Scalloping - Silicon - Dry etching

Scalloping - Silicon - Dry etching

Material Name: Silicon
Record No.: 5
Primary Chemical Element in Material: Si
Sample Type: Wafer
Uses: Etching
Etchant Name: None
Etching Method: Dry etching
Etchant (Electrolyte) Composition: No data
Procedure (Condition): No data
Note: A second method, known as the Bosch Process, is a switched-gas process that utilizes alternate steps of etching with SF6 as the feed gas, and polymer deposition, usually with c-C4F8 as the feed gas. The switched process is the one more commonly used, and it requires fast acting mass-flowcontrollers to switch between etch and deposition steps, which occur every few seconds. An ICP etching tool is commonly used for the process and bias power is turned on only during the etch step and is kept low—only a few watts. Source power during the etch step depends on the total gas flow (to be discussed below). During the deposition step, polymer is being formed on the horizontal surface as well as on the sidewall. The etch step then removes the polymer from the horizontal surface, and proceeds with etching of silicon, while the remaining polymer on the sidewall, even though it is being eroded, provides protection against lateral etching. The resulting sidewalls show striations (Fig. 1), and may be an issue when smooth sidewalls are needed (e.g., mirror surface).

It is desirable to use etch mask and etch stop materials (if needed) that are resistant to the fluorine plasma. SiO2, Al2O3, and photoresist etch masks etch very slowly with selectivities with respect to silicon of 250:1, >10,000:1, and 50:1, respectively. SiO2 and Al2O3 can also be used as an etch stop material. Care must be used when choosing the etch stop and etch mask material such that the material chosen does not sputter during the etch to deposit nonvolatile products. This will cause “grass” to form in the etched areas due to micromasking.
Reference: Vincent M. Donnellya and Avinoam Kornblit, Plasma etching: Yesterday, today, and tomorrow, J. Vac. Sci. Technol. A 31(5), Sep/Oct 2013, pp. 050825-1 - 050825-48.

Figure 1: Scalloping associated with a switched (Bosch) process. The scallops are the result of alternate etch and deposition steps.