Desiccant substrate package

A method for protecting substrates used in the manufacture of semiconductors, memory products, and other electronic devices from the effects of moisture during transport and storage is disclosed. This method involves the use of a cassette or box made from polycarbonate or another material having similar hydroscopic properties, treating the cassette or box to reduce its moisture content, and surrounding the cassette or box, and the substrates held therein, with a moisture barrier. This results in a package which will keep the substrates dry and eliminates the need for a separate desiccant within the package.

BACKGROUND OF THE INVENTION 
I. Field of the Invention 
The present invention relates to packaging for storing and shipping 
substrates used in the manufacture of semiconductors, memory products and 
other electronic devices. More specifically, the present invention relates 
to packaging which protects such substrates and electronic devices from 
the harmful effects of moisture during transport and storage. 
II. Description of the Prior Art 
Substrates used in the manufacture of semiconductors, memory products, and 
electronic devices tend to be very fragile and sensitive. Manufacturing of 
such devices is generally done in clean room environments. Much research 
and development has gone into the design of substrate handling equipment 
as well as transport carriers and packages for such substrates. Much of 
this research and development revolves around steps which can be taken to 
protect the substrate and prevent physical damage. See, for example, U.S. 
Pat. No. 5,398,481 granted on Mar. 21, 1995 to Takeuchi et al. 
Substrates used in the manufacture of semiconductor and memory products can 
be damaged in a variety of ways. They are easily scratched, cracked or 
chipped. Very significant problems can arise from chemical reactions which 
can occur when such items are stored in the presence of moisture. 
Various packaging techniques have been used to prevent the problems 
associated with moisture. For example, U.S. Pat. No. 4,553,020 issued on 
Nov. 12, 1985 to Val describes a hermetically sealed, encapsulation 
package for electronic components and integrated circuits. This packaging 
includes a base and cover. The base includes a separate moisture retention 
element which retains any water molecules remaining within the package 
after sealing or which may enter the package through a leak. This 
retention element is made of an absorbent and porous glass type material 
in the zeolite group capable of retaining water molecules. Alternatively, 
the retention element can be formed by a material which reacts chemically 
with water. Examples cited in the Val patent include silicon, titanium, 
zirconium, tantalum, vanadium, aluminum and tin. In each case, supplying 
power to the moisture retaining element is used to improve its 
performance. In the case of zeolite, applying power constitutes a 
degassing treatment which reduces the water molecules in the moisture 
retaining element to a minimum. In the case of the other materials listed 
above, using power to heat the moisture retaining element at predetermined 
intervals after closure of the package ensures that the atoms of the 
reactive metal diffuse toward the surface and through any oxide layer 
which has already been formed to renew the moisture retaining capacity of 
the moisture retaining element. 
Another example of a technique developed to protect substrates and related 
electronics devices from moisture is described in U.S. Pat. No. 5,274,914 
which issued on Jan. 4, 1994 to Kitamura et al. In this patent, the item 
to be packaged, along with a silica gel desiccant, is place inside a 
moisture-proofing bag which is heat-sealed. The bag can be made of a 
laminate film having a polyethylene layer, a polyester layer, a carbon 
conductive layer, an acrylic resin layer, and a vinylidene chloride layer. 
The bag is, of course, sealed before intrusion of moisture occurs. 
U.S. Pat. No. 5,287,962, which issued to Nomi et al on Feb. 22, 1994, also 
represents an effort to deal with the susceptibility of semiconductors to 
moisture because of the permeability of the molding compounds. The Nomi 
patent describes a packaging medium which consists of a shipping means for 
carrying a semiconductor device, a flexible dry-pack bag for containing 
the shipping means, and a vacuum seal indicator. The devices to be 
packaged are baked until dry just prior to packaging. The dry-pack bag is 
vacuum-sealed around the device to be packaged. No desiccant is provided. 
This system relies on the assumption that the device is dry before it is 
placed in the package and will remain dry as long as the vacuum seal is 
not broken. 
Each of the systems recognize the need for substrates and associated 
electronic devices to be kept dry. The Val and Kitamura patents utilize 
separate desiccants which can serve to increase processing and packaging 
costs or serve as a contaminant. A real need exists for a substrate 
packaging system which is economical and does not introduce any potential 
contaminants which could damage the substrate. 
SUMMARY OF THE INVENTION 
One object of the present invention is to provide an economical substrate 
packaging system which is capable of reducing the adverse effects of 
moisture. 
A further object of the present invention is to provide such a packaging 
system which controls moisture levels without the use of a separate 
desiccant. 
Another object of the present invention is to provide a packaging system 
which can contain a plurality of substrates in a protected fashion in a 
moisture-controlled environment. 
These and other objects are achieved by providing a packaging system 
consisting of a cassette and a sealable bag. The cassette is preferably 
made of polycarbonate or some other durable material having similar 
hydrophilic characteristics. The sealed bag provides a moisture-proof 
barrier. The system also requires some suitable means for drying the 
cassette prior to use so that the material of the cassette acts as a 
desiccant during transport and storage of substrates in the package. Such 
means could include a source of warm air, a source of nitrogen, a vacuum 
chamber, or the like. 
One packaging method contemplated by the invention would be to dry the 
cassette to reduce the moisture content of the material from which the 
cassette is made. When sufficiently dried, the substrates are loaded into 
the cassette and the cassette and substrates are sealed inside the bag. 
Another method contemplated by the invention involves loading the 
substrates into the cassette, drying the cassette, and then sealing the 
cassette inside the bag. Still another method might involve the use of a 
cassette consisting of a wafer carrier and a separate box, either or both 
of which can be dried and thus prepared to serve as a desiccant before 
being sealed in the bag with the substrates.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a typical prior art packaging system used to store and 
transport substrates. This packaging system includes a substrate carrier 2 
and a box 3 having a bottom 4 and a top 6. The box 3 is designed to hold 
the substrate carrier 2 and any substrates held therein. Further 
information related to the packaging system shown in FIG. 1 can be found 
in U.S. Pat. No. 4,966,284 which issued on Oct. 30, 1990 in the name of 
Gregerson et al. 
Those skilled in the art will recognize that the prior art also includes 
the use of substrate carriers, like carrier 2 shown in FIG. 1, without a 
separate box. Likewise, the prior art includes boxes designed to hold 
substrates without the need for a separate carrier. Throughout this 
application, the term "cassette" is used generically to cover substrate 
carriers used separately or with a box and boxes used separately or with a 
carrier. As used herein, the word "cassette" is intended to cover any 
portable device designed to hold and protect one or more substrates during 
transport or storage. 
The purpose of the present invention is to protect substrates from the 
harmful effects of moisture during transport and storage. This is achieved 
by making the cassette, or at least a part thereof, out of a hydrophilic 
plastic material. One suitable plastic material is polycarbonate. 
Cassettes made out of hydrophilic materials such as polycarbonate can be 
treated in a number of ways to reduce the moisture content of the 
material. FIG. 2 shows, by way of example, a substrate carrier 2 in 
combination with a gas source 10. The gas source 10 can be used to create 
a flow of gas around the carrier 2 to dry out the hydrophilic material 
from which the carrier 2 is made. The gas could, for example, be nitrogen 
or even heated air. Bathing the carrier 2 in such a gas serves to reduce 
moisture content of the hydrophilic material. 
FIGS. 3 and 4 show alternative techniques for reducing the moisture content 
of the hydrophilic material from which the carrier 2 is made. The carrier 
2, for example, can be placed inside a vacuum chamber 16 as shown in FIG. 
3. Evacuation of the chamber 16 using a pump 14 will serve to draw 
moisture from the hydrophilic material to reduce the total moisture 
content. The carrier 2 could also be placed in an oven 17 as shown in FIG. 
4. Using heating elements 18 to warm the oven 17 to a temperature between 
100.degree. F. and 150.degree. F. will serve to reduce the moisture 
content of the carrier 2. This temperature range, while high enough to 
draw the moisture out of the hydrophilic plastic material, is not high 
enough to melt the plastic or otherwise undermine the structural integrity 
of the carrier 2. 
FIGS. 2-4 show the cassette being dried without the substrates in place. 
This may be preferred to prevent the substrates from being damaged by the 
drying process. However, the substrates can be present in the cassette 
during the drying operation as long as the particular drying process used 
will not adversely affect the substrates. Many substrates can easily 
withstand temperatures in the 100.degree. F. to 150.degree. F. range and 
are not adversely affected by nitrogen, warm air or vacuum treatment. 
FIGS. 5 and 6 each show cassettes surrounded by a moisture-proof barrier 
20. This moisture-proof barrier 20 is added after the cassettes have been 
dried to encapsulate the cassette and its contents. Given this 
arrangement, the dried cassette acts as a desiccant absorbing any water 
molecules within the moisture-proof barrier 20. The moisture-proof barrier 
20 prevents water molecules from passing through the barrier or at least 
limits the number of molecules passing through to a level where such 
molecules will also be absorbed by the cassette. 
The moisture-proof barrier 20 can be made of any one of a number of 
suitable materials. Definite advantages are achieved by using a laminated 
plastic film which can be heat sealed. One such laminate includes a layer 
of polyethylene, a layer of polyester, a carbon conductive layer, an 
acrylic resin layer, and a vinylidene layer. 
When the packaging system of the present invention is used, a cassette 
comprising a substrate carrier 2, or a box 3, or both, is provided. At 
least a portion of the cassette is made of a hydrophilic plastic material. 
The material is treated as described above and then is sealed, along with 
the substrates, in a moisture-proof barrier 20. The substrates may be 
inserted into the cassette before the cassette is treated or between the 
treatment of the cassette and encapsulation of the cassette in a 
moisture-proof barrier 20. When this procedure is used, the cassette acts 
as a desiccant and works in combination with the moisture-proof barrier 20 
to protect the substrates from moisture.