EECS143,Sp 99 N. Cheung
Midterm Exam #1 Solutions
Problem 1
(a) The observed indicates the growth rate is slower than the Deal-Grove model after 2000 Å of oxide is
grown. Even if we take the limit that growth rate is proportional (time)1/2 , the oxide will be thicker
than 4000 Å after 4 hours of oxidation. The Si surface layer must have a faster oxide rate than the bulk
of the wafer.
Conjecture 1 : The processed Si wafer was oxidized first to an oxide thickness of 100 Å and then have the
oxide dissolved in HF. FALSE
Dopant segregation can enhance oxidation rate but the small oxide growth cannot change the surface layer
dopant concentration due to dopant segregation.
Conjecture 2 : The processed Si wafer has a highly doped surface layer ( doping > 1019/cm3) which is less
than 1000 Å thick. TRUE
The Si wafer has a highly doped surface layer ( N > 1019/cm3) which is less than 1000 Å thick ( i.e., 0.46 ×
2000 Å ≈1000 Å). The underneath substrate is lightly doped. The Si wafer has a highly doped region
underneath a lightly doped surface region. Oxidation rate is higher when the doping concentration is higher
than 1019/cm3, mainly through the linear term B/A. After this layer of highly doped Si is consumed, the
growth rate slows down.
Conjecture 3 : The processed Si wafer has a thin layer of poly-Si layer on top surface. TRUE
Initial oxidation of poly-Si is very fast. After poly-Si is all consumed, the oxidation rates slows down.
(b) SiO2volume
Si volume = 5×1022
2.3×1022=2.17
Volume of sphere ∝ radius3 or radius of SiO2
radius of Si = (2.17)1/3 = 1.29
Therefore , radius of SiO2 sphere =1.29 µm.
(c) LOCOS Advantages :
(i) Self-aligned channel stop ; (ii) oxide topography more planar than opening an oxide window.
LOCOS Disadvantages: The "bird's beak" region wastes device area.
Problem 2 (a)
Parameter Electrical Channel Length L
Implant Dose ↑↓
Substrate conc. NB ↑↓
Sidewall Angle θ ↑↑
Gate material changed from poly-Si to Tungsten ↑
Implant ions changed from Phosphorus to Arsenic
(same energy) ↑
(b) Restore crystallinity of damage Si caused by ion bombradment
Position implanted dopants into substituitional sites of Si lattice so that the dopants gives "shallow"
dnor or acceptor energy levels [ i.e. dopant activation]
