on the substrate from the gas phase inside a
high temperature MOCVD reactor. The typical
range of gases used is as follows:
Atomic nitrogen source: ammonia ■
(NH3)
Dopants: gaseous molecules contain- ■
ing boron and phosphine
Etch gases: nitrogen trifluoride (NF ■
3);
sulphur hexafluoride (SF6 ); carbon tet-
rafluoride (CF4)
Process diluant: hydrogen (H ■ 2)
Pumping and vent gas: nitrogen (N ■ 2)
Other recipe ingredients: silane (SiH ■
4),
nitrous oxide (N2O), methane (CH4),
helium (He)
can poison the LED device, significantly
reducing its light output.
Each MOCVD process chamber consumes approximately ten tonnes per
annum of ultra-pure ammonia gas,
which is kept in excess in the chamber
atmosphere during deposition, while
hydrogen is consumed at approximately
half this rate. The MOCVD deposition
process is slow, each batch takes several
hours, hence sustained flows of ultra-high purity gas are required. With the
new generation of LED fabs planned to
include 50 to 100 or more MOCVD reactors, a cost-effective high volume/high
purity delivery of gases must be achieved
in order to support these fabs. Table 3
lenging properties of ammonia which must
be managed.
Moisture control: While specifications
typically require moisture levels of 50 parts
per billion or lower, ammonia’s hygroscopic nature means that keeping the gas
dry through filling of shipping containers,
delivery to the customer and connection to
piping for distribution to the process tools,
is a significant challenge. Overcoming this
requires careful design, installation and quality control. Generally, the highest semiconductor industry cleanliness standards are
applied including the use of electro-polished
stainless steel, high-purity leak-tight valves
etc. to minimize risk of moisture contamination and to speed up system dry-down. All of
these measures also drive up cost.
Other ESG
2%
Depreciation and
other
35%
N2
13%
Wafer
50%
H2
22%
NH3
63%
Metal Organics
10%
Gases
5%
Figure 5. Approximate break down of the cost structure of LED chip manufacturing, not
including the device packaging (ESG = electronic specialty gases)
An approximate break down of the cost
structure of LED chip manufacturing (not
including the device packaging) is shown
in Figure 5.
Ammonia (NH3) and hydrogen (H2) rep-
resent the largest portions of gas costs
and are also critical to the performance
of the LED device, since moisture and
oxygen contamination in trace quantities
shows gas delivery schemes that can
meet both flow and purity specifications
that are available to customers today,
and their applicability to LED fab scale.
Managing Ammonia
In order to provide the quality and quantity of gas to enable cost-effective high-volume LED fabrication, there are two chal-
Vaporization
Ammonia is a low vapor pressure gas,
with a high latent heat of vaporization.
Shipped as a liquid, a significant amount
of energy is required to deliver a suitable
pressure (typically >100 psig) and flowrate
(100s to >5000 slm) of vapor to the process.
Since moisture tends to concentrate in the
liquid phase, high vaporization rates can
also drive higher moisture contamination
into the gas phase in aerosol droplets. To
avoid carryover of moisture to the process,
a significant amount of liquid NH3 (typically
at least 10%) is left in the container to be
returned and reprocessed or disposed of,
further increasing cost.
The combination of these interacting
problems creates practical limits on supply
via conventional gas packages. Using container heating, supply rates today are limited
to around 1500 slpm per system, hence new
supply modes must be deployed in order
to meet the demands of new fabs in a cost
efficient manner.
MOCVDs
< 25
25
50
100
200
Ammonia
Heated Drums
or ISO tank
ISO tank supply to vaporizer
On Site Purification
Nitrogen
Liquid Nitrogen
On Site Generator
Hydrogen
Compressed Gas
Trailer
Electrolyzer
On Site Generator
Table 3. Table 3 shows gas delivery schemes that can meet both flow and purity specifications that are available to customers today, and their applicability to LED fab scale.
Managing Hydrogen
As with photovoltaics, delivering high
purity hydrogen (99.999%) cost-effectively
to large fabs is impacted primarily by the
regional variations in the source and specification of the hydrogen supply. In Europe,
liquid hydrogen is commonly available and
provides an economic supply mode for
many high flow demands. In China, compressed hydrogen by tube trailers is available
but costs are driven by the distance to the
