GaAs - Dry Etching

GaAs - Wet Etching

Material Name: GaAs
Recipe No.: 10345
Primary Chemical Element in Material: Ga
Sample Type: Wafer
Uses: Etching
Etchant Name: None
Etching Method: Wet etching
Etchant (Electrolyte) Composition: Several wet chemical digital etching techniques were attempted to determine one that provided consistent and reproducible results. An important step in the etching process is to completely separate the chemicals used for each part of the digital etch step (oxidation step and oxide etching step). That is, it is necessary to completely rinse the H2O2 before applying the acid, and vice versa. Residue of either H2O or acid on the GaAs wafer will mix with the next application of acid or H2O2 and provide a normal acid: H2O2 etchant that destroys the controlled, self-limiting digital etching process. Because H2O2 and many acids contain water, the use of deionized water (DI H2O) was selected to provide the necessary rinsing between each step. To prevent dilution of the chemical agents, an N2 blow dry is used to remove any water remaining after rinsing.

A new method was developed to provide separately fresh oxidizing and etching solutions to the sample during each etch step to minimize chemical cross contamination. Since contamination due to incomplete rinsing must be eliminated to maintain the digital etching chemistry, a photoresist spinner was modified to perform the digital etching procedure. The modification of the photoresist spinner consisted of replacing exposed metal parts (wafer chuck, vacuum shaft, and spinner baseplate) with oxidizer and acid resistant high density polypropylene machined to original specifications. With the spinner, fresh oxidizing and etching solutions can be sprayed from above onto the sample during each etch step. The spinning motion removes the applied solution quickly, and allows the application of a continuously fresh stream of solution. DI H2O rinsing of the wafer can also take place while spinning, providing a continuously fresh water stream to remove the oxidizing and etching solutions. In this manner, fresh solutions of H2O2, acid, and DI H2O are applied to the sample during the digital etch, thereby eliminating cross contamination between H202 and the acid.

Process development has demonstrated that successful digital etching requires the use of uncontaminated oxidizing and etching solutions with complete rinsing between the oxidizing and acid etching steps. Spin rinsing with fresh DI H20 for 15 s at 1000 rpm was found to be sufficient to remove both the oxidizing and etching solutions from the wafer before applying the next solution in the digital etch process. However, it was determined that spinning the wafer during applications of the oxidizing and etching solutions was not necessary, as soaking the wafer during these steps was determined to be sufficient. The measured digital etch rates of this soak and spin rinsing method are equivalent to those obtained using the continuous spin technique. Soaking is desirable because it provides a large reservoir of solution to perform the necessary oxidation and etching without depleting the solution. Because these experiments were performed manually, the static chemical soak and spin rinsing technique was used to maintain repeatability and reproducibility, as it was difficult to insure complete coverage with manual spraying, whereas a static flood was easy to accomplish and maintain during the given soak times. The digital etching experiments conducted in this work used a combined chemical soak and spin rinsing procedure as outlined in Table 1.
Procedure (Condition): No data
Note: A new room temperature wet chemical digital etching technique for GaAs is presented which uses hydrogen peroxide and an acid in a two-step etching process to remove GaAs in approximately 15 A increments, In the first step, GaAs is oxidized by 30% hydrogen peroxide to form an oxide layer that is diffusion limited to a thickness of 14 to 17 A for time periods from 15 to 120 s. The second step removes this oxide layer with an acid that does not attack unoxidized GaAs. These steps are repeated in succession until the desired etch depth is obtained. Experimental results are presented for this digital etching technique demonstrating the etch rate and process invariability with respect to hydrogen peroxide and acid exposure times.
Reference: Gregory C. DeSalvo, et al., Wet Chemical Digital Etching of GaAs at Room Temperature, Journal of The Electrochemical Society, 143 (11), 1996, pp. 3652-3656.

Table 1: Experimental procedure used to perform a single wet chemical digital etching cyde.

Table 2: Digital etch rate dependence on acid or base using a 10 cycle digital etch step repetition with 1 min acid soaks and 1 min H202 soaks.

Table 3: Digital etch rate dependence on the number of digital etch cycles performed using 1 min HCI:H20 (l:1) soaks and 1 in H202 soaks.

Table 4: Digital etch rate dependence on digital etch soak time using a ten cycle digital etch repetition.