A Simple Photoacoustic Microscopy [PAM) Experiment Elizabeth H. Me1 a n d Edward M. Eyrlng University of Utah. Salt Lake City, UT 84112 Photoacoustic spectroscopy (PAS) is a method invented by Alexander Graham Bell ( 1 ) for determining the absorption spectra of gases, liquids, or solids. The modern, analytical chemical version of solid state PAS has been described most frequently by Rosencwaig (2). Typically, a monochromatic beam of lieht is choooed a t -50 Hz and directed through a glass w i d o w onto Sample surface in a sealed cell. f t h e surface absorbs the incident light, it is heated and in turn heats the boundary layer of gas in contact with the surface. This cyclic heating and cooling produces asound wave in the gas having a frequency equal to that of the incident light intensity modulation and an amplitude proportional to the ahsorhance of the samole surface. This sound wave is customarily detected by a s'imple microphone inside the cell, and the sienal is broueht uo out of the environmental noise bv a &k-in amplifier. ippli'cations have been remarkable for their diversitv touchine. for example. on such subiects as in situ detection of detection of surfiie changesof electrodes (i), tetracycline on human skin (4), investigations of fluorescence in solutions ( 5 ) , radiationless decay measurements in solids (6).and correlations of absorption edge energies of liquid cr&als with their transition tkmperatures (7): The feasibility of performing photoacoustic spectroscopy on a microscopk scale in a particularly simple manner has been demonstrated recently by Luukkala and Penttinen (8). The onlv new conceot in ohotoacoustic microsco~v .. (PAM) is quite simple: the incident electromagnetic radiation is focused to a verv small soot size. The s a m ~ l ethen can be rastered under t i e light beam in two dimekions. By recording the PA sienal amplitude a t each spot a c o m ~ l e t microscopic e image of ihe sample can be obtained. T h e PAM technique is ootentiallv of areat interest for the semiconductor industry, because P A Menables the manufacturer to monitor the oresence of electrical shorts or leaks in integrated circuits a t an early stage of device fabrication. T h e mathematical description of the photoacoustic effect in~ solids done hv and Gersho (RG -~ was ~initinllv ~ ~ ~ Rosencwaie Theory) (9). A one-&mensioial photoaco;stic cell (Fig. 1) is the model and a sinusoidallv modulated lieht source with wavelength X is assumed i d d e n t on the s&d sample. T h e temoerature in the s a m ~ l ae t omition x and time t, d x , t ) , can then be evaluated from the iollowing equations for thermal diffusion
BACKING MATERIAL
BOUNDARY LAYER
I/OF GAS
a
~
~~~
Figure 1. Cross-sectionalview ol a cylinnical phmoacavstlc Cell,showing h poslti~of the backing material and the solid sample on a onedimensional axis. With the fact that the temperature and fluxcontinuity condition at the sample surface,e.g. S8(0,t) = '.(O.t) and
respectively, the general solution of the thermal diffusion eauations can be obtained (2). Rosencwaie and Gersho usine t i e following definition ofthermal diffusion length for material j, pj = fi classified samples into two categories: (1)When p6 >> 1 wi:ie 1 is the sample thickness and pa is the ootical absorption lenah. the sample is 'Lopticallytransparent." (2) when ps
