Chlorine-Based ICP Etching for Improving theLuminance Efficiency in Nitride LEDs
H. Ogiya, T. Nishimiya, M. Hiramoto, S. Motoyama and O. Tsuji
Research and Development Department, Samco Inc.,
36 Waraya-cho, Takeda, Fushimi-ku, Kyoto 612-8443, Japan

Keywords: PSS, ICP, LED, Sapphire, GaN, Etching
Abstract
In this research, formation of fine cone-shaped patterns
called PSS (Patterned Sapphire Substrate), and reverse-taper
etching of GaN sidewalls were carried out using
chlorine-based ICP-RIE (Inductively Coupled Plasma Reactive Ion Etching) in order to improve the external
luminance efficiency of GaN-LED (Gallium Nitride - Light
Emitting Diodes). Dry etching systems with a special ICP coilcalled SSTC (Symmetrically Shielded Tornado Coil) [1] aresuited for the formation of PSS, and by controlling parameterssuch as the flow rates of Cl2, BCl3, and Ar, process pressure,
ICP RF power, and bias electrode RF power, a cone profile
with a curved surface can be formed without forming micro
trenches. Also it was confirmed that an etching rate of 100
nm/min, and a selectivity of 1.3 over photo resist (PR) wereachieved. Furthermore, reverse-taper etching of GaN wascarried out with the same system, and smooth, 70° sidewalls
were obtained employing a Ni mask.

INTRODUCTION
In order to improve luminance efficiency of GaN- LEDs
epitaxially grown on sapphire, crystal quality improvement,internal quantum efficiency improvements through device
structure changes, and external quantum efficiency
improvements through light extraction innovation, arerequired.
ELO (Epitaxial Layer Overgrowth) is a common method
for improving crystal quality, epitaxial structure
improvement is effective for increasing internal luminanceefficiency and reflection on the epitaxial layer/sapphireinterface and sidewalls is effective for improving lightextraction.
PSS is used for light reflection on the epitaxial
layer/sapphire interface, by means of concave-convex
nano-patterning on sapphire substrates. This patterning aims
to gather light towards the light extraction plane byscattering and diffraction of light on the bottom plane of theLED device, leading to improvement in the external

luminance efficiency. PSS is not only effective for more
efficient light extraction but also helps with improvement ofthe GaN crystal quality.
However, the sapphire substrate is very hard, chemicallyresistant, and is therefore very difficult to process. To form
fine cone-shape patterns on the substrate, the plasma
etching method is very effective, but it is accompanied byseveral problems such as a low etching rate, poor mask
selectivity, and substrate heating. In this work, by using
the SAMCO model RIE-330iPC ICP-RIE system with theinternally developed SSTC ICP source [1], with aproprietary heat resistant processing of the photo resist (PR)mask, these inherently difficult problems in forming PSS
were solved.
Regarding light reflection on the sidewalls of LEDdevices, there is a way of etching the GaN sidewall thatresults in a reverse-taper shape, which improves theexternal luminance efficiency by preventing light escaping
via the sidewalls. This sidewall taper also can be produced
using ICP-RIE. Here we explain the details of the research.
EXPERIMENTAL DETAILSThe PSS etching carried out in our research wasperformed on SAMCO’s RIE-330iPC system fitted withSAMCO’s internally developed SSTC ICP source with13.56 MHz ICP power at 2 kW and 13.56 MHz bias power
at 1 kW. Based on a chlorine chemistry, additive gases
such as BCl3, Ar, and CHF3 were used. The bias substrateelectrode was equipped with an ESC (electrostatic chuck)and its temperature was kept constant at RT (RoomTemperature) using a water medium, and the clamped wafer
tray temperature was equilibrated to the ESC usingbackside helium. Process parameters such as the flow ratesof BCl3, Ar, and CHF3, process pressure, and the ICP andbias RF powers were controlled and the feature profile,sapphire etching rates, and selectivity over PR wasmeasured using a SEM. UV curing and hard baking of thePR was carried out to increase its heat resistance.

RESULTS AND DISCUSSION
A photo resist mask is used for the formation of the PSS.
The highest process temperature of photo resist in general isaround 120°C, but high RF power is required for theetching of sapphire that is a difficult material to dry etch.
With high RF power, and the accompanying ion
bombardment, heat from the plasma to the substrateincreases, and the PR mask deforms (or burns) due to theincreased substrate temperature as is shown in Figure 1,
making it impossible to obtain a well defined cone-shaped
profile.

“Direct cooling” of the sapphire substrate is carried out
using mechanical or electrostatic clamping. A one-wafer per
batch system will suffer from low throughput, and amultiple-wafers-per batch system with direct wafer coolingwould have a very complicated wafer clamping mechanism.
In contrast to wafer direct wafer cooling, a tray-transfer
type system will have a simpler mechanism, but without theability of cooling each wafer, “PR burning” will result. To
prevent this problem, surface hardening of the PR by UV
curing and inner hardening by high temperature baking wascarried out. As shown in Figure 1(b), the UV and thermal
processed PR mask did not burn and an excellent PSS
profile was obtained.
It is critical to control the PSS profile and the important
profile factors are the feature height, width, taper angle,
bottom profile, top profile, and taper profile. Without
optimizing these factors, an “air gap” can be generated
during the GaN epitaxial process (see Figure 2(b)), leading
to an increase in the crystal imperfections, or causing theepitaxial layer quality to become degraded, leading to a
decline in the internal quantum efficiency.

(a) PSS fabricated using standard photo resist

Furthermore, with disordered cone-shape profiles,
scattering and diffraction of light will not take place as
designed and this will lead to degraded external quantum
efficiency. In addition, as shown in Figure 2(a), in the caseof a “two-step” profile, differences in the crystal growth
direction increase the probability of imperfections occurringaround the top of the profile. Precise PSS profile control isrequired to suppress these problems.
It is crucial to control the etch selectivity of the sapphireover the photo resist mask in order to control the height ofcone-shape. As shown in Figure 3(a), if the selectivity overthe mask decreases, the cone-shape height also decreases
and the top of the profile turns round. If the selectivity
increases, the cone-shape height increases and the top
becomes more pointed.

(b) PSS fabricated following processing of
the resist for increased heat resistance.
Figure 1．Comparison of PSS formation following resist processing
for increased heat resistance.

There are two ways to prevent this heat-induced problem:(i) directly cool the sapphire substrates to keep the PR mask
below its deformation temperature, and (ii) increasemaximum processing temperature of the PR mask.

All the etching parameters, for example, the ICP RFpower, bias RF power, pressure, and gas flow rates,
influence the selectivity control, but if these parameters
were changed, it could also result in a lower sapphire etchrate or micro trenches. Therefore, it was decided instead toadd CHF3 to the etchant gas mixture in order to improve theetch selectivity of sapphire over the PR mask withoutchanging the other process parameters.

Two-Step Profile(a)etching to proceed in the lateral direction, the pressure wasset to a relatively high 7 Pa, and the bias power was set tothe lowest power at which etching would continue (~50W).

(a) Selectivity 0.7

Micro Trench(b) Selectivity 1.0(b)Figure 2．PSS shape and crystal defects. PSS profiles that produce
epitaxial defects: (a) Two-step profile causes differences in thecrystal growth direction, increasing the probability of imperfections
around the top of the profile. (b) Micro-trenches on the fringe ofthe dot cause an “air gap” to form during crystal growth, increasing
the probability of imperfections above the trench.

As shown in Figure 4, by employing CHF3 it was possible
to improve the sapphire to PR selectivity without decreasing
the sapphire etching rate. CH polymer is formed andfunctions as a protective film when CHF3 is added to theetchant gas mixture. This polymer film is also formed on thesapphire surface, but the thin film reacts with the oxygen
supplied from the sapphire structure and is removed from thesapphire surface. Thus, with the improvement of the PR heat
resistance and of the sapphire to PR selectivity by adding
CHF3 gas, an etching rate of more than 100 nm/min, and theformation of cone-shape profiles were both achieved.
The reverse taper formation of the GaN sidewall during
etching employs Cl2 and Ar gases. In order for the GaN(c) Selectivity 1.3
Figure 3．Changes in resist mask selectivity result in changes in PSS
shape and height

Figure 5(a) shows that when the ICP RF power is low,lateral etching is weak and a step is formed on the sidewall.
Increasing the ICP power to 800W (Figure 5(b)) results inremoval of the step, and results in a virtually straightreverse-tapered sidewall, in this case around 70°. Because
the angle varies depending on the GaN crystal plane,
control beyond this cannot be achieved. The GaN etchingrate was 140nm/min when using a Ni mask, which is thepreferred mask material. Because of lateral mask erosion,
SiO2 masks do not allow formation of a reverse taper profile
as dramatic as can be achieved with Ni masks.

CONCLUSIONS
Using ICP-RIE etching based on chlorine chemistry,
optimal cone-shaped patterns were formed on sapphire
substrates and reverse taper etching of GaN was carried out.
By processing the resist mask for higher temperature
resistance, an excellent PSS profile was obtained using thesimpler tray processing method. Formation of a 70° reverse
taper sidewall profile was achieved by optimizing
parameters for the GaN reverse taper etching using a Nimask. Future challenges include finer patterning of the PSSand concave-convex nano-patterning of the GaN surface.

Sapphire/PR Selectivity

2.8

Sapp./PR Sele.
PSS:W

1.6

2.7
2.6

1.4

2.5

1.2

2.4

12.3

0.8

2.2

0.6

2.1
Start PR Width：2.04μm

0.4

20.2

1.9

0

1.8
05

10

1520

PSS:Width（μm）

21.8(a) ICP=500W

25CHF3 Addition (sccm)
Figure 4. By adding CHF3 to the etchant mix, the sapphire to resist
selectivity can be improved and the PSS height and shape can becontrolled. The addition CHF3 forms a CH polymer that functions as aprotective film. The polymer film is also formed on the sapphiresurface, but the film reacts with oxygen generated from sapphire andis quickly removed from the surface. By this mechanism thesapphire etching rate does not decrease and yet the etch selectivity canbe improved.

ACKNOWLEDGEMENTS
The authors wish to thank Mr. Nakagami who focused on
development of the SSTC, and Mr. Takahashi and Mr.
Maruno who provided experiment support.
REFERENCES[1]S.Nakagami ,H.Nakano,T.Tatsuta, M.Sawai and O.Tsuji: 18thInternational Symposium on Plasma Chemistry, 32P-72, August26-31(2007), Kyoto University, Japan(b) ICP=800W
ACRONYMS
LED: Light Emitting DiodePSS: Patterned Sapphire Substrate
SEM: Scanning Electron Microscope
SSTC: Symmetrically Shielded Tornado CoilFigure 5．GaN-LED reverse taper etching. Increasing the ICP power
results in a flat and smooth reverse taper profile. Etching angle was 70ºand is dependent on the GaN crystal plane.