phys. stat. sol. (c) 1, No. 10, 2573 – 2576 (2004) / DOI 10.1002/pssc.200405065
Batch reactive ion etching of gallium nitride using photoresist as a mask Mark Dineen*, Sean Lee**, Ligang Deng, Andrew L. Goodyear, and Colin Welch Oxford Instruments Plasma Technology, North End, Yatton, Bristol, UK Received 1 March 2004, revised 17 August 2004, accepted 18 August 2004 Published online 20 September 2004 PACS 81.05.Ea, 81.65.Cf, 85.40.Hp Though advances had being made in the growth technology of gallium nitride (GaN), the device wafer sizes remain at a maximum of 2”. The consequence of this is that in a production environment single wafer processing severely impacts on throughput and therefore the profitability. Dry etching is an essential part of gallium nitride device (C.R.Eddy, Jr., Etch Process of III–V Nitrides, RES 4S1, G10.5 (1999) [1]) processing because of the high bond strength and lack of simple wet etch process. This paper discusses the methods developed to enable the Reactive Ion Etching (RIE) of batches of GaN. Photoresist was used to define mask patterns on 2” GaN wafers. Various batch sizes were processed in an Oxford Instruments Plasma Technology Plasmalab133 RIE tool. Etch rates in excess of 50 nm/min were achieved for batches of six and fifteen two inch GaN wafers. Uniformities of etch across these batches are +/–3% and +/–5%, respectively. © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Introduction
The nitride family of semiconductors has gone through an amazing growth and development in the last decade and is now poised to play a vital role in a wide range of advanced semiconductor devices. Primarily in the fabrication of an array of optical and electronic devices including light emitting diodes (LEDs), transistors,and laser devices for CD and DVD players. As a result of many of the properties that make nitride semiconductors very attractive in the above mentioned electronic and optoelectronic device applications, simplicity in the production capabilities has deem to gain much attention. However, the process has also placed some restriction on acceptable masking materials with more robust silicon dioxide and nitride often being chosen over photoresist. This is often not desired in production operation, as such, a research and development work has been performed in this paper to realize the production-quality etch processes for the nitrides in the most cost effective way. In this work, the progress in developing etch processes for the nitrides is reviewed. The first ion assisted technique addressed is the conventional reactive ion etching processes [2]. The review then moves on to discuss the batch etching efforts of nitride. The highlights of these efforts are then summarized.
* **
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M. Dineen et al.: Batch reactive ion etching of gallium nitride using photoresist as a mask
Experimental Top gas inlet
RIE
13.56 MHz
Fig. 1
∼
Temperature controlled lower electrode
Experimental arrangement.
The experiments were carried out on the Plasmalab 133 RIE tool from Oxford Instruments (Fig. 1). In these reactors the plasma density and energy of impinging ions on the substrate are tightly coupled with higher applied RF powers resulting in higher densities and energies All samples were patterned on photo resist using photolithography and was etched using Cl2/BCl3/Ar chemistry. The samples have been loaded into the chamber with different loads to study the most optimal condition for batch etching. There were batches of 6 two inch wafers and 15 two inch wafers loaded across the 320 mm lower electrode (Fig. 2, Fig. 3, respectively). The chamber was pumped down to a base pressure of 1 × 10–6 Torr. Etch rates were determined by measuring the depth of the etched features with a Dektak stylus profilometer before and after removal of the mask. Etch rates and uniformities were also being studied against the total load in the chamber.
Fig. 2 Batch of 6 two inch wafers loaded across the 320 mm lower electrode.
© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Fig. 3 Batch of 15 two inch wafers loaded across the 320 mm lower electrode.
phys. stat. sol. (c) 1, No. 10 (2004) / www.pss-c.com
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Results
One of the most obvious findings that we got from batch production apart from the single wafer etching is the loading effect of first order [3]. The loading effect is the dependence of etch rate on the amount of available surface area; concentration of active species decreases as the etchable surface area increases.
Fig. 4
Etch uniformity vs. radius.
Fig. 5
Batch etching rate vs. loading.
Because of its quadratic dependence [4] on the wafer radius, the loading effect must also be taken into account in preparing the transition to larger-diameter areas (Fig. 4). As is well known, the etch rate and its uniformity are sensitive functions such as the total gas flow, the area exposed to the plasma and to the common parameters: ratio of the gas flows, induced dc bias, and RF power itself. The effects of etch rate to the increasing loads has been demonstrated in Fig. 5.
Fig. 6
Etch rates vs. RF power.
Fig. 7 Etch rates vs. pressure.
In all cases nitride etch rates are seen to increase monotonically with increasing RF power (Fig. 6, the work has shown that similar etch rate is achievable at lower power with changes in Cl2 flow) and decrease with increasing pressure (Fig. 7) [5]. All of these works have been performed with resist masks that were slightly thicker in order to withstand the high ion energies used in processing. In order to have a control over the loading effect, optimal condition has been derived and at a lower RF power to deuce surface damages with 40% Cl2, 900 W and 20 mTorr.
© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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M. Dineen et al.: Batch reactive ion etching of gallium nitride using photoresist as a mask
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Conclusions
We have demonstrated batch etching of GaN applying RIE in capacitively coupled plasma and achieved a constant rate for batch etching in production application. Although the use of photoresist has shown some restriction to the better process performance such as higher etch rates, it has however lead to shorter preparation time for the mask material thus greatly improved production capabilities. Our work has shown a remarkable ‘well preserved’ photo resist after etching (Fig. 8).
Fig. 8 ‘Well preserved’ photoresist after etching.
References [1] [2] [3] [4] [5]
C. R.Eddy, Jr., Etch Process of III–V Nitrides, RES 4S1, G10.5 (1999). S. J. Pearton, C. R. Abernathy, F. Ren, and J. R. Lothian, J. Appl. Phys. 76, 1210 (1994). C. J. Mpgab, J. Electrochern. Soc. 124, 1262 (1977). B. N. Chapman and V. J. Minkiewicz, J. Vac. Sci. Technol. 15, 329 (1978). C. R. Eddy, Jr. and B. Molnar, in: Gallium Nitride and Research Materials, edited by F.A. Ponce, R.D. Dupuis, S. Nakamura, and J.A. Edmonds, MRS Proc. Vol. 395 (Materials Research Society, Pittsburgh, PA, 1996).
© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
