1. Chapter 7 Oxidation
半導體製程
材料科學與工程研究所
張翼 教授

2. Silicon Dioxide layer uses
Surface passivation
Doping barrier
Surface dielectric
Device dielectric: Field oxide,Gate oxide

3. Figure 7.1 Surface passivation with silicon dioxide layers.
Silicon dioxide is very hard and dense can be used as passivation layer:Preventing dirt,scratches,
chemical reactions.
Surface contaminants on the surface end up in the oxide during the oxide
growth, can oxide the surface and then remove the oxide to rid the surface of unwanted mobile ion contaminations

4. Figure 7.2 Silicon dioxide layer as dopant barrier.
Silicon dioxide layer can block the dopant form reaching the silicon surface.
(very low hole density)
Silicon dioxide thermal expansion coefficient is similar to silicon, wafer will
not warp during high temperature process

5. Figure 7.3 Dielectric use of silicon dioxide layer.
Silicon dioxide can be used as insulator between metal
and silicon. However, it must be think enough not to induce induction in the silicon surface.
Induction:oxide between metal is thin that electrical charge in the metal line induces charges on the metal surface.
Field oxide: oxide that is thick enough not to induce the charge on the wafer surface.

6. Figure 7.4 Silicon dioxide as field oxide and in MOS gate.
Field oxide: oxide that is thick enough
not to induce the charge on the wafer surface.

7. Figure 7.4 Gate oxide: oxide as dielectricNf
Gate oxide: oxide as dielectric
With thickness thin enough to
allow induction of a charge in
the gate region under the oxide
can be as thin as 35 to 80 Å range

8. Figure 7.5 Silicon dioxide layer in solid-state capacitor.
Can be used as an dielectric layer between
the silicon wafer and a surface conduction layer

9. Figure 7.6 Silicon dioxide thicknesses chart.

10. Figure 7.7 Reaction of silicon and oxygen to form silicon dioxide.

11. Figure 7.8-(1) Silicon dioxide growth stages: (a) initial

12. Figure 7.8-(2) Silicon dioxide growth stages: (b) linear
X=αt

13. Figure 7.8-(3) Silicon dioxide growth stages: (c) parabolic↓↓
→ Si + O2 = SiO2

14. Figure 7.9 Linear and parabolic growth of silicon dioxide.
Oxides less than 1000 Å are formed by linear mechanism(e.g.Gate oxide)
Thicker oxides (e.g. masking oxides) are formed by parabolic relationship

15. Figure 7.10 Parabolic relationship of SiO2 growth parameters.

16. Figure 7.11-(1) Silicon dioxide thickness versus time and temperature in (a) dry oxygen.
2000 Å at 1200oC
requires 6 min
4000 Å at 1200oC
requires 220 minutes

17. Figure 7.11-(2) Silicon dioxide thickness versus time and temperature in (b) steam(H2O).
Using H2O as oxidizing gas
oxides grow faster
since hydroxyl ion OH-
diffuses faster in the
oxide faster than oxygen
Higher oxide growth rate!

18. Figure 7.12 Reaction of silicon and wafer vapor to form silicon dioxide and hydrogen gas.
Wet Oxidation :oxides grow with H2O
Dry Oxidation : oxides grow with oxygen
After wet oxidation, H2 are trapped in the oxides,
making it less condense, after heating in inert atmosphere,
such as nitrogen, the two oxides become identical

19. Figure 7.13 Oxidation of <111> and <100> silicon in steam.
Larger number of
atoms on the surface
allows a faster growth
rate of oxide on silicon

20. Oxidation of Polysilicon
Polysilicon oxidation rate can be faster, slower or similar to single crystal silicon depending on the deposition method, deposition temperature, deposition pressure, the type and concentration of the doping, and the grain structure of the polysilicon.

21. Figure 7.14 Differential oxidation of silicon.
Polysilicon-
Depending on the surface
condition of the wafer,the
oxide thickness on the wafer
is different on different areas,
this is called differential oxidation

22. Figure 7.15 Oxidation methods.

23. Figure 7.16 Cross section of single horizontal tube furnace with three heating zones.(also called diffusion furnace)
Conduction and
radiation between
coil and tube
occurred
Ceramic liner -
as heat sink to
foster more
even heat
distribution
along the tube
-Quartz reaction
tube
Separate power
supply for different
heating zone
Thermocouple
Positioned along
the tube send back
the temperature
information to the
Controller to control
the temperature within +-0.5C

24. Figure 7.17 Tube furnace. Scavenger connected to the exhaust
system which contains a scrubber to
remove toxic gases
Constant nitrogen flow
during loading
/unloading
to keep dirt
out and prevent
oxidation
Tubes are
vertically on
top of each other
(usually 3-4 tubes)

25. Figure 7.18 Temperature levels during oxidation.
Full load of wafers can drop the tube temperature as much as 50oC
Need to fast recovery time for the heating system without introducing
wafer warping condition or over shoot

26. Wafer Warping
Large diameter wafers processed at higher temperatures (above 1150oC) tend to warp due to rapid heating and cooling causing wafers to be useless
Temperature ramping and slow loading are two methods to reduce wafer warping
Temperature ramping: wafers are inserted into the the furnace at lower temperature and after the temperature is stabilized, slowly increased to the process temperature. After process cycle, the furnace is cooled to lower temperature before wafers are removed
Slow loading:Slow loading of the wafers into the furnace at a rate of 1 in/min

27. Figure 7.19-(1) (a) Operating principle of mass flow meter
Gases need to delivered
at specific flow rate,
pressure and for a
specific time
When no gas is
flowing, both sensors
are at the
Same temperature
With gas flow,
difference between
two sensors is related
to the amount of heat mass
(not volume) that moved down stream
which can be related to a steady amount
of material flows through the meter

28. Figure 7.19-(2) (b) cutaway

29. Oxidant sources
Dry oxidation:compressed dry oxygen not contaminated with water vapor (water vapor will increase oxidation rate), a preferred method to grow very thin gate oxide (<1000Å) for MOS devices
Water vapor sources:Several choices depends on the required thickness and cleanliness

30. Figure 7.20 Bubbler water vapor source.
steam
Drawback:
Control of water vapor
flux is difficult due to
water level and water
temperature fluctuation
As carrier gases pass
through the vapor, it
becomes saturated with
water vapor
heat water to boiling temperature
98-99oC

31. Figure 7.21 “Dryox” (dry steam) water vapor source.
Wet oxidation in steam
Draw back:
Hydrogen is explosive
To prevent explosion:
1.separate O2
and H2 lines
2.flowing excess oxygen to form
Non-explosive
Water molecules
3.hot element to
burn off excess
H2 at source cabinet and scavenger
Two gases mix
and form steam
at high temperature
Improved control over thickness and cleanliness over liquid system
Due to 1. gases are very clean and dry; 2.flow amount can be precisely
controlled by MFC
Dryox is preferred oxidation method for production for all advanced devices

32. Chlorine added oxidation
Improves cleanliness and device performance
Chlorine reduces MIC, structural defects in oxide and wafer surface,and reduces charges at oxide silicon interface
Use Cl2, HCl,or TCE,TCA in dry oxygen gas stream
TCA is preferred due to safety and easy delivery

33. Figure 7.22 Wafer boat and cradle.

34. Figure 7.23-(1) Manual wafer handling devices. (a) Vacuum pickup

35. Figure 7. 23-(2) Manual wafer handling devices
Figure 7.23-(2) Manual wafer handling devices. (b) limited grasp tweezer

36. Figure 7.23-(3) Manual wafer handling devices. (c) flip transfer boats

37. Figure 7.23-(4) Manual wafer handling devices. (d) Auto pick and place

38. Figure 7.24 Transfer tube loading of wafers.
Time, Temperatures, gas sequences, and push-pull
Rates ( recipe) are programmed into a host computer

39. Figure 7.25 Vertical tube furnace.
Greater contamination control (no particle generation due to boat scraping the sides of the tube), particle density reaches 0.01/cm2 range
Larger wafer size (>200mm)
(horizontal tube has a limit)
Smaller foot print
(less expensive clean room)
Laminar flow(uniform with
non separation of the gases
into layers and without
turbulence ) is difficult to keep
in larger diameter wafer tube
Can rotate wafer to get
better uniformity (only
60% variation of the
horizontal furnace)

40. Rapid thermal processing (RTP)
Fast Ramp Furnace: fast ramp,low batch furnace, tens of
degree per minute heating rate (Conventional:few degree per minute), reduces ramp up and ramp down time, save cost
RTP: Radiation heating ,
Heat sources include:graphite heaters, microwave, plasma and;
tungsten halogen lamps
Tungsten halogen lamps are most popular
Very short heating, the body of wafer never get heated, Reduce dopant diffusion
Reduce thermal budget, saves energy
Minimize total heating/cooling time, reduces dislocations
Single wafer process for large wafer with uniform requirement
RTO (rapid thermal oxidation):grow thin oxide for MOS (<100Å)with smaller feature sizes

41. Figure 7.26 RTP design (Source: Semiconductor International, May 1993).
Thermal couple
contact from
the back (slow
response time)
and optical
Pyrometer are
used for
temperature
detection
Can add heated
annular ring
to keep the
edge of the
wafer in the
right
temperature
range

42. Figure 7. 27 Example RTP time/temperature curve
Figure 7.27 Example RTP time/temperature curve. (Source: Semiconductor International, May 1993).
Weakness: temperature
uniformity,wafer edge is
particularly bad
Different emissivity due to
different layers causing
nonuniformity in
temperature
during heating

43. Figure 7.28 Oxidation of silicon by RTO (Source: Ghandi, VLSI Fabrication Principles).

44. Problems with high temperature oxidation
High thermal budget
Growth of dislocations in the bulk of the wafer causing device performance problems
Growth of hydrogen induced dislocations along the edge of openings in layers on the surface (surface dislocation) causing electrical leakage along the surface and degradation of silicon layers grown on the wafer for bipolar circuits
Growth of dislocation is a function of the temperature and the time the wafer spends on the high temperature
These factors lead to high pressure, lower temperature oxidation

45. Figure 7.29 High-pressure oxidation.
Quartz tube encasing in
stainless steel and sealed
with oxidant at atm
1 atm increase in pressure
allows 30oC drop in
temperature, the system
allows 300 to 750oC drop
in temperature, this will minimize dislocation growth
Can use high temperature high pressure growth but at shorter time

46. High pressure oxidation
Good for very thin gate oxide growth because thin gate oxide requires structure integrity (no holes) and higher dielectric strength (high pressure grown oxide has better dielectric strength)
Good for solving bird’s peak problem during LOCOS process

47. Figure 7.30-(1) Bird’s beak growth: (a) no pre-etch

48. Figure 7.30-(2) Bird’s beak growth: (b) 1000 Å pre-etch

49. Figure 7.30-(3) Bird’s beak growth: (c) 2000 Å pre-etch (From Ghandhi, VLSI Fabrication Principles)

50. Figure 7.31 Oxidation process flow.
Surface cleaning is importance
for high temperature growth:
1.Contaminants may diffuse into
the wafer
2.Thin native oxide may alter
thickness and integrity of
grown oxides
Cleaning process:
Mechanical scrubber
RCA wet cleaning
HF-last process

51. Figure 7.32 Oxidation process cycles.
(need dry nitrogen)
(thin oxide) (thick oxide>1200Å)
Added Chlorine to improve oxide quality for thin gate oxide
(can be in one step or be preceded or followed by a dry oxidation cycle)

52. Figure 7.33 Anodic oxidation.
Oxygen created and forms
silicon dioxide on the wafer surface
Similar to
chemical plating
Can be used for
very thin oxide
formation
or KNO3
Oxide formed is less condense
Silicon diffused to the surface to
form oxide, when oxide was etched
away, it may leaves silicon with
different dopant concentration levels

53. Figure 7. 34 Nitridation of <100> silicon
Figure 7.34 Nitridation of <100> silicon. (Wolf, Silicon Process)
(rapid growth followed
by flat grown mechanism)
For <100Å, oxide
quality is poor and
hard to control,can
not be used for gate
Dielectric, silicon
Nitride can be used,
since it is denser
and good diffusion
barrier
Can also use silicon
oxygen nitride (SiOxNy)
or ONO oxide/nitrite/oxide
Silicon exposed to
ammonia between
950 to 1200oC