Adhesive bonding and miscellaneous chemical manufacture – Differential fluid etching apparatus – With microwave gas energizing means
Reexamination Certificate
2000-04-25
2001-08-14
Dang, Thi (Department: 1763)
Adhesive bonding and miscellaneous chemical manufacture
Differential fluid etching apparatus
With microwave gas energizing means
Reexamination Certificate
active
06273992
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The invention relates to processes for the manufacture of semiconductor devices and more particularly to processes related to the analysis of impurities in silicon wafers.
(2) Description of Prior Art
The manufacture of very large scale integrated (VLSI) circuits involves hundreds of discrete processing steps beginning with the introduction of blank silicon wafers. The quality an purity of the starting silicon wafers is, without question, one of the most crucial factors in the performance of the semiconductor devices in the finished product. The current high density, high performance, low cost technology makes widespread use of the metal oxide silicon field effect transistor which depends upon a thin silicon oxide gate insulator. This gate oxide is grown by thermal oxidation of the surface of the silicon wafer.
Trace metallic Impurities within the wafer surface and in the chemicals used to grow layers thereon or to clean or treat oxide layers thereon have a deleterious effect on the performance of the gate oxide as well as on it's reliability. Because of these serious consequences great strides have been taken to provide the highest quality control of the starting material. Additionally, the processing of defective wafers can result in enormous yield losses.
Fortunately, great strides have been by taken by silicon wafer manufacturers to provide reliable substrates. Analytical methods have been found widespread use to properly qualify and characterize silicon wafers. Among these are atomic absorption spectroscopy, emission spectroscopy, inductively coupled mass spectrometry, and X-ray fluorescence.
A well known sampling method which has been developed and cited by Maeda, et. al., U.S. Pat. No. 4,990,459 is a vapor phase decomposition (VPD) technique. The VPD technique extracts and concentrates trace levels of metallic contaminants from the surface of a test wafer by decomposing a layer of silicon oxide with HF vapors. The residue, which contains the non volatile impurities is then collected in a small droplet of a suitable acid such as hydrofluoric acid. The droplet is systematically moved across the entire wafer surface so that all the residue is collected. The recovered droplet is then analyzed by the well known analytical methods mentioned hereinbefore.
Referring to
FIG. 1
there is shown a cross section of a prior art sampling technique using VPD, as cited by Maeda et.al. a test wafer
10
having a silicon oxide layer
11
on its surface is placed into a closed chamber
12
. A pool of aqueous HF
13
, located elsewhere within the chamber
12
, emits HF vapors
14
which fill the chamber and, in time, decompose the silicon oxide layer
11
. The wafer is then removed and any residue on the polished side of the wafer
10
is collected by a manual method involving the passage of a collection droplet across the wafer surface by tilting the wafer, thereby rolling the drop over the entire surface.
In an earlier patent by the present inventors, Petvai, et.al. U.S. Pat. No. 5,569,328, the sample collection technique was greatly improved by providing automating the movement of the collection droplet. An inert carrier is used to contain the droplet as well as increase the contact area of the droplet. Not only is the reliability and reproducibility of sample collection improved by this apparatus, but the cycle time and the risk of external contamination are greatly reduced. The wafer is mounted on a table having a programmable rotation. The apparatus provides a robotic arm which transports the wafers from a cassette to a VPD chamber where HF vapors decompose the silicon oxide layer. The wafer then passes to the droplet collection station where the sample is collected by a droplet on a pre-loaded sample carrier delivered from a carousel. The entire apparatus operates in an internal class
1
environment.
The long time required to decompose the silicon oxide layer by the use of vapor etching technique illustrated by
FIG. 1
affects the cycle time and thereby limits the production capability of the apparatus. This is especially true when thicker, thermally grown, silicon oxide layers are examined. The flash mist method provided by the current invention greatly increases the decomposition rate of the oxide layer.
In order to place the embodiments of this invention into a proper perspective, a brief review of the prominent details of the acid droplet fluid scanner apparatus cited by Petvai, et.al. is now given utilizing
FIG. 2
which corresponds to FIG. 4 of that patent.
Wafers, loaded in a cassette, are introduced into the system
41
which encloses a class
1
particle environment, through a small systems interface
42
and placed on cassette stand
44
. A pickup fork
46
, under robotic control
50
transports a test wafer (not shown) from the cassette stand
44
, first to a bar code reader
51
, where the wafer is identified, and thence to a VPD etching chamber
52
wherein the silicon oxide layer is decomposed. The robotic arm
50
then delivers the wafer to a rotatable table
54
which is fitted with a vacuum chuck. A translating arm mechanism
58
retrieves a droplet carrier from a carrousel (not shown) and positions the carrier on the fork
57
near the edge of the mounted wafer. A precision liquid handler
61
on the robotic arm
50
retrieves a pre-measured volume of liquid and delivers it to the droplet carrier. The wafer table
54
is rotated in a prescribed sequence as the translational arm
58
moves the captured droplet toward the center of the wafer, thereby traversing the entire wafer surface and collecting any residue for the analysis. At the completion of this cycle, the liquid handler
61
retrieves the droplet from the droplet carrier and deposits it back either into a cup in the carousel
60
of an auto sampler, where it is retained for analysis, or onto an inert membrane fitted onto a carrier fixture designed for any of the analytical equipment of choice. The wafer is delivered to the receiving cassette
62
. The computer system
47
with accompanying keyboard
48
and mouse
49
are also shown.
SUMMARY OF THE INVENTION
It is an object of this invention to provide an improved method for decomposition of a silicon oxide layer on a silicon wafer which is much faster than prior art methods thus enabling it's usage for real-time production lines.
It is another object of this invention to provide an improved method for decomposition of a silicon oxide layer on a silicon wafer which can be incorporated in a fast cycle time, production capacity, fully automated multi-wafer testing apparatus.
It is yet another object of this invention to provide a method and apparatus for producing ultra clean wafer surfaces as a final step in blank wafer production, and as a surface preparation step for most manufacturing processes in semiconductor manufacturing lines.
It is another object of this invention to describe an efficient and reliable processing station for decomposing a silicon oxide layer on a silicon wafer and collecting an analysis sample of residues by fluid scanning which can be applied as a fast cycle time, production capacity automated wafer testing apparatus.
It is yet another object of this invention to provide a method and apparatus for depositing ultra thin uniform liquid films on flat substrates.
It is yet another object of this invention to provide a method and apparatus for depositing ultra clean thin liquid films, said films being free of all detectable metallic impurities on flat substrates.
These objects and others which will become apparent are accomplished by an apparatus which creates an ultra clean flowing mist of liquid droplets. The mist flow is directed towards the center of a cooled substrate, whereupon it spreads radially over the surface of the substrate. The larger droplets in the stream are drawn to the surface of the cooled substrate by Bernoulli action of the flowing gas.
The deposited liquid layer of aqueous HF reacts with the silicon oxide layer at a much faster rate than the HF v
Bohnenkamp Leslie Jane
Buet Michael P.
Petvai Steve I.
Ackerman Stephen B.
Dang Thi
Saile George O.
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