Surface Damage Induced by Dry Etching

S. W. Pang, S. T. Sung, K. K. Ko, S. Thomas III, and E. W. Berg

University of Michigan, Ann Arbor, Michigan 48109-2122, USA

For devices with submicrometer dimensions, dry etching is necessary for pattern transfer to maintain vertical profile. In addition to the demands of vertical profile and controllable etch rate, the etch-induced damage has to be minimized in order to realize the advantages of these small devices. However, the high energy ions and energetic particles exist in the plasma reactors can potentially induce damage and degrade device performance. Etch-induced surface damage has been evaluated using electrical characterization of Schottky diodes, transmission lines, and conducting wires after dry etching. Surface analyses using Auger electron spectroscopy (AES), transmission electron microscopy (TEM), and atomic force microscopy (AFM) were used to detect changes in surface stoichiometry, defect distribution, and morphology after etching. Photoreflectance and reflectivity measurements were used to evaluate the effects of etching on optical properties.

It is found that higher ion energy, higher ion flux, or lower etch temperature causes more surface damage. Enhanced defect diffusion has been observed from TEM for GaAs etched at higher temperature, while the electrical characteristics of the devices improved due to lower defect density by annealing. Using AES, the etched surface is found to be residue-free. Dry etched mirrors have high reflectivity mirrors up to 93%. The reflectivity is independent of the ion energy or the ion flux used as long as smooth sidewalls and vertical profile were maintained.

Damage removal using low-energy chlorine species has been demonstrated to be effective to reduce defects on the etched surface and sidewalls. The low energy chlorine species can also passivate the surface damage with minimal etching if native oxide is allowed to form on the etched surface prior to passivation. Complete recovery of the electrical characteristics of both the Schottky diodes and unalloyed transmission lines was observed with a 30 s Cl2 plasma passivation at 25¡ãC. The results suggest that surface damage can be minimized by etching with low ion energy, low ion flux, high etch temperature, low pressure, high Cl2 concentration, and with damage removal or passivation using low energy chlorine species.

Figure 1. Thermal wave signal and ideality factor as a function of removal thickness from the dry etched surface. The sample was etched with 50 W microwave power and 100 W rf power in a Cl2 plasma. The rf power was lowered to 1 W for damage removal.

Figure 2. The calculated defect density, the measured barrier height , and the predicted barrier height at different microwave powers. The process conditions were 10 sccm Cl2 at 1 mTorr, 150 V |Vdc|, and 8 cm below the ECR source.

Figure 3. Reflectivity and sidewall damage depth as a function of rf power. The samples were etched using 1/9 Cl2/Ar, 50 W microwave power, 0.5 mTorr, and with rf power ranging from 70 to 200 W.

Figure 4. Forward I-V characteristics showing complete recovery after the samples were etched and passivated with a Cl2 plasma for time ranging from 0.5 to 2 min. Samples were first etched in a Cl2/Ar plasma with 20% Cl2, 50 W microwave power, and 200 W rf power at 0.5 mTorr. The Cl2 plasma for passivation was generated with 50 W microwave power at 2 mTorr and 25¡ãC.

Figure 5. Damage depth determined by changes in rc with removal of dry etched surface by wet etching. The etch condition for curve A was microwave and rf power of 50 W, Cl2/Ar at 1/9 sccm, 1 mTorr pressure, and 13 cm source distance. For curve B, the rf power was increased to 200 W. For curve C, only Ar flowed at 10 sccm.

Figure 6. InP surface roughness is reduced at higher microwave power due to the higher ion density leading to an increase in the etch rate. The etch conditions were 100 W of rf power, Cl2/Ar gas flow at 10/10 sccm, 2 mTorr pressure, 8 cm source distance, and 30 ¡ãC stage temperature.

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Last Updated: November 19, 2007

E-Mail: pang@eecs.umich.edu 

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