U.S. patent application number 08/730015 was filed with the patent office on 2002-03-14 for thin film semiconductor device having a buffer layer.
Invention is credited to HAYASHI, HISAO, KATO, KEIJI, SHIMOGAICHI, YASUSHI.
Application Number | 20020030188 08/730015 |
Document ID | / |
Family ID | 17773443 |
Filed Date | 2002-03-14 |
United States Patent
Application |
20020030188 |
Kind Code |
A1 |
HAYASHI, HISAO ; et
al. |
March 14, 2002 |
THIN FILM SEMICONDUCTOR DEVICE HAVING A BUFFER LAYER
Abstract
A thin film semiconductor device having improved operating
characteristics and reliability of a thin film transistor formed on
a glass substrate. The thin film semiconductor device has a thin
film transistor 3 formed on a glass substrate 1 containing alkali
metal. The surface of the glass substrate 1 is covered by a buffer
layer 2. The thin film transistor 3 formed on this buffer layer 2
has a polycrystalline semiconductor thin film 4 as an active layer.
The buffer layer 2 includes at least a silicon nitride film and
protects the thin film transistor 3 from contamination by alkali
metals such as Na and has a thickness such that it can shield the
thin film transistor 3 from an electric field created by localized
alkali metal ions (Na.sup.+).
Inventors: |
HAYASHI, HISAO; (KANAGAWA,
JP) ; SHIMOGAICHI, YASUSHI; (KANAGAWA, JP) ;
KATO, KEIJI; (KANAGAWA, JP) |
Correspondence
Address: |
SONNENSCHEIN NATH & ROSENTHAL
P.O. BOX 061080
WACKER DRIVE STATION
CHICAGO
IL
60606-1080
US
|
Family ID: |
17773443 |
Appl. No.: |
08/730015 |
Filed: |
October 11, 1996 |
Current U.S.
Class: |
257/58 ;
257/E29.276; 257/E29.295 |
Current CPC
Class: |
H01L 29/78603 20130101;
H01L 29/78606 20130101 |
Class at
Publication: |
257/58 |
International
Class: |
H01L 031/20; H01L
031/0376; H01L 031/036; H01L 029/04 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 13, 1995 |
JP |
P07-291788 |
Claims
What is claimed is:
1. A thin film semiconductor device comprising: a glass substrate
containing an alkali metal; a buffer layer covering the surface of
said glass substrate; and a thin film transistor formed on said
buffer layer and having a polycrystalline semiconductor thin film
as an active layer, wherein said buffer layer includes at least a
silicon nitride film and protects said thin film transistor from
alkali metal contamination and has a thickness such that it can
shield said thin film transistor from an electric field created by
localized alkali metal ions.
2. A thin film semiconductor device according to claim 1, wherein
said silicon nitride film has a thickness of at least 20 nm.
3. A thin film semiconductor device according to claim 1, wherein
said buffer layer has a thickness of at least 100 nm.
4. A thin film semiconductor device according to claim 1, wherein
said thin film transistor has a bottom gate structure comprising a
gate electrode, a gate insulating film and a polycrystalline
semiconductor thin film superposed in order from the bottom.
5. A thin film semiconductor device according to claim 4, wherein
said polycrystalline semiconductor thin film has channel region
located directly above the gate electrode, high concentration
impurity regions located on either side of said channel region and
low concentration impurity regions located between said channel
region and said high concentration impurity regions, said low
concentration impurity regions being shielded from an electric
field arising in said glass substrate by said buffer layer.
6. A thin film semiconductor device according to claim 4, wherein
said gate insulating film contains a silicon nitride layer and is
superposed with said buffer layer and the two synergetically
protect and shield said thin film transistor.
7. A thin film semiconductor device according to claim 6, wherein
the total thickness of the mutually superposed gate insulating film
and buffer layer is at least 100 nm.
8. A thin film semiconductor device according to claim 7, wherein
the total thickness of said gate insulating film and buffer layer
is at least 200 nm.
9. A thin film semiconductor device according to claim 1, wherein
said buffer layer is of a two-layer structure made up of a silicon
nitride film and a silicon oxide film.
10. A thin film semiconductor device according to claim 1, wherein
a pixel electrode is formed connected to at least a part of said
thin film transistor.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to a thin film semiconductor device
used as a driving substrate of an active matrix liquid crystal
display panel or the like. More particularly, it relates to a thin
film semiconductor device using ordinary glass as a substrate and
made by low temperature processes. Still more particularly, it
relates to technology for preventing adverse affects of alkali
metals contained in the glass.
[0002] A thin film semiconductor device is a device wherein a thin
film transistor is formed on an insulating substrate, and because
they are ideal for example for driving substrates of active matrix
liquid crystal display panels their development has been being
advanced vigorously in recent years. Particularly when using a thin
film semiconductor device in a large-area liquid crystal display
panel, it is essential to reduce the cost of the insulating
substrate, and glass substrates are being employed instead of the
relatively high quality quartz substrates used in the past. When a
glass substrate is used, because its heat resistance is relatively
low, the thin film transistors must be formed by low temperature
processes of below 600.degree. C. Now, as a semiconductor thin film
constituting active layers of the thin film transistors, amorphous
silicon and polycrystalline silicon have been used. However, from
the point of view of the operating characteristics of the thin film
transistors, polycrystalline silicon is superior to amorphous
silicon. For this reason, the development of polycrystalline
silicon thin film transistors made by low temperature processes has
been being advanced in recent years.
[0003] When polycrystalline silicon is used as an active layer of a
thin film transistor formed on a glass substrate, contamination
caused by alkali metals such as sodium (Na) contained in the glass
substrate has been a problem. Polycrystalline silicon is more
sensitive to alkali metal contamination than amorphous silicon, and
with polycrystalline silicon such contamination has an adverse
influence on the operating characteristics and reliability of the
thin film transistor. For example, if an alkali metal diffuses into
the gate insulating film of a thin film transistor the device
characteristics change. When at a high temperature a bias is
applied and an operating test is carried out, the device
characteristics change greatly because alkali metal in the gate
insulating film moves and polarizes and concentrates in localities.
Consequently, when thin film transistors have been formed on a
glass substrate, the practice of forming in advance as a base layer
a silicon nitride film (SiN.sub.x) or a phosphorus-containing glass
(PSG) as a buffer layer has been carried out. By this buffer layer
being interposed, the vertical diffusion of alkali metal from the
glass substrate toward the gate insulating film is suppressed and
contamination of the gate insulating film is prevented.
[0004] However, it has become clear that just preventing vertical
movement of alkali metal is not sufficient. That is, horizontal
diffusion of alkali metal included in the glass substrate occurs
due to bias of the driving voltage impressed on the thin film
transistor, and alkali metal ions polarize and concentrate locally.
An electric field is created by local polarization of charges of
alkali metal ions, and this reversely has an adverse affect on the
operating characteristics of the thin film transistor. It has
become clear that as a result of this the threshold voltage and the
leak current of the thin film transistor undergo fluctuations. It
is extremely difficult to prevent this horizontal movement of
alkali metal in the glass substrate. For this reason, for example
in U.S. Pat. No. 5,349,456 a method for removing Na from a glass
substrate is disclosed. However, this method is not always
practical because it greatly diminishes the merit of using a low
cost glass substrate.
SUMMARY OF THE INVENTION
[0005] Accordingly, it is an object of the invention to solve the
problem described above and provide a thin film semiconductor
device comprising a thin film transistor formed on a glass
substrate wherein an electric field arising as a result of
horizontal diffusion of alkali metal in the glass substrate is
effectively and cheaply prevented from adversely affecting the
operating characteristics of the thin film transistor.
[0006] To achieve the above-mentioned object and other objects, a
thin film semiconductor device according to the invention comprises
as a basic construction a glass substrate containing an alkali
metal, a buffer layer covering the surface of the glass substrate
and a thin film transistor formed on the buffer layer with a
polycrystalline semiconductor thin film as an active layer. As a
characterizing feature of the invention, the buffer layer includes
at least a silicon nitride film and protects the thin film
transistor from alkali metal contamination and has a thickness such
that it ca n shield the thin film transistor from an electric field
created by localized alkali metal ions. In one form of the
invention, the thin film transistor has a bottom gate structure
wherein a gate electrode, a gate insulating film and a
semiconductor thin film are superposed in order from the bottom. In
this case, the semiconductor thin film has a channel region located
directly above the gate electrode, high concentration impurity
regions located on either side of the channel region and low
concentration impurity regions interposed between the channel
region and the high concentration impurity regions. The low
concentration impurity regions are shielded from an electric field
forming in the glass substrate by the buffer layer. Preferably, the
gate insulating film includes a silicon nitride layer and is
superposed with the buffer layer and the two synergetically protect
and shield the thin film transistor. In this case, the total
thickness of the mutually superposed gate insulating film and
buffer layer is over 200 nm. The buffer layer is preferably a
two-layer structure made up of a silicon nitride film and a silicon
oxide film. In a specific construction, a pixel electrode is formed
connected to at least a part of the thin film transistor and the
thin film semiconductor device can be used in a driving substrate
of an active matrix display panel.
[0007] In the invention, a buffer layer is interposed between a
glass substrate and a thin film transistor. This buffer layer
includes at least a silicon nitride film, and blocks vertical
movement of alkali metal and thereby suppresses contamination of
the gate insulating film. The silicon nitride film has a fine
composition, and by making its thickness above 20 nm it is possible
to substantially completely prevent Na and the like from passing
through it. Also, in addition to the silicon nitride film this
buffer layer includes for example a silicon oxide film and has a
two-layer structure. Because film stresses in the silicon oxide
film are smaller than in the silicon nitride film it is possible to
make the thickness of the buffer layer as a whole large and thereby
electrically separate the thin film transistor from the glass
substrate. By making the thickness of the buffer layer at least 100
nm it is possible to electrically shield the thin film transistor
from the glass substrate. Therefore, it is possible to shield the
thin film transistor from adverse affects of an electric field
formed as a result of horizontal diffusion of alkali metal inside
the glass substrate. As a result, it becomes possible to maintain
the reliability and operating characteristics of the thin film
transistor even when a glass substrate containing alkali metal is
used.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a schematic sectional view of a first preferred
embodiment of a thin film semiconductor device according to the
invention;
[0009] FIG. 2 is a schematic sectional view of a second preferred
embodiment of a thin film semiconductor device according to the
invention;
[0010] FIG. 3 is a schematic sectional view of a third preferred
embodiment of a thin film semiconductor device according to the
invention; and
[0011] FIG. 4 is a schematic perspective view of an example of an
active matrix liquid crystal display panel assembled using a thin
film semiconductor device according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0012] Preferred embodiments of the invention will now be described
in detail with reference to the accompanying drawings. FIG. 1 shows
a first preferred embodiment of a thin film semiconductor device
according to the invention, and is an example wherein a thin film
transistor of N-channel type and of top gate structure is formed on
a glass substrate. As shown in FIG. 1, this thin film semiconductor
device is made using a glass substrate 1 containing an alkali metal
such as Na. The upper surface of the glass substrate 1 is covered
by a buffer layer 2. A thin film transistor 3 is formed on the
buffer layer 2. The thin film transistor 3 is a field effect
transistor having a polycrystalline semiconductor thin film 4
consisting of polycrystalline silicon or the like as an active
layer. The thin film transistor 3 has a top gate structure, and a
gate electrode G is formed by patterning on a gate insulating film
5 on the polycrystalline semiconductor thin film 4. As a result, a
channel region Ch is formed directly below the gate electrode G
with the gate insulating film 5 therebetween. A small amount of a
P-type impurity is diffused into this channel region Ch part of the
polycrystalline semiconductor thin film 4 for threshold value
adjustment. A source region S and a drain region D impregnated with
an N-type impurity at a high concentration are provided on opposite
sides of the channel region Ch. The thin film transistor 3 having
this construction is covered with an interlayer insulating film 6
consisting of PSG or the like. Contact holes are formed in the
interlayer insulating film 6, and through these contact holes
interconnection electrodes 7S, 7D are electrically connected to the
source region S and the drain region D respectively. In this
example an N-type impurity is injected to form an N-channel type
thin film transistor 3, but of course the invention is not limited
to this and can also be applied to a P-channel type thin film
transistor.
[0013] As a characterizing feature of the invention the buffer
layer 2 includes at least a silicon nitride film, and protects the
thin film transistor 3 from alkali metal contamination. The silicon
nitride film (SiN.sub.x) has a relatively fine composition, and by
making its thickness at least 20 nm it is possible to substantially
completely block the vertical upward diffusion of alkali metals
such as Na contained in the glass substrate 1. Also, this buffer
layer 2 has a thickness such that it can shield the thin film
transistor 3 from an electric field resulting from localized alkali
metal ions (Na.sup.+) and the like. For example the buffer layer 2
has a two-layer structure made up of the silicon nitride film
(SiN.sub.x) and a silicon oxide film (SiO.sub.2) and has a total
thickness of at least 100 nm.
[0014] The electric field shielding function of the buffer layer 2,
which is a characterizing feature of the invention, will now be
described in more detail. When the thin film transistor 3 is
operated, there are times when for example a ground potential (0V)
is impressed on the interconnection electrode 7S on the source
region S side and a positive bias voltage is impressed on the
interconnection electrode 7D connected to the drain region D. When
this kind of bias is applied to the device, Na.sup.+ ions, which
are positive charges, are excluded from the vicinity of the drain
region D and move horizontally to the vicinity of the source region
S. As a result, as shown in FIG. 1, positive charges (Na.sup.+)
concentrate in the vicinity of the source region S near the surface
of the glass substrate 1 and a positive region 8 is formed.
Meanwhile, in the vicinity of the drain region D near the surface
of the glass substrate 1, because the charge equilibrium breaks
down by an amount corresponding to the exclusion of the Na.sup.+, a
negative region 9 is formed. In this way an electric field
resulting from localization of Na.sup.+ forms in the vicinity of
the surface of the glass substrate 1. The operating characteristics
of the thin film transistor 3 are adversely affected by this
electric field, resulting in fluctuation of its threshold voltage
and increase of its leakage current. To avoid this, in this
invention the buffer layer 2 is interposed between the thin film
transistor 3 and the glass substrate 1. Because this buffer layer 2
has a two-layer structure made up of SiN.sub.x and SiO.sub.2 and
has an ample thickness, it substantially completely shields the
thin film transistor 3 from electric fields forming in the glass
substrate 1. Furthermore, because the buffer layer 2 includes an
SiN.sub.x film, it substantially completely blocks vertical
movement of Na in the same way as in the related art and thereby
prevents contamination of the gate insulating film 5.
[0015] FIG. 2 shows a second preferred embodiment of a thin film
semiconductor device according to the invention, and shows an
example of a bottom gate structure. The basic structure is the same
as that of the first preferred embodiment shown in FIG. 1, and
corresponding parts have been given the same reference numerals to
facilitate understanding. As shown in FIG. 2, a thin film
transistor 3a has a bottom gate structure wherein a gate electrode
G made of metal or the like, a gate insulating film 5 and a
polycrystalline semiconductor thin film 4 are superposed in order
from the bottom. The thin film transistor 3a having this
construction is protected and shielded from the glass substrate 1
by a buffer layer 2. The thin film transistor 3a is covered by an
interlayer insulating film 6, and an interconnection electrode 7S
and a pixel electrode 10 are formed on the interlayer insulating
film 6. The pixel electrode 10 is electrically connected to the
drain region D of the thin film transistor 3a through a contact
hole. A thin film semiconductor device having this construction can
be used for example in a driving substrate of an active matrix
liquid crystal display panel. That is, the thin film transistor 3a
is formed as a switching element of a pixel electrode 10.
[0016] With the bottom gate structure also, as with the top gate
structure shown in FIG. 1, when a bias is impressed on the drain
region D side the influence of this bias causes a polarization of
the charge distribution in the glass substrate 1 to arise and a
positive region and a negative region form. Therefore, the buffer
layer 2 is provided to shield the thin film transistor 3a from the
influence of electric fields forming in the glass substrate 1.
Because in the case of the bottom gate structure the gate electrode
G made of metal or the like is interposed between the
polycrystalline semiconductor thin film 4 and the glass substrate
1, the proportion of the semiconductor thin film 4 affected by
electric fields forming inside the glass substrate 1 is less than
in the case of the top gate structure. That is, even if a biased
presence of Na were to occur inside the glass substrate 1 below the
channel region Ch, because in addition to the buffer layer 2 there
is a shielding effect of the gate electrode G, the channel region
Ch itself is not so affected by the electric field in the glass
substrate 1. Furthermore, in the case of the bottom gate structure,
because with respect to the bias between the source region S and
the drain region D the gate voltage impressed on the gate electrode
G is always at a potential level between the source region and the
drain region, biased presences of charges inside the glass
substrate 1 would not be expected to occur as much as in the case
of the top gate structure.
[0017] FIG. 3 is a partial sectional view of a third preferred
embodiment of a thin film semiconductor device according to the
invention. This third preferred embodiment is basically the same as
the second preferred embodiment shown in FIG. 2, and corresponding
parts have been given the same reference numerals to facilitate
understanding. The point of difference is that in this third
preferred embodiment the thin film transistor has an LDD (Lightly
Doped Drain) structure. As shown in FIG. 3, the thin film
transistor 3a has a bottom gate structure wherein a gate electrode
G, a gate insulating film 5 and a polycrystalline semiconductor
thin film 4 are superposed in order from the bottom. The
polycrystalline semiconductor thin film 4 has a channel region Ch
located directly above the gate electrode G, high concentration
impurity regions (N+) located on opposite sides of the channel
region Ch and low concentration impurity regions (N) located
between the channel region and the high concentration impurity
regions. A high concentration impurity region (N+) constitutes a
drain region D, and a low concentration impurity region (N)
constitutes an LDD region. In FIG. 3 only the drain region D side
of the thin film transistor 3a is shown, and the source region S
side is omitted. In this example at least the LDD region is
shielded from an electric field forming in the negative region 9 of
the glass substrate 1 by a buffer layer 2. When an LDD region is
formed away from the gate electrode G, unlike the example shown in
FIG. 2, as in the case of the top gate structure shown in FIG. 1
the semiconductor thin film is influenced by charges inside the
glass substrate 1. For this reason, in this example the buffer
layer 2 for weakening the influence of charges is provided between
the glass substrate 1 and the polycrystalline semiconductor thin
film 4. In this example the gate insulating film 5 includes a
silicon nitride layer and is superposed with the buffer layer 2 and
the two synergetically protect and shield the thin film transistor
3a. The total thickness of the mutually superposed gate insulating
film 5 and buffer layer 2 is over 200 nm. Because in the bottom
gate structure the buffer layer 2 and the gate insulating film 5
are superposed and a synergetic electric field shielding effect is
obtained in this way, it is possible to electrically separate the
LDD region from the negative region 9 of the glass substrate 1
substantially completely.
[0018] FIG. 4 is a schematic perspective view showing an example of
an active matrix liquid crystal display panel assembled using the
thin film semiconductor device shown in FIG. 2 or FIG. 3. As shown
in FIG. 4, the liquid crystal display panel is made up of a driving
substrate 101 made of glass, a facing substrate 102 also made of
glass and a liquid crystal 103 held between the two. A pixel array
part 104 and a driving circuit part are formed on the driving
substrate 101. The driving circuit part is divided into a vertical
driving circuit 105 and a horizontal driving circuit 106. Also,
terminal parts 107 for outside connections are formed on a
peripheral part of the driving substrate 101. The terminal parts
107 are connected to the vertical driving circuit 105 and the
horizontal driving circuit 106 by way of interconnections 108. The
pixel array part 104 comprises mutually intersecting gate lines 109
and signal lines 110. The gate lines 109 are connected to the
vertical driving circuit 105 and the signal lines 110 are connected
to the horizontal driving circuit 106. Pixel electrodes 111 and
thin film transistors 112 for switching these are formed at the
intersections of the lines 109, 110. Although not shown in the
drawing, facing electrodes and color filters are formed on the
inner surface of the facing substrate 102. In this invention an
ordinary glass material is used as the driving substrate 101, and
the thin film transistors 112 and the pixel electrodes 111 are
formed on the driving substrate 101 after the surface thereof is
covered with a buffer layer. Also, the vertical driving circuit 105
and the horizontal driving circuit 106 are formed at the same time.
Therefore, because it is possible to use a cheap glass material, it
is possible to make a large-area active matrix liquid crystal
display panel at a relatively low cost. At this time, because a
buffer layer having both an alkali metal contamination preventing
function and an electric field shielding function is used, there is
no risk of the glass substrate adversely affecting the reliability
or operating characteristics of the thin film transistors.
[0019] As described above, according to the invention, a buffer
layer is interposed between a glass substrate and a thin film
transistor. This buffer layer includes at least a silicon nitride
film, and as well as protecting the thin film transistor from
alkali metal contamination it has a thickness such that it can
shield the thin film transistor from an electric field created by
localized alkali metal ions. Consequently, it is possible to avoid
suffering the affects of electric fields inside the glass substrate
and obtain stable thin film transistor operating characteristics.
Also, because it is possible to prevent alkali metal contamination
of the thin film transistor, its reliability is improved.
* * * * *