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Source: SEMICONDUCTOR MANUFACTURING HANDBOOK

CHAPTER 11

WET ETCHING

Peng Zhang
Air Products and Chemicals, Inc.
Allentown, Pennsylvania

11.1 INTRODUCTION

In semiconductor manufacturing, the etch process is responsible for transferring a pattern from a
mask layer to the underlying layer. A part of the underlying film is removed by chemical reaction
with a reagent material called etchant, while the other part remains intact, protected underneath the
mask layer (Fig. 11.1). The basic requirement for an effective etch process is the high selectivity
between the mask and the etched layers. In other words, there should be minimal etching on the mask
layer. One of the most common mask layers is the photoresist.
There are mainly two methods of etching—wet chemical and dry plasma etching. During wet
etching, the etchants are liquid chemical mixtures that react with the substrate to produce soluble
products to be dissolved away. Dry etching employs plasma gas to etch the substrate both chemically
and physically. Dry etching has started replacing wet etching in many applications, particularly in
advanced microelectronics.
When the semiconductor industry was getting started, wet etching was the only etching method.
Nowadays, it is still quite common in the semiconductor manufacturing including compound semi-
conductor and microelectromechanical systems (MEMS) applications. There are several factors con-
tributing to the popularity of wet etching. First, wet etching technology is mature and well established.
It is simple to implement and there is no additional cleaning step needed. In addition, the etch selec-
tivity is usually very high with no damage to the underlying substrate. Probably the most important
factor is that wet etching is much cheaper than dry etching, which relies on expensive etching gases
and chambers.
Except when etching crystalline materials, the wet etching process is isotropic, i.e., the etch rate
is equal in all directions. This limits the use of wet etching in advanced microelectronic manufac-
turing. As shown in Fig. 11.2a, with isotropic etching, undercutting occurs underneath the mask
layer. As the geometries of transistors are being pushed to smaller and smaller sizes, the thickness of
the film being etched becomes comparable to the lateral pattern dimension. The undercutting of the
etched film becomes intolerable and the etch process difficult to control. Therefore wet etching is not
a viable method to etch features in the µm and sub-µm ranges. As an alternative, dry etching or reac-
tive ion etching (RIE) is an anisotropic etching process where energetic ions hit the substrate surface
along paths perpendicular to the surface by directional bombardment while secondary reactions form
protective residues on sidewalls to reduce lateral etching. As a result, high-aspect-ratio vias or con-
tacts can be achieved. Nowadays, dry etching is the main etching process in advanced microelec-
tronics manufacturing.

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