Resist thickness

Throughput decreases as resist thickness increases. A typical relationship is... 

Each wafer has 100 chip sites with 0.25 cm2 active area. The daily production level is to be 2500 finished wafers. Find the resist thickness to be used to maximize the number of good chips per hour. Assume 0.5 < f < 2.5 as the expected range. First use cubic interpolation to find the optimal value of t, t. How many parallel production lines are required for t, assuming 20 h/day operation each How many iterations are needed to reach the optimum if you use quadratic interpolation ... 

Plot of objective function (number of good chips per hour) versus resist thickness, f( im). 

Sensitivity is defined as the exposure energy necessary for 50% resist thickness remaining in the exposed areas. Contrast values are assessed by measuring the slope of the linear portion of the curve obtained by plotting the thickness of the relief image as a function of the logarithm of the exposure energy (15). The film thickness was measured with a Nanospec AFT film thickness monitor (Nanometrics). 

When the concentration of the azide exceeds 30 wt%, sensitivity decreases (Figure 2) and the contrast becomes worse (Figure 3). This is due to the increase of the optical density of the resist. Large optical density prevents the light from penetrating into the resist (3,11). Also, the resist thickness remainig after development is maximum at the 30 wt% azide concentration (Figure 2). From these results, it was concluded that the azide-styrene resin resist which contains 30 wt%... 

Figure 8 shows the effect of the alkaline concentration in TMAH solutions on the contrast and sensitivity of the new resist. Sensitivity of the resist increases as the alkaline concentration increases, however, the contrast is maxima (4.72) at 0.83% TMAH solution. This means that the higher concentration over 0.83% cannot distinguish the difference of the dissolution rate between the unexposed and exposed resist film. For instance, the higher concentrated developer also attacks the exposed areas and the loss of resist thickness occurs. The alkaline concentration in TMAH solution, therefore, is optimized at 0.83%. This developer concentration was subjected to the following lithographic evaluation. 

Figure 12. Resistive property of sensitized PMMA decrease of resist thickness by plasma (Apparatus OAPA-300 RF 200 W vac 0.55 torr gas CF4 95% and Ot 5% flow rate 1.2 L/min)...

Tortuosity is a long-range property of a porous medium, which qualitatively describes the average pore conductivity of the solid. It is usual to define x by electrical conductivity measurements. With knowledge of the specific resistance of the electrolyte and from a measurement of the sample membrane resistance, thickness, area, and porosity, the membrane tortuosity can be calculated from eq 3. 

An additional complication associated with the standing wave effect occurs for the exposure of resist over topography. When a resist is spin-coated onto a substrate containing steps, the resist thickness varies from one area to another on the wafer. Since the standing wave effect is a strong function of resist thickness, exposure variations resulting from variation in resist thickness in the vicinity of the step result in changes in linewidth. 

By convolving the spreading Gaussian beam treatment with the experimentally determined relationship between resist thickness and input dose, Heidenreich was able to calculate line heights and profile widths at the substrate in good agreement with experiment. 

The output trace from the laser end-point detection system for a typical resist development cycle is depicted in Figure 9. The temporal distance between maxima can be related to a change in resist thickness through Equation 11... 

Figure 49. Scanning electron micrograph of images printed in IBM s experimental Mid-UV resist using a Perkin Elmer Micralign 500 in the UV-3 mode. The resist thickness is 1.1 fim the period of the grating is Sum.

Materials Resist composition Resist thickness Resist Tg /im C... 

With an MLR system, linewidth variations caused by resist thickness differences can be eliminated because of the planarization step. The reflections from either topographic structures or multiple film layers can be eliminated by using an absorbant material for the bottom layer. Either an inherently absorbant polymer material can be chosen or a dye can be added... 

Rothman (52) investigated the planarization of polyimide films over features tens of micrometers in size and separation. Bassous and Pepper (55) studied planarization of PMMA and AZ1350J over features pertinent to Si wafer processing. A mechanical stylus was used to determine the topography of the wafer and the corresponding surface variation of the resist thickness as shown in Figure 29 where a 1.7 - pm thick AZ1350J layer was spun on steps of different space and width combinations. 

A simple test to estimate the interfacial layer thickness is to measure the thickness of the bottom layer before and after spinning, exposure, and development of the top layer. The difference is taken to be the thickness of the interfacial layer for comparison purposes. In reality, the mixing is continuous and the development of the top layer stops inside the interfacial layer instead of at its edges precisely. Furthermore, the test in Reference 26 relies on the IBM Film Thickness Analyzer to measure the resist thickness for convenience. Since this tool operates on the principle of spectral reflectivity changes caused by film thickness changes, a uniform refractive index is important. When some part of the interfacial layer still remains, the measurement can be erroneous in principle. 

Figure 34. The circles show interfacial thickness as a function of resist concentration spun at 3500 rpm and prebaked at 55 C The solid dots show interfacial thickness also as a function of concentration but the spin speed is changed to maintain a constant resist thickness of 0.41 0.01 pm.

Resolution in a contact image is set by diffraction between the mask and the bottom of the resist. Thick resists, or gaps between mask and resist, degrade resolution. In practice, the minimum usable linewidth W(m) can be approximated from... 

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