Thin film stress measurement

There are a number of subtle effects that have to be considered when making thin film stress measurements on silicon wafers First of all, the crystal orientation of the wafer Influences the resulting stress. The same thermal CVD silicon dioxide film thickness on the same substrate indicates larger tensile stresses on (100)-oriented wafers as compared with (111 (-oriented wafers. 

UHV-compatible thin film stress measuring apparatus based on the cantilever beam principal. Review of Scientific Instruments 61, 3859-3862. 

Mixed-frequency deposition of the nitride is one possibility to adjust the stress in the deposited layer [122]. The ratio of the deposition times in the high-frequency (375 kHz) and low-frequency (187.5 kHz) plasma can be varied during the process. For the layer used here 95% high-frequency deposition time was chosen (Fig. 4.14e).The stress was measured on wafer-level with a thin-film stress analyzer. The stress value was determined by recording the curvature of the wafer after thin-film deposition. A tensile stress of 75 5 MPa was measured for the layer. 

The expression (3.63) can be used for the determination of thin film stress without a priori knowledge of either stress-free d—spacing of the set of crystallographic planes being interrogated or the elastic constants of the film material. The procedure exploits the linear relationship between d and sin that was identified in (3.63), and it requires that measurements... 

Physical Properties. Raman spectroscopy is an excellent tool for investigating stress and strain in many different materials (see Materlals reliability). Lattice strain distribution measurements in siUcon are a classic case. More recent examples of this include the characterization of thin films (56), and measurements of stress and relaxation in silicon—germanium layers (57). 

X-ray diffraction consists of the measurement of the coherent scattering of x-rays (phenomenon 4 above). X-ray diffraction is used to determine the identity of crystalline phases in a multiphase powder sample and the atomic and molecular stmctures of single crystals. It can also be used to determine stmctural details of polymers, fibers, thin films, and amorphous soflds and to study stress, texture, and particle size. 

Stress in crystalline solids produces small shifts, typically a few wavenumbers, in the Raman lines that sometimes are accompanied by a small amount of line broadening. Measurement of a series of Raman spectra in high-pressure equipment under static or uniaxial pressure allows the line shifts to be calibrated in terms of stress level. This information can be used to characterize built-in stress in thin films, along grain boundaries, and in thermally stressed materials. Microfocus spectra can be obtained from crack tips in ceramic material and by a careful spatial mapping along and across the crack estimates can be obtained of the stress fields around the crack. ... 

Marshall, D.B. and Evans, A.G., Measurement of adherence of residually stressed thin-films by indentation. I. Mechanics of interface delamination. J. Appl. Phys., 56(10), 2632-2638 (1984). 

For times less than the transit time of the wave, the current is proportional to the stress at the input electrode in a linear approximation. For times greater than the wave transit time, the current is proportional to the stress difference between the electrodes. Thus, the thin-film nature of PVDF provides a means to measure stress differences, and, given mechanical tolerances that limit loading times to a few nanoseconds, measurements are difficult to... 

Remarkably some polymers are ferroelectric. Polyvinylene fluoride, as a thin film (approximately 25 /mi thick) is used in Shockwave experiments to measure stress. The pressure range over which this polymer operates is an order of magnitude larger than that of quartz or lithium niobate. 

It should be noted that approximately 1% of the APS used in the last experiment appears to be APS monomers before dilution. Could this low monomer concentration be responsible for adhesion It should be noted that the residual monomers present in this highly-oligomerized solution correspond to a 0.001 vol % solution if one were to remove the oligomerized APS. As a result of this observation, the dependence of the adhesion of thin films to native-oxide silicon wafers as a function of the concentration of the APS under conditions of T H stress was investigated. Adhesion studies were performed using APS solutions with concentrations that varied from the industry standard of 0.1 vol % down to 0.00001 vol %. The test wafers were prepared and exposed to T(200) and T(500) conditions as discussed above. Adhesion was measured by 90° peel test, as discussed above. The results of this study are presented in Fig. 13. The Lx-axis is APS concentration which decreases from left to right. The y-axis is the adhesion in the units of g mm"The three curves are the results at T(0), T(200) andT(500). 

In the first of these techniques an approximation to uniform rate of shear throughout the sample is achieved by shearing a thin film of the liquid between concentric cylinders. The outer cylinder can be rotated (or oscillated) at a constant rate and the shear stress measured in terms of the deflection of the inner cylinder, which is suspended by a torsion wire (Figure 9.2) or the inner cylinder can be rotated (or oscillated) with the outer cylinder stationary and the resistance offered to the motor measured. 

When a thin film of one material is deposited on a stress-free substrate of another material, one may find the thin film to be under tensile or compressive stress. The nature of this stress is typically evaluated by depositing the thin film on a silicon wafer and measuring its deflection. As illustrated in Figure 5, a tensile stress will deflect the substrate upward, and a compressive stress will deflect it downward. 

One commercial wafer deflection gauge is available, and is sketched in Figure 6. The degree of light reflection is used to indicate the amount of wafer deflection. The only difficulty with this technique occurs when relatively low stress films are measured. For normal films (i.e., thermal CVD silicon dioxide) and a stress of 109 dynes/cm2, a typical 100-mm silicon wafer (0.62-mm thick) with a 1-jum thick film will deflect 10 jum at its center. The Ionic Systems gauge claims a 0.03-pm sensitivity, so the typical stress can be measured readily. For smaller stresses 108 dynes/cm2, it may be useful to use a thinned wafer to make deflection measurements. 

There is also evidence that film stress depends on film thickness, although there are conflicting reports. Therefore, it is prudent to measure stress using films equal in thickness to those eventually to be used. For very thin films and small stresses, quite small deflections may have to be measured. 

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