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Top-down view for F = xz + yz'

Short answer

F = xz + yz’ is built as F = NOT(G) where G = NOT(xz+yz’); pull-down: (x series z) || (y series z’), pull-up: (x||z) series (y||z’), 12 transistors. Positive photoresist dissolves where UV-exposed, so the developed pattern matches the mask clear areas; it provides high resolution.

Draw a top down view and schematic for the following function: F = xz + yz'. Describe with suitable diagram about positive photoresist used in photolithography.

Schematic and top-down view of F = xz + yz'

Static CMOS gates are inverting, so F is made as F = NOT(G), where G = NOT(xz + yz'). One inverter produces z' and another produces F from G.

 Pull-up (PMOS)              Pull-down (NMOS)
 VDD                                G
  |                            +----+----+
 (x || z)                      |         |
  |                            x         y
 (y || z')                     |         |
  |                            z         z'
  G                            |         |
                               +----+----+
                                    |
                                   GND
 z --[INV]-- z'
 G --[INV]-- F

Pull-down: (x in series with z) in parallel with (y in series with z'). Pull-up (the dual): (x parallel z) in series with (y parallel z'). In all, 12 transistors (inverter for z' = 2, gate = 8, output inverter = 2).

Simplified top-down (stick) view of the gate part:

 VDD  =============================== metal
        |   |     |   |
      +-+---+-----+---+-+  p+ diffusion
      | x   z     y   z' |  poly gates run
      |  |   |     |   | |  vertically
      +-+---+-----+---+-+  across both rows
        |_______ G _______   metal output
      +-+---+-----+---+-+  n+ diffusion
        |   |     |   |
 GND  =============================== metal

Legend: the vertical poly lines x, z, y, z' cross the p+ row (top) and the n+ row (bottom); each crossing is a transistor. Metal joins the drain areas to the output G.

Positive photoresist

A positive photoresist is a light-sensitive polymer that becomes soluble in the developer where it has been exposed to UV light. The unexposed part remains and protects the layer below.

 Exposure (UV through mask)      After development
   UV  UV      UV  UV             _____       _____
 [####]     [####]  mask         |resist|     |resist|
 ===resist===========            ===SiO2============
 ======SiO2==========            gap = window opened
 ======Si wafer=======
  • The pattern on the wafer is the same as the mask (clear areas of the mask become openings).
  • Advantages: better resolution, easier to remove, widely used for fine features.
  • Example: Novolac resin with diazonaphthoquinone (DNQ) sensitiser.

✓ Answer checked by Claude Opus 5.5 (high effort)

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