Homebrew PCBs

Here is a article on printing photo resist. Why would they deposit 
photo resist instead of a direct resist. 

R��sum�� / Abstract
This article presents a technique for resist deposition using a 
novel fluid ejection method. An ejector has been developed to 
deposit photoresist no silicon wafers without spinning. Drop-on-
demand coating of the wafer reduces waste and the cost of coating 
wafers. The novel piezoelectric fluid ejector is based on a 
variation of the design of a flextensional transducer that excites 
axisymmetric resonant modes in a damped circular membrane. The 
ejector is made by bonding a thin piezoelectric ring to a thin, 
fully supported, circular membrane. The ejector design is optimized 
for maximum flexure at the lowest order resonant frequency using 
finite element modeling. The resist, is placed behind one face of 
the membrane which has small orifice (50-150 ��m diameter) in its 
center. By applying an ac signal across the piezoelectric element, 
continuous or drop-on-demand ejection of the resist is achieved. 
Shipley 1400-21, 1400-27, 1805, and 1813 resists were used to coat 
sample 3 wafers. Later, these wafers were exposed and developed. The 
deposited resist film was 3.5 ��m thick and had a surface roughness 
of about 0.2 ��m. The ultimate goal is to deposit resist films with a 
thickness of the order of 0.5 ��m, and a surface roughness of the 
order of 30 A��. Such goals can be attained by using micromachined 
multiple ejectors presently under development, or with better 
control liver the deposition environment. In the micromachined 
configuration, thousands of ejectors will be made into a silicon die 
and thus allow for full coating of a wafer in a few seconds. Coating 
in a clean environment will allow the lithography of circuits for 
microelectronic applications