uct u ral changes. Figure 3. A p pea r ance of F T /I R -6300FV & Po l ar i za t ion m o dula t io n u n it (u pper)S ystem layout o f p ola r i z ation modulation un i t (lowe r) JASCO INC. 28600 Mary's Cour t ， E a s to n ， M D 21601US A M easurement E xampl e 1. PMMA thin f ilm on an Al mirror. The resu l t of PMMA measurement of a t h in f ilm on an A l mirror is shown i n Fig. 4. The spectr u m h as good S/N withou t the i n f lue n ce of H，O. Figure 4. M easu r ed spec t r u m of PMMA thin f i lm 2. PMMA thin f i lm on Al mirrorMeasuremen t of a nat i ve oxidation fil m on an Al mirror i s shown i n Fig.5. The thickness of the n ative ox i da t ion fi l m obtained using ellipsometry (JASCO： M-220) is 46.9 A and i t is confirmed that the measurement of thickness a t the A level was made in approx. 1 minute using PM-IRRAS. JASCO INC. 28600 Mary's Cour t ， E a s to n ， M D 21601US A