5. How are e-beam resists exposed? How can the optimum exposure dose be determined?
By using very short-wavelength electrons for the exposure of the resists, an excellent resolution of up to 2 nm can be achieved (spot beam). Exposure is carried out by conventional e-beam equipment following the principle of direct writing or shaped beam procedure.
For mix&match processes of the AR 7000 resists, additional exposure with i-, g-line steppers or, respectively, contact exposure in the particular spectral UV range can be realized.
The data on radiation sensitivity given in our product information were defined in standard processes and are guide values. Each user has a different process, so that the optimum exposure dose must be found by own experiments. Silicon wafers and mask blanks alone differ to a great extent in their sensitivity (PMMA mask: 15 µC/cm² – PMMA wafer 80 µC/cm²). Likewise, the influence of the accelerating stress is big.
The exposure dose (dose to clear), which develops a big area without structures in an adequate development time (independent of coating thickness, 0,5 µm: 30 – 60 s) on a PMMA positive resist, should be increased by 10 – 20 % for the purpose of structuring. For negative resists, the full development time of the unexposed areas should be around 30 – 40 s for 0,5 µm. The exposure dose, which causes a layer composition of > 90 % in this process, should likewise be increased by 10 – 20 % for the structuring. If a shorter development time is chosen (stronger developer), this affects the sensitivity, because stronger crosslinking is needed (higher dose).