Molybdenum Etchants Study Surajit Sutar University of Notre Dame

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1 The purpose of this Molybdenum etching study is to find a Molybdenum metallization process without involving a Lift-Off. Molybdenum, a refractory metal, when evaporated by an E-beam, causes a significant rise in the temperature of the evaporation chamber. This could present problems during the subsequent Lift-Off due to the possible baking of the photoresist during evaporation. An alternative process would be to deposit Molybdenum all over the surface of the wafer, pattern the Molybdenum surface in a photolithography process, and then to etch off Molybdenum where not needed. This process requires suitable and preferably selective etchants for Molybdenum. Preparation of the samples: Molybdenum was deposited on Si and InAs (PEDI-90, top InAsP removed) wafers. The wafers were patterned using AZ5214E photoresist. Samples like these were then treated with the etchant and were step-profiled to get the etch depth after removing the photoresist. The metal deposited varied in thickness. Could be taken between 700 and 1000 A 0. A brown Molybdenum oxide layer is usually formed in exposure to air. That could have affected the Molybdenum thickness and subsequent etching too. Etchants studied: 1. x HF: x H 2 O a. 1 HF: 1 H 2 O b. 1 HF: 5 H 2 O No etching was observed in 1 minute 2. x HF: x H 2 O 2 : x H 2 O No etching was observed in 5 minutes. All the compositions give a very rough surface. The etching is not uniform. The places most etched are dark (probably oxide), and at other places Molybdenum becomes fragmented. The average surface roughness is very high. The samples were soaked in HCl (both concentrated and diluted) to remove the brown oxide film. But 3 hours of soaking yielded nothing. The etchant seemed to attack both InAs and Si.

2 a. 1 HF: 2 H 2 O 2 b. 1 HF: 2 H 2 O 2 : 5 H 2 O µm µm c. 1 HF: 2 H 2 O 2 : 10 H 2 O nm nm d. 1 HF: 1 H 2 O 2 e. 1 HF: 1 H 2 O 2 : 1 H 2 O f. 1 HF: 1 H 2 O 2 : 5 H 2 O nm nm Surface Roughness Å 40 Å Å 30 Å Å 400 Å Å 200 Å Time (minutes) Surface Roughness nm 3.1 nm nm 61.4 nm nm 3192 nm

3 g. 1 HF: 1 H 2 O 2 : 10 H 2 O Time (minutes) h. 1 HF: 1 H 2 O 2 : 25 H 2 O µm Time (minutes) i. 1 HF: 1 H 2 O 2 : 50 H 2 O nm Time (minutes) j. 1 HF: 1 H 2 O 2 : 200 H 2 O 20 0 nm Time (minutes) Surface Roughness 90 (on Si) nm 79.2 nm 90 (on InAs) 5.25 µm IT DOES ATTACK InAs!!

4 2. 15 NH 4 OH: 10 H 2 O 2 : 100 H 2 O 100 Etching Rate for Molybdenum on Si (nm) NH 4 OH : 10 H 2 O 2 : 100 H 2 O Molybdenum thickness nm The Molybdenum thickness could have been nm. After 3 minutes, there were places where the photoresist (5214E) seemed to have been worn off at the edges of features. It means either the etchant attacks the PR or the etching is isotropic. The etching reaction is marked with brown film being formed at the places etched. 1 HNO 3 : 1 H 2 SO 4 : 3 H 2 O Doesn t seem to attack Silicon. But attacks InAs rapidly (from both the top and bottom of the sample). Takes off around 80 nm of Molybdenum in 10 seconds. 1 HNO 3 : 10 H 2 O No etching was observed after 20 minutes of treatment. 1 HNO 3 : 1 HCl: 1 H 2 O Attacks InAs rapidly after etching Molybdenum.

5 Straight H 2 O 2 (nm) Molybdenum Etch with H 2 O 2 Si Substrate Molybdenum 50 y = x R= ss1013etch2 The etching was done with Photoresist (5214E) as an etch mask, and it was observed that for longer etch times (greater than 3 minutes), the molybdenum under the photoresist was etched at the edges. It suggests adhesion problems between Molybdenum and the photoresist or H 2 O 2 attacking the photoresist. Also, the etch rate depends upon the strength of the H 2 O 2 (the actual percentage of H 2 O 2 in the solution).

6 Reactive Ion Etching (RIE) with CF 4 and O 2 (nm) CF 4 /O 2 RIE Etch Molybdenum y = x R= y = x R= y = x R= RIE Etch 26 sccm CF, 5 sccm O, mt, 100 W, ~210 V DC bias Si substrate Molybdenum PECVD grown SiO Etch 2ss1013etch1 The etch rate as found from the above figure is approximately Å/s. The etchant doesn t attack InAs, but with the DC bias ~ 200 V, it is expected to damage the surface and introduce defects. Conclusions: 1. The mineral acid etchants (HNO 3, H 2 SO 4 etc) are found to have high etch rates, but they attack InAs and possibly, other III-IV compounds. 2. The NH 4 OH etchant and the H 2 O 2 etchant can be used with a photoresist (5214E) etch mask for shorter etch times (< 3 minutes). For longer etches, SiO 2 can be used as an etch mask. Note: NH 4 OH could attack InAs if the ph of the solution is low. 3. The RIE with CF 4 and O 2 etches Si and SiO 2. It could introduce defects in InAs. 4. After etching, usually a thin brown oxide film is formed on the surface. This could be taken off by treatment with diluted HCl.