Summary Unit4

Summary Unit4 - Unit 4: Wafer Cleaning and Gettering...

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
Unit 4: Wafer Cleaning and Gettering Contaminants = device defect o At t=0 reduce yield o At t>0 reliability issues later on Sources? o Particles (dust): people, suspended matter in air (micron scale) o Photoresist residue o Alkali metal ions (group1): Na+, K+ etc, low work function Come from humans and air Change doping conc, modifies gate conc which could cause change in V th o Heavy metals: gold, chromium, iron (don’t have affinity to oxidize much) Form recombination centres, which distorts intrinsic conc, creating internal currents to restore equilibrium Three approaches: o Avoid them (clean rooms) o Remove them (clean wafer) o Isolate them (gettering) Problem 4.3: As MOS devices scaled to smaller dimensions, gate oxides must be reduced in thickness, as gate oxide thickness decreases do MOS devices become more or less sensitive to sodium conc? As the gate oxide thickness decreases, COX increases, so the same amount of mobile charge QM will have less effect on VTH as oxides get thinner. Therefore MOS devices are less sensitive to sodium contamination. Using the same expression for VTH as in part a), we observe that as the oxide thickness decreases, (COX increases), to maintain the same VTH, NA will have to increase. NA will actually have to increase by the square root of the oxide thickness decrease to keep VTH constant. Level 1 contamination reduction: clean factories o Very low dust conc in air, wear suits to avoid alkali metal contamination o Class 1000 > class 10000 o Class 10000: <=10000 particles per cubic feet o Moving from class 10 to 100: defect density x 10 o Moving from class 100000 to 10000: defect density /10 o Hepa: high efficiency particle (mechanical + electrostatic filtering)
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 4

Summary Unit4 - Unit 4: Wafer Cleaning and Gettering...

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online