The Inductively Coupled Plasma (ICP) Tool
based Material Etching
Rate Etching of Waveguide and Mirror
InP Grating Etching or
Crystal (PhC) Etching
Red LED and Solar Cell Etching
InP MicroLens Etching
Instruments Plasma Technology provides a range of high performance, flexible
tools to semiconductor processing customers involved in research and
development, and production. We specialise in three main areas:
Dry etching is now widely used in the fabrication of optoelectronic and
electronic devices involving III-V materials, due to the need for careful
control of the critical dimensions of components. Fast etch rates,
repeatability, uniformity, clean chemistries, vertical profile, low device
damage are some of the most desirable aspects of the etching process.
Inductively coupled plasma (ICP) etching is ideally suited to these
requirements, since it provides a high ion density; hence fast etch rates, while
allowing separate control of ion density and ion energy, giving a low damage
Instruments Plasma Technology (OIPT) has developed a wide range of ICP etch
processes for III-V semiconductors to meet these demands.
In this article, we will focus on etching process for InP and related
materials, discuss various etching chemistries and system requirements for
different applications and provide an update of the latest new process
The Inductively Coupled Plasma (ICP) Tool
The system used for these processes is the Oxford
Instruments Plasma Technology Plasmalab System 100 ICP etcher (OIPT CS1
hardware). A schematic of the etch chamber is given in Figure 1 and the full
system is shown in Figure 2.
Figure 1. Schematic of the Plasmalab System100 ICP180
Figure 2 Plasmalab System 100 ICP180
RF power (13.56MHz) is applied to both the ICP source (up to 3000Watts) and
substrate electrode (up to 600Watts) to generate the etch plasma. An
electrostatic shield around the ICP tube is used to ensure that the ICP power is
purely inductively coupled (i.e. 'true-ICP'), hence eliminating sputtering of
tube material and minimising unnecessary highenergy ion damage to devices. Ion
energy at the substrate is monitored by measurement of the DC bias generated on
the lower electrode, and is controlled mainly by the RF power supplied to this
Wafers are loaded into the chamber via a loadlock to maintain good stability
of chamber vacuum and hence repeatability of etching results.
The wafers being etched are either mechanically or electrostatically clamped
to the temperature-controlled lower electrode. Helium pressure is applied to the
back of the wafers to provide good thermal conductance between the chuck and the
wafer. Where necessary, smaller samples are attached on 4" Silicon carrier
wafers with thermally conductive glue.
The Plasmalab System100 ICP has control of substrate temperature
to accuracy of ±1°C over a temperature range of - 5°C to +400°C, through the use
of electrical heater elements and a coolant circulating circuit. This can be
extended to –150°C to +400°C with the addition of a supply of liquid nitrogen.
Substrate temperature has a marked effect on the etch result, as it controls the
volatility of the etch species and hence influences the chemical component of
the process, affecting not only etch rate, selectivity and profile, but also
surface roughness. The system can be operated over a pressure range from 1mT to
100mT allowing accurate control process chamber pressure.
InP based Material Etching
High Rate Etching of Waveguide and Mirror Facet
For the high rate etching of mirror facet and waveguide, the key requirements
are fast etch rates to depths of up to 10µm and 5µm respectively, controllable
etch depth, highly anisotropic profile, no notching at buried layers of InGaAsP
(or similar), and smooth sidewalls and etched surface.
CH4/H2/Cl2 chemistry is the most popular
process for this kind of application. If the temperature of the wafer is allowed
to increase to near 200°C then the etch rate of the commonly used
CH4/H2 process increases, however, profile control becomes
difficult due to increased undercutting. Addition of Cl2 to this
mixture allows highly anisotropic etch profiles, due to the low volatility of
InClx. This therefore allows accurate profile control through
adjustment of CH4/Cl2 ratio. Etch rates of >1.5µm/min
and selectivities of >15:1 to SiO2 or SiNx masks can be
achieved. Figure 3 shows a 10µm deep mirror facet etched using this
Table 1. CH4/H2/Cl2
process performance summary
Etched surface and
|*Oxide mask profile is required to be better than 80
Figure 3. InP based material etched using
CH4/H2/Cl2 process. Etch rates of >1.5µm/min
and selectivity of >15: are achieved.
This chemistry has the advantage that it etches a wide range of materials,
i.e. those containing In, P, Ga, As, Al, Sb etc, with low selectivity (~0.5-1:1)
between each other, hence etched profiles have no notching at interfaces between
materials. It also produces less polymer contamination than the
CH4/H2 chemistry due to the lower CH4 content
of this process and much faster etch rate. There is no additional wafer heating
required, as the InP based wafer is heated solely by the high density plasma
itself. With accurate control of the plasma parameters, process repeatability is
better than ±3%, and no wafer clamping is required.
This technique enables batch processing for high throughput production
applications, e.g. 4x2" wafer loaded per run, since the wafers can simply rest
on a carrier plate and do not need to be individually clamped and helium cooled.
Another variant of this process is the CH4/Ar/Cl2
chemistry which has also been shown to produce excellent etch results using this
However, often the demands of production dictate that the chamber must stay
as clean as possible, ideally with no polymer deposition, even at the expense of
the etch anisotropy and sidewall smoothness if necessary. This requires that the
process does not contain CH4. A common approach is to use a
Cl2 based etch chemistry with a heated electrode (≥150°C in order to
effectively remove the InClx etch product from the wafer surface).
Accurate wafer temperature control is recommended for this process. If the
sample gets too hot the InClx'evaporates' from the surface easily and
hence produces undercutting. On the other hand, at too low a temperature InClx
is nonvolatile resulting in slow etch rates, low selectivity and surface
roughness. Often N2 is added to increase the physical component of
the etching and to passivate the surface, hence reducing surface roughness and
improving profile control. Etch rates of >1 µm/min and selectivity to
SiO2 of >10:1 have been achieved using this process. Figure 4
shows a typical 5µm deep etch result. This is a H+ free process which
may give less damage to device, since H+ often forms a passivation
layer at the etched surface that may affect device performance.
Figure 4. Cl2/N2 etched
The CH4/H2/Cl2 and
Cl2/N2 processes listed above can also be used to create
device mesas, with either vertical or sloped profiles achieved by suitably
adjust process parameters.
An alternative technique which allows processing at lower temperatures of
~100-150°C involves the use of HBr chemistry, since the etch product of InBrx
becomes volatile at a lower temperature than InClx. Figure 5 shows a typical 5µm
deep etch result at an etch rate of 0.8µm/min and a selectivity of >10:1 to
SiO2. Again, good temperature control is recommended due to the
sensitivity of etch results to wafer temperature.
Figure 5. HBr waveguide etch
The HBr process can also etch InP with photoresist (PR) as a mask as shown in
Figure 6 since it requires lower temperature compare to Cl2
chemistry. Typically an etch rate of >1µm/min and a selectivity of 14:1 are
achieved. This process required hard baking of photoresist mask before etching
in order to reduce photoresist burning. Advantages of this process include
potential elimination of the use of hard masks and significantly reduce process
complexity and cost.
Figure 6. InP etches using photoresists as a mask
A Cl2/H2 process has been developed recently. In this
process, the lower electrode is set at room temperature. The wafer is placed on
top of a carrier wafer without additional thermal contact. No wafer clamping is
required. Therefore it is a simple process. The etch mechanism is similar to the
CH4/H2/Cl2 process - the wafer is heated by the
plasma itself. The advantage of this process is the absence of CH4,
therefore no polymer depositing in the chamber. It is a clean and also
environmental friendly process. In this process, the gas ratio of
Cl2/H2 is very important. High gas ratio leads to high
etch rate but also gives an undercut etching profile. Figure 7 shows the results
of Cl2/H2 etch in ICP mode. The etch rate is 850nm/min
with selectivity to nitride mask of > 10:1.
Figure 7. InP/InGaAs sample etched using
Cl2/H2 process at room temperature.
InP Grating Etching or Shallow Etching
Although InP etching process can be replaced by faster, cleaner etch
chemistries in ICP mode for the majority of applications, however, the CH4/H2
process is still widely used for InP DFB(distributed feedback lasers) grating
etching, due to the requirements of shallow, accurately controlled etch depth
(typically <200nm). Also the frequent use of photoresist masks, often
delicate e-beam resists, for grating definition requires room temperature
etching. In an ICP tool this process is typically performed with no ICP power,
i.e. only lower electrode power is applied, enabling a slow 'RIE mode' of
etching. Figure 8 shows the result of a RIE mode grating etch in an ICP tool to
a depth of 100nm at an etch rate of 20nm/min.
Figure 8. CH4/H2 grating etch
CH4/H2 process in RIE mode is a popular for shallow InP
etch (etched depth less than 1000nm). Since it is a room temperature process,
photoresist can be used as mask. However, CH4/H2 forms a
large amount of polymer in the chamber and also deposit at etched top surface
and sidewall. Often a short O2 clean step is added into the process
following the etching in order to remove the residual polymer. Figure 9 shows
the result of a RIE mode shallow InP etch to a depth of less than1000nm at an
etch rate of 20~40nm/min.
Figure 9 shallow InP etch using CH4/H2 process, (a)
single step process showing some polymer deposit on the etched top surface and
sidewall. (b) Two step process, no more polymer residual on etched surface.
CH4/H2 chemistry is also commonly used for
InGaAs/InAlAs selective etching due to the requirements of shallow etch depth,
and selectivity between InGaAs and InAlAs.
and HBr in ICP mode processes can also be used for shallow etch. If the sample
is pre-heated to above 150 degree by the lower electrode, it has been shown to
be possible to reduce the etch rate from >1µm/min to 0.2µm/min4 by choosing
low ICP power. A typical etched profile is shown in Figure 10.
Figure 10. Controllable etch rate for shallow etching
InP Photonic Crystal (PhC) Etching
Etching of InP photonic crystal waveguide structure is very challenging,
since it requires high aspect ratio with feature sizes under half a micron. The
most popular structure is two dimension hole type with hole size less than
All InP etched process motioned above can be employed to etch PhC. P Strasser
from ZTH Zurich developed an etching process using ICP180. The conclusion
from his work is that Cl2/N2/Ar is the best chemistries
for PhC etch. This is a polymer free process, and also provides a square foot as
shown in Figure 11. The wafer temperature is set at above 200°, Cl2
is a etch gas, Ar is used as a dilute gas and N2 gives passivation at
the sidewall. An aspect ratio of >15:1 was achieved. Figure 10 shows an
etched depth of 2.9µm and etch rate of 1.75µm/min achieved for 190nm diameter
hole size, which gives aspect ratio ~16:1. The small sample pieces have to be
glued on to the carrier plate and backside Helium cooling is required.
Figure 11. PhC etched in InP. The holes have a diameter of
180nm and etched depth is 2.9µm. (With kind permission of P Strasser, etc.
Communication Photonics Group ETH Zurich)
InP Via Hole Etching
The requirements for InP via hole etching are somewhat different, i.e.
fastest possible etch rates to depths of up to 150µm, near vertical or slightly
sloped etch profile, resist masked (ideally), flat smooth base, but no concern
about sidewall smoothness. These requirements can be met through the use of an
HBr/BCl3 based etch process at moderate to high temperatures
(120-180°C). The photoresist mask must be thoroughly hardbaked to a high
temperature (>150°C) to ensure that it survives the etch process without
reticulation. Figure 12 shows a 100µm deep via hole etched using this technique.
Etch rate was >2.75µm/min and selectivity to photoresist >15:1.
Figure 12. HBr/BCl2 process for InP Via hole
InP/InGaP/AlInP Red LED and Solar Cell Etching
InP/InGaP/AlInP based material combinations are widely used for making red
LED's or Solar cell's. Requirements for both red LED and solar cell products are
high yields and low cost. Therefore a batch process is essential, also
photoresist is chosen for simplified process and low cost.
BCl3/Cl2/Ar/CH4 is used. The optimised process is unclamped. Table
temperature is kept at 20~30degree, give a etch rate of 450nm/min with
selectivity to photoresist mask of 3:1 and etched profile as shown in Figure
Figure 13. InP based Solar cell etched using
BCl3/Cl2/Ar/CH4, Photoresist was used as a etch
InP MicroLens Etching
Microlenses which are commonly used for advanced photonic applications are
formed in photoresist using one of two techniques. The simplest technique
involves forming squat cylinders of resist using conventional lithography. The
substrate is then heated above the glass reflow temperature of the photoresist
(i.e. 130-150°C), allowing it to reflow.
This will create a spherical surface, with the radius that may be calculated
from the volume of resist and the area of contact with the substrate. The lens
profile is then transferred into the substrate material by ICP dry etching,
often with 1:1 selectivity.
Figure 14 shows a SEM image of a microlens etched into InP to a depth of
20µm. This was created by resist reflow combined with ICP etching. In this case
it is possible to adjust the selectivity between the InP and the photoresist
either by changing the gas mixture used for the process or by adjusting the ICP
power and/or DC bias between the plasma and the substrate. Increasing the
selectivity (so the photoresist etches more slowly) will increase the curvature
of the finished lens. As the gas mixture used for this process includes chlorine
there is the likelihood of post-etch 'bubbles' forming on the etched surface
when the wafer is removed from the tool, due to the hydrophilic nature of
chlorine. OIPT has developed a proprietary technique that avoids this
effect and provides a smooth etched surface.
Figure 14. Microlens etched into InP (a small amount of
photoresist is visible on the left SEM image, highlighting the etch
InP based material etching is a vital technology for the fabrication of
optoelectronic and electronic devices.
Instruments Plasma Technology's System100 ICP etcher (OIPT CS1 hardware)
provides wide ranges of III-V material etching solutions. Highly vertical (or
controlled slope) etched profile, smooth sidewall, with good selectivity to
oxide, nitride or PR mask, and controllable etch rate can be achieved.
Source: Oxford Instruments Plasma Technology.
For more information on this source please visit Oxford
Instruments Plasma Technology.