5.5.1 Summary: General Aspects of Silicon Technology

Essentials of the bipolar transistor:
band diagram bipolar transistor
High emitter doping (NDon for npn transistor here) in comparison to base doping NAc for large current amplification factor g = IC/IB.
NDon/NAc » k = injection ratio.  
       
g   »   NDon
NAc		   ·   æ
ç
è  		 1  –  dbase
L		 ö
÷
ø
 
 
Small base width dbase (relative to diffusion length L) for large current amplification.  
Not as easy to make as the band-diagram suggests!  
     
Essentials of the MOS transistor:  
Basic MOS transistor

band diagram of inversion condition
Band diagram for inversion
Gate voltage enables Source-Drain current  
Essential process. Inversion of majority carrier type in channel below gate by:
  • Drive intrinsic majority carriers into bulk by gate voltage with same sign as majority carriers.
  • Reduced majority concentration nmaj below gate increases minority carrier concentration nmin via mass action law
  
   
nmaj · nmin  =  ni2
 
       
  • An inversion channel with nmin > nmaj develops below the gate as soon as threshold voltage UTh is reached.
  • Current now can flow because the reversely biased pn-junction between either source or drain and the region below the gate has disappeared.
 
         
The decisive material is the gate dielectric (usually SiO2). Basic requirement is:  
High capacity CG of the gate electrode - gate dielectric - Si capacitor = high charge QG on electrodes = strong band bending = low threshold voltages UG  
QG  =  CG · UG  
It follows:
  • Gate dielectric thickness dDi Þ High breakdown field strength UBd
  • Large dielectric constant er
  • No interface states.
  • Good adhesion, easy to make / deposit, easy to structure, small leakage currents, ...
  
Example:
U = 5 V, dDi = 5 nm  Þ  E = U/dDi = 107 V/cm !!

er(SiO2)  =  3.9
Integration means:
It ain't easy!
1. Produce a large number (up to 1.000.000.000) of transistors (bipolar or MOS) and other electronic elements on a cm2 of Si
2. Keep thoses elements electrically insulated from each other.  
3. Connect those elements in a meaningful way to produce a system / product.  
         
An integrated bipolar transistor does not resemble the textbook picture at all, but looks far more complicated Þ.  
Integrated bipolar transistor
This is due to the insulation requirements, the process requirements, and the need to interconnect as efficiently as possible.  
The epitaxial layer cuts down on the number of critical diffusions, makes insulation easier, and allows a "buried contact" structure.  
         
Connecting transistor / elements is complicated; it has to be done on several levels  
Multi layer metalilzation
Materials used are Al ("old"), Cu ("new"), W, (highly doped) poly-Si as well as various silicides.  
Essential properties are the conductivity s of the conductor, the dielectric constant er of the intermetal dielectric, and the resulting time constant t = s · er that defines the maximum signal transmision frequency through the conducting line.  
         
Integrating MOS transistors requires special measures for insulation (e.g. a field oxide) and for gate oxide production  
Integrated MOS transistors
Since a MOS transistor contains intrinsically a capacitor (the gate "stack"), the technology can be used to produce capacitors, too.  
         
CMOS allows to reduce power consumption dramatically.  
Inverter Integrated CMOS
The process, however, is more complex: Wells with different doping type need to be made.  
         
Using the third dimension (depth / height) might become necessary for integrating "large" structures into a small projected are (example: trench capacitor in DRAMs Þ).  
Planarization in DRAMs
Unwanted "topology", however, makes integration more difficult.  
Planarized technologies are a must since about 1995! Þ  
         
It ain't neither easy nor cheap!    
Property Number
Feature size 0,2 µm
No. metallization levels 4 - 7
No. components > 6 · 108 (Memory)
Complexity > 500 Process steps
Cost (development and 1 factory) ca. $ 6 · 109
Typical wafer size for new factories (2007) : 300 mm diameter, 775 µm thickness, flatness in lower µm region
Process integration circle
Chip size a few cm2, much smaller if possible
Yield Y = most important parameter in chip production = % of chips on a wafer that function (= can be sold).  
Y = 29 % is a good value for starting production  
Chip making = running about 20 times (roughly!!) through "materials" - "structuring" loop.  
About 400 - 600 individual processing steps (= in / out of special "machine") before chip is finished on wafer  
More than 30 processing steps for packaging (after separation of chips by cutting)  
Simple estimate: 99.9% perfection for each processing step meansY < 70 %.  
Dirt in any form - as "particles" on the surface of wafer, or as "contamination" inside the wafer is almost always deadly
Blood cells on chip Metal particle on chip
Particles with sized not much smaller than minimum feature sizes (i.e. < 10 nm in 2007) will invariably cover structures and lead to local dysfunction of a transistor or whatever.
Point defects like metal atoms in the Si lattice may precipitate and cause local short circuits etc. from the "inside", killing transistors  
One dysfunctional transistor out of 1.000.000.000 or so is enough to kill a chip!  
   
Being extremely clean is absolutely mandatory for high Yields Y!    
Use cleanrooms and hyper-clean materials!    
It won't be cheap!    
Moore's law predicts exponentially growth of "chip complexity" with a high growth rate - how far will it reach?
Moore's law break down?
Problems and costs are growing exponentially with every new generation.
It follows: The market must grow exponentially too, if you want to make a profit.  
It follows: Large amounts of money can be easily made - or lost.  
Falling behind thecompetition in your technology and yields means certain death for companies without a monopoly in some product.
Questionaire
Multiple Choice questions to all of 5
   
     
With frame Back Forward as PDF

© H. Föll (Electronic Materials - Script)