Semiconductor device manufacturing: process – Chemical etching – Liquid phase etching
Reexamination Certificate
2000-08-07
2002-02-12
Niebling, John F. (Department: 2812)
Semiconductor device manufacturing: process
Chemical etching
Liquid phase etching
C438S631000, C438S697000, C438S718000, C438S753000, C438S959000, C438S974000, C438S977000, C438S337000, C216S038000, C216S079000, C216S099000
Reexamination Certificate
active
06346485
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an improvement on the method of removing, through chemical etching, a damaged layer that is generated on the surface of a monocrystalline silicon wafer during a process of producing the wafer.
2. Description of the Related Art
A conventional process of producing a mirror-polished semiconductor wafer typically comprises the steps of slicing a monocrystalline ingot of silicon or the like to obtain a semiconductor wafer; and chamfering, lapping, acid etching, mirror-polishing, and cleaning the sliced semiconductor wafer. Depending on the required specifications, the sequence of steps is changed; some steps are repeated a plurality of times; or other steps such as heat treatment and grinding are added to or replace the above-described steps. Thus, a variety of kinds of steps are performed in accordance with the specifications.
Among the above-described steps, acid etching is performed for the purpose of removing a surface damaged layer introduced in the course of mechanical machining steps such as slicing, chamfering, and lapping. In the acid etching step, the surface of a wafer- is etched to a depth of a few to a few tens of microns through use of mixed acid aqueous solution composed of hydrofluoric acid, nitric acid, acetic acid, and water. However, acid etching involves the following problems:
1) The flatness of a wafer after lapping—which is indicated by thickness variation represented by, for example, TTV (Total Thickness Variation) (&mgr;m) or LTV
max
(Local Thickness Variation) (&mgr;m) —deteriorates as the etching amount increases.
2) A waviness of a millimeter order or an uneven region called “peel” is generated on an etched surface.
3) Harmful NO
x
is generated due to etching. In consideration of these problems, alkali etching is used in some cases.
Alkali etching has the following advantages:
a) Flatness established by lapping is maintained after etching.
b) Generation of harmful gas is suppressed. However, alkali etching has the following disadvantages:
i) If foreign mater enters pits locally existing on an etched surface and having a depth of a few microns and a diameter of a few to twelve or thirteen microns, the foreign matter causes generation of particles and/or contamination in a subsequent step.
ii) Since deep pits exist and surface roughness (Ra) increases, a polishing stock removal in a subsequent step of mirror-polishing (mechano-chemical polishing) must be increased.
iii) Since an etched surface has a sharp uneven shape compared to a surface etched through acid etching, the unevenness itself serves as a source of particles.
Accordingly, particles generated in a subsequent step and a polishing stock removal in a mirror-polishing step can be decreased if etching treatment can be performed while flatness attained through lapping is maintained, so as to remove a mechanically formed damage layer, improve the surface roughness, efficiently decrease the depth of deep pits locally formed due to the etching, and smooth the uneven shape of the surface.
SUMMARY OF THE INVENTION
The present invention has been accomplished to solve the above-mentioned problems, and an object of the invention is to provide a method of processing a semiconductor wafer which can remove a mechanically formed damage layer, improve surface roughness, and efficiently decrease the depth of locally formed deep pits, while the flatness of the wafer attained through lapping is maintained, in order to produce a chemically etched wafer (CW) having a smooth and flat etched surface that hardly causes generation of particles and contamination.
Another object of the invention is to provide a semiconductor wafer processed through the above-described processing method.
To achieve the above object, the present invention provides a method of processing a semiconductor wafer sliced from a monocrystalline ingot. The method comprises at least the steps of chamfering, lapping, etching, mirror-polishing, and cleaning and is characterized in that in the etching step alkali etching is first performed and then acid etching is performed, and that an etching amount of the alkali etching is greater than an etching amount of the acid etching.
In the etching step of the processing method of the present invention, after the step of lapping alkali etching is first performed in order to remove a mechanically formed damage layer, while the flatness of the wafer attained through lapping is maintained, and subsequently, acid etching is performed in order to decrease the depth of locally formed deep pits remaining after the alkali etching and to improve the surface roughness and the sharp uneven shape.
At this time, the etching amount of the alkali etching must be set greater than the etching amount of the acid etching because of the following reasons. That is, in order to decrease the depth of locally formed deep pits remaining after the alkali etching, the etching amount of the alkali etching must be increased to a certain level, which is greater than the etching amount of the acid etching required for decreasing the rate of generation of faults such as stain stemming from unevenness in etching and for improving flatness.
Preferably, before being subjected to the acid etching a wafer that has undergone the alkali etching is immersed into aqueous solution of hydrogen peroxide.
The surface of a wafer that has undergone the alkali etching is active and hydrophobic, so that foreign matter easily adheres and dirties the wafer. However, if the surface of the wafer is oxidized through immersion into aqueous solution of hydrogen peroxide and thus made hydrophilic, particles hardly adhere to the wafer surface.
Preferably, the etching amount of the alkali etching is 10-30 &mgr;m, and the etching amount of the acid etching is 5-20 &mgr;m
In the alkali etching, there is a tendency that the depth of locally formed deep pits remaining after the alkali etching decreases with increasing etching amount, and that the surface roughness becomes higher with increasing etching amount of the alkali etching. Therefore, the etching amount of the alkali etching is maintained within the above-described range. In the acid etching, as the etching amount increases, the stain generation rate decreases considerably although the flatness deteriorates. Therefore, the etching amount of the acid etching is maintained within the above-described range.
Preferably, the etchant used in the alkali etching is an aqueous solution of NaOH or KOH, and the etchant used in the acid etching is a mixed acid aqueous solution composed of hydrofluoric acid, nitric acid, acetic acid, and water.
When such etchants are-used, etching is performed effectively and reliably in both the alkali etching and the acid etching, and the respective etching amounts can be controlled with relative ease. In addition, the etching can be performed at low cost.
In the present specification, each specific value used in relation to etching amount represents the sum of the thicknesses of layers removed, through etching, from opposites surfaces of a wafer.
Preferably, the acid etching is reaction-controlled acid etching.
When the acid etching is of a reaction-controlled type, the flatness can be further improved through suppression of waviness, while realizing a decrease in the depth of deep pits locally remaining after the alkali etching and improvement of the surface roughness and the sharp uneven shape.
Preferably, in the reaction-controlled acid etching, there is used an etchant obtained through addition of 20-30 g/l of silicon into a mixed acid aqueous solution composed of hydrofluoric acid, nitric acid, acetic acid, and water.
When such etchant is used, etching is performed effectively and reliably, and the etching amount can be controlled with relative ease. In addition, the etching can be performed at low cost.
The present invention provides another method of processing a semiconductor wafer sliced from a monocrystalline ingot. The method comprises at least the steps of chamfering, lapping, etching, mirro
Kato Tadahiro
Kudo Hideo
Miyazaki Seiichi
Nihonmatsu Takashi
Yoshida Masahiko
Hogan & Hartson L.L.P.
Niebling John F.
Shin-Etsu Handotai & Co., Ltd.
Zarneke David A
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