Semiconductor device manufacturing: process – Chemical etching – Liquid phase etching
Reexamination Certificate
2001-12-12
2003-05-13
Whitehead, Jr., Carl (Department: 2813)
Semiconductor device manufacturing: process
Chemical etching
Liquid phase etching
C438S752000, C438S906000
Reexamination Certificate
active
06562728
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-275255, filed Sep. 11, 2001.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a surface treatment method of semiconductor substrate used in an electronic device having an SiGe/Si hetero junction.
2. Description of the Related Art
A semiconductor substrate utilizing germanium in formation of pn junction is preferably used in infrared detectors of high sensitivity and diodes. A SiGe substrate (or film) or Ge containing substrate (or film) incorporated in these electronic devices must be brought into contact with an Si film (or substrate). In this case, before the SiGe substrate (or film) or Ge containing substrate (or film) is brought into contact with the Si film (or substrate), it is important to clean the surface of the SiGe substrate (or film) or Ge containing substrate (or film) to remove impurities from the surface (impurity metal, oxide, carbide, etc.).
If surface cleaning of the SiGe substrate (or film) or Ge containing substrate (or film) is insufficient, impurity elements such as oxygen, carbon, and metal are left over on the surface, and when the Si film is deposited on the surface, stacking faults or point defects may be generated from these impurities. When the defect density on the pn junction boundary increases, it forms recombination centers, which cause the leak current in the pn junction when an inverse bias is applied to the pn junction. If a semiconductor substrate of such junction structure having a high defect density is incorporated in an electronic device (transistor or diode), probability of causing malfunction is high.
A hetero junction bipolar transistor (HBT) and a field effect transistor are known as electronic devices operating at high speed, and are widely used in communications and high speed signal processing. To further enhance the operation speed of such devices, a silicon-germanium film (SiGe film) is used.
In the HBT as shown in
FIG. 6
, if impurities are left over on the hetero junction boundary or surface of SiGe film
101
, a leak current may be generated at the pn junction potential. Such leak current is one of the causes of failure of electronic device.
Impurities (foreign matters) left over on the SiGe surface or Ge surface are classified into three types as follows.
1) Oxides such as SiO
2
and GeO
2
2) Carbon impurities such as hydrocarbon
3) Metal impurities such as Na, K, and Fe
Oxides are films naturally formed on the substrate surface when the surface of SiGe or Ge contacts with the atmosphere. Such oxide films are called natural oxide films, and the film thickness is about several nm to 20 nm.
Electronic devices using SiGe substrates or Ge substrates are manufactured in a device production plant using Si substrates. As chemical cleaning technology employed in a device production plant for Si, generally, the RCA method is known (W. Kern and D. A. Puotinen, RCA Vol. 31, 1970, 187).
The typical RCA method is executed in the following procedure (a to 1).
a. Cleaning in ultrapure water for several minutes.
b. Immersing in mixed solution of NH
4
OH, H
2
O
2
, and H
2
O (1:2:7) at 75° C. for more than several minutes.
c. Cleaning in ultrapure water for several minutes.
d. Immersing in 1% hydrofluoric acid at room temperature for several minutes.
e. Cleaning in ultrapure water for several minutes.
f. Immersing in mixed solution of HCl, H
2
O
2
, and H
2
O (1:2:7) at room temperature for more than several minutes.
g. Cleaning in ultrapure water for several minutes.
h. Immersing in 1% hydrofluoric acid at room temperature for several minutes.
i. Cleaning in ultrapure water for several minutes.
j. Immersing in mixed solution of H
2
SO
4
, H
2
O
2
, and H
2
O (1:2:7) at room temperature for more than several minutes.
k. Cleaning in ultrapure water for several is minutes.
1. Drying by spin rotation.
Although the RCA method has been proved to be excellent for cleaning of Si substrate, but it is inappropriate for cleaning of SiGe substrate (or film) or Ge containing substrate (or film). That is because when the surface of SiGe substrate (or film) or Ge containing substrate (or film) is cleaned by RCA method, the surface is seriously roughened. If Si layer or metal layer is deposited on the SiGe substrate (or film) or Ge containing substrate (or film) thus having lost flatness, the defect incidence rate of final products of electronic devices is heightened, and the yield is lowered.
BRIEF SUMMARY OF THE INVENTION
It is hence an object of the invention to present a surface treatment method of semiconductor substrate capable of removing impurities from the surface of a substrate having a surface layer composed of Ge or SiGe, achieving the flat surface and reducing the defect density on the hetero junction boundary.
The surface cleaning method of semiconductor substrate according to an aspect of the invention comprises (a) immersing a substrate having Ge or SiGe mixing Ge and Si at least in the surface layer in a solution of hydrofluoric acid, and removing impurities from the surface of the substrate without overetching the surface of the substrate, (b) pouring pure water on the substrate to wash away the solution of hydrofluoric acid applied in the step (a) from the surface of the substrate, and (c) immersing the substrate in a solution of hydrogen peroxide, and removing foreign matters from the surface of the substrate without overetching the surface of the substrate.
Further, a step (d) may be also a step of pouring pure water on the substrate to wash away the solution of hydrogen peroxide applied in the step (c) from the surface of the substrate, and removing the water applied on the surface of the substrate and then drying the substrate. On the dry surface of the substrate, a next thin film layer is deposited.
Prior to the step (a), preferably, the substrate is washed by running water. As a result, particle deposited on the surface are washed away from the substrate.
The solution of hydrofluoric acid used at step (a) preferably contains hydrofluoric acid by 1 mass % or more to 5 mass % or less. If the concentration of hydrofluoric acid exceeds 5 mass %, the surface of the substrate is overetched. On the other hand, if the concentration of hydrofluoric acid is less than 1 mass %, it takes too much time in processing, and impurities cannot be removed sufficiently. At this step (a), oxides such as SiO
2
and GeO
2
are removed from the surface of the substrate.
In the step (c), an oxide film of 1.0 nm or less in thickness is formed on the surface of the substrate. Such thin oxide film effectively prevents carbide foreign matters in the atmosphere (especially hydrocarbon) from sticking to the surface of the substrate.
In the step (b) or (d), a washing vessel is filled with pure water, the substrate is immersed in pure water in the washing vessel, and pure water is further supplied into the washing vessel while pure water is discharged from the washing vessel to form a flow of pure water in the washing vessel, and the surface of the substrate contacts with a running flow of pure water. As a result, the solution of hydrofluoric acid is removed from the substrate, and the solution of hydrofluoric acid is prevented from being mixed with the solution of hydrogen peroxide at the next step.
Further, putting substrates in a carrier, and arranging a vessel containing a solution of hydrofluoric acid, a vessel containing running flow of pure water, and a vessel containing a solution of hydrogen peroxide sequentially in series, preferably, in the step (a), the carrier containing the substrates is conveyed into the vessel of solution of hydrofluoric acid, the carrier containing the substrates is immersed in the solution of hydrofluoric acid, and the carrier containing the substrates is lifted from the solution of hydrofluoric acid, in the step (b), the carrier containing the substrates is transferred from the vessel of solution of hydrofluoric acid into the vessel of runni
Jr. Carl Whitehead
Mitsubishi Heavy Industries Ltd.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Smoot Stephen W.
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