Development of High Performance Anisotropic Silicon Etchant
Tsuguhiro Tago（SANYO Semiconductor Manufacturing Co., Ltd. Niigata factory）
Tetsuo Aoyama, Mayumi Kimura, Kenji Isami（Hayashi Pure Chemical Ind., Ltd.）
In recent years, processing silicon substrate itself is desired to make three-dimensional structure for special application semiconductor device.
This process is to etch silicon substrate (100) with anisotropic etching chemical to make trenches along with crystal orientation.
Our newly developed chemical is epochal enough to give you solution to conquer slow etching rate of silicon, furthermore to make effective prevention of aluminum corrosion in the process.
Micro machining technology (MEMS) is hopefully expanding and anisotropic etching of silicon is introduced to make desired three-dimensional structure.
Our newly developed chemical is applicable for this process also.
- Necessity of New Process Development
As stated above, the processing of silicon substrate is indispensable to device for special application demand in the coming future .
In our development, finding countermeasures for silicon wet etching issues which can not give enough silicon etching speed at all, and furthermore for corrosion on aluminum or aluminum alloy which is the most considerable issue with present anisotropic chemical.
The market has other difficulties as high price of existing anisotropic chemical and very few information about chemical component to make similar chemical, because the chemical is developed by joint of demander and chemical supplier.
In order to break through those issues, SANYO and Hayashi started developing new chemical with technical experience of new aluminum etching chemical development last year.
- Details of Silicon Etching Chemical
At first, subjects of present etching process are examined. Present etching chemical bases TMAH(Tetra Methyl Ammonium Hydroxide) including 1.4% silicon.
The process flow to make trench with this chemical is
BHF <remove natural oxide layer> → TMAH <anisotropic silicon etching> → BHF <rinsing>
Reaction of silicon etching has two major categories as following.
a) Anisotropic etching with TMAH
Si dissolves by forming silicon hydroxide.
Si + 4OH – → Si (OH)4
b) Silicon etching with HF and HNO3
Silicon is oxidized and its layer (SiO2) is dissolved by HF.
Si + 2HNO3→ SiO2+ 2HNO2
SiO2+ 6HF → H2SiF6+ 2H2O
Our study found that reaction of silicon oxidation in above formula b) can be disregarded.
Therefore, as the reason of slow etching speed, obstruction by silicon oxide, insufficient decomposition of TMAH and insufficient OH –are expected.
After verification of these two points, a minute amount of oxide layer and fluorine are detected and found insufficiency of OH –.
In order to solve these issues, additive is contained to enhance two capability of TMAH itself.
●Enhancing ability of removing oxide layer
●Enhancing electric conductivity of etching chemical to generate high reactive OH –
These two abilities succeeded solving the subject of silicon etching.
Secondary, issue of aluminum anti-corrosion is examined.
The chemical reaction of Aluminum and TMAH proceeds as below and dissolves as aluminum hydroxide.
Al + 3OH – → Al(OH)3
The following is the chemical reaction of Al and TMAH, Al and additive.
The additive works as catalyst and it is not consumed.
Therefore this chemical can keep stable pH, and it can make longer bath life.
Silicon compound is added in above mentioned TMAH, because adding silicon compound in TMAH of present chemical is known by our past examination.
Reaction mechanism of aluminum and silica is not well known. Generation of aluminum silicate by decomposition of silicon compound and its reaction is expected.
And also silicon compound reacts with TMAH and makes pH lower.
Accordingly, the solubility of aluminum hydroxide and aluminum silicate decreases and deposits on aluminum surface. Then OH –can not react on the surface of aluminum by these two obstacles.
As the result, this phenomenon gives anti-corrosion of aluminum.
High performance silicon etching chemical can be developed by adding additive and silica into TMAH. The results are shown as follows.
● Enable to make anisotropic etching of silicon
● Price: Cost Down 30% in weight succeeded
● Patent application completed
● Anti-corrosion of aluminum (0 nm/min.)
● Long life and environmental sustainability
● Enable to make anisotropic etching easily even for deteriorated silicon layer damaged by dry etching
● Pre-process (with BHF, HF etc.) unnecessary by changing concentration of additive
● Inquiries from dozens of companies in our hand