Resistance thickness effect

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. 

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. 

With any resist, the ideal conditions for obtaining high resolution and good linewidth control are a flat surface and a thin resist (<400 nm). The flat surface means that the resist will have very little variation in thickness and, as a result, there will be little variation in resist linewidth. However linewidth variations stemming from resist thickness variations do occur when lines traverse a step as a result of exposure and development effects. Indeed, such... 

Fig. 7 Effect of the (average) current density on the deposit thickness distribution subject to the resistive substrate effect. Conductivity = 0.55 Q cm-1 (1.8 M Sulfuric Acid). 200 mm wafer. 1000 A copper seed. Time-step growth simulations...

Accordingly, the effective resist thickness for the case of absorption only causing a variation in intensity with depth in the resist [i.e., the case where I(z) decays exponentially] can he calculated from Eq. (12.128) as... 

For a 125 mm wafer, the air flow at the very edge is no longer laminar once the rotation rate exceeds about 3700 rpm. At 6000 rpm, the transition occurs at a radial position of 49 mm the outer 14 mm is in the transition or turbulent flow regime on a 125 mm wafer at this speed. In the turbulent region, the evaporation rate depends on radial position and varies with the spin speed to a power other than one-haIf. The result is expected to be a non-uniform resist thickness. While this effect is probably not important for the wafers in use today, it may become important in the future. The critical speed for a 200 mm wafer is only 1430 rpm. Above this value, at least part of the wafer will be in the turbulent air flow regime. 

Apart from resistance, capacitive effects should also be taken into consideration. The best design of a Luggin probe is one with a narrow capillary at its tip with thin walls to prevent shielding, but with thick walls in the main body which widen rapidly away from the tip to reduce resistance in the control loop [21]. 

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