106 - Regular

Improvement of LED Luminance Efficiency by Sapphire Nano PSS Etching
H. Ogiya, T. Nishimiya, M. Hiramoto, S. Motoyama, O. Tsuji, and P. Wood
Research and Development Department, Samco Inc.,
36 Waraya-cho, Takeda, Fushimi-ku, Kyoto 612-8443, Japan075-621-7841, motoyama@samco.co.jp
Abstract:

SAMCO has investigated 3 methods to achieve this:GaN-based blue LEDs (with 450-460 nm light emission)

1) Direct chucking of sapphire substrate; 2) Indirect

are generally produced by depositing a GaN film on achucking using tape or grease; 3) Heat resistancesapphire substrate and then fabricating them into LED

enhancement of the dry photoresist mask.

devices. Patterned sapphire substrates (PSS) are fabricatedMethod 1 – direct chucking, performs temperatureand used in GaN-based LEDs in order to increase thecontrol by clamping wafers to a cooled electrostatic chuckinternal quantum efficiency by reducing GaN crystal(ESC) or mechanical chuck and introducing high pressure

defects. They also have the benefit of improving external

helium on the backside of the wafer to maintains the wafer

quantum efficiency by increasing the reflection area.

temperature below 100℃. Method 2 – indirect chucking,Regarding electroluminescence (EL) intensity, performing

uses a tray and a heat conductive sheet or grease to achieve

PSS etching improves the luminous efficiency by aboutsimilar results. However, when using either of these

13% compared to processing without PSS. It is well-known

methods, failure in cooling during the dry etching processthat luminous efficiency of LED devices increases by

can occur due to insufficient clamping of the ESC orcarrying out the PSS process (Figure 1).

mechanical chuck, or peeling/delamination of the heat
conduction sheet from the tray. To overcome theseproblems, we have developed a dry etching process thatuses a positive photoresist with heat resistant Novolak resin(ZPP-L100 manufactured by ZEON Japan).
In Method 3, firstly, a UV-cure was carried out using alow-pressure mercury lamp primary with active
wavelengths of 185nm and 254nm in order to prevent the1.8µm

photoresist from softening and drooping. Then, the sample
was post-baked at 250℃ to avoid degradation of thephotoresist caused by the absorption of heat from plasma
during the dry etching process. As a result, the sapphire dryetching process was completed without photoresist burning

Figure 1. Typical micro-PSS etch pattern.
A dry etching process using a photoresist mask has beenutilized for PSS processing because the required etching
precision cannot be achieved with wet etching. However,since sapphire dry etching requires a high RF power, thewafer absorbs heat from the plasma and can reachtemperatures of up to 250℃, causing photoresist burningand deterioration, and harming the etching process.
Therefore, to prevent photoresist deterioration, it isnecessary to keep the substrate temperature below 100℃.

or drooping.
The result of this experiment shows that through the useof a heat-resistant photoresist mask, etching defects caused
by insufficient direct chucking or indirect cooling can beeliminated. Thus, stable sapphire etching can be realized.
The heat resistant photoresist technology is also
beneficial for nano PSS, which further improves theluminous efficiency of LEDs. We carried out etchingexperiments to create nano PSS, aiming to achieve a higher
internal quantum efficiency than standard micro-PSS byreducing GaN crystal imperfections. In these nano PSS

1.6	  diameter were fabricated on the sapphire wafer as shown in

1.5	  The size of the nano-PSS pattern was set toabout half of the emission wavelength of 450nm. Then wegrew GaN epitaxial layers on the nano-PSS, measured EL
characteristics of 450~460nm, and compared the luminous
efficiency (Figure 3).

The results showed that the EL

emission intensity of nano-PSS is 45% higher than of flat
sapphire substrates. It is also 30% higher than the ELemission intensity of the conventional micro-PSS (Figure1.4	  1.3	  Normalized E-­‐L	  intensity

Figure 2.

(a.u.)	 

etching experiments, sapphire patterns of 200nm in1.2	  1.1	  1.0	  0.9	  0.8	  0.7	  4).

0.6	  Flat	  Micro-­
non-­PSS	 
PSS	 

200nm

Nano-­PSS	 

E-L intensity

Wavelength(a.u.)(nm)

Flat (non-PSS)

1.00

450

Micro PSS

1.13

460

Nano PSS

1.45455

Figure 4. Comparisons between each PSS pattern and its
E-L intensity
Our aim is to optimize the nano-patterns on sapphire

Figure 2. Nano-PSS

substrates to further improve the luminous efficiency ofGaN LEDs.

E-­L	  intensity	  (a.u.)	 

3000	  Meanwhile, we are also evaluating

alternative applications for heat-resistant photoresist
technology. For example, currently SiO2, SiN and metal

2500	  masks are used for plasma etching processes requiring high2000	  substrate temperatures. As we have demonstrated here,1500	  

heat-resistance photoresist could significantly improvefabrication efficiency in other applications.

1000	  500	  Acknowledgement:
We are grateful to Professor Ikko Tadatomo of Yamaguchi0	  0	  50	  100	  150	  200	  Current	  (mA)	 
Figure 3. E-L luminous efficiency of LED with nano-PSS
pattern (height 250nm) and injected current.

University for helpful discussion of Nano PSS Etching.