Electronic apparatus having thin film transistors

Details Electronic apparatus having thin film transistors Electronic apparatus having thin film transistors

1999-01-19

2001-03-13

Sikes, William L. (Department: 2871)

Liquid crystal cells, elements and systems

Particular excitation of liquid crystal

Electrical excitation of liquid crystal

C349S005000, C349S006000

Reexamination Certificate

active

06201585

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed relates to an electronic apparatus in which an optical engine (optical system) incorporates a liquid crystal display (hereinafter abbreviated as LCD) having a circuit that is constituted of thin-film transistors (hereinafter abbreviated as TFTs). In particular, the present invention relates to an electronic apparatus such as a projector that uses a transmission-type LCD.
2. Description of the Related Art
In recent years, the technique of forming TFTs on a substrate, such as a glass substrate, by using a semiconductor thin film, (typically a silicon thin film) has made rapid progress. Active matrix LCDs, that have a circuit using such TFTs as switching elements and that perform optical modulation by a liquid crystal, are now attracting much attention as next-generation display devices.
Among the electronic apparatuses now attracting a lot of attention and which use an active matrix LCD as a display medium, are transmission-type and reflection-type projectors.
FIG. 3A
outlines a basic optical engine of a three-panel transmission-type projector.
In
FIG. 3A
, reference numeral
301
denotes a lamp light source;
302
and
303
are dichroic mirrors that reflect only red light and green light, respectively;
304
-
306
are full-reflection mirrors;
307
-
309
are transmission-type LCDs corresponding to R (red), G (green), and B (blue), respectively;
310
is a dichroic prism; and
311
is a projection lens.
Strong light is emitted from the lamp light source
301
, and only the R component beam is reflected by the dichroic mirror
302
. The R component beam is changed in traveling direction by the full-reflection mirror
306
and enters the R-LCD
307
.
Only the G component beam is reflected by the G dichroic mirror
303
and enters the G-LCD
308
.
The B component light, that has passed through the dichroic mirrors
302
and
303
, is reflected by the full-reflection mirrors
304
and
305
and enters the B-LCD
309
.
Light beams carrying video information, that have passed through the R, G, and B-LCDs
307
-
309
, are recombined by the dichroic prism
310
and projected onto a screen by the projection lens
311
.
FIG. 3B
is a schematic sectional view of each of the transmission-type LCDs
307
-
309
. In
FIG. 3B
, reference numerals
312
and
313
denote pixel TFTs. A light beam
314
carrying video information that is applied from above is interrupted by black masks
315
and
316
.
Since the number of carriers in a semiconductor increases when it is excited by light, it is necessary to prevent a light beam from being applied directly to the active layers of TFTs by using black masks as mentioned above. However, since a substrate
300
on which the TFTs
312
and
313
are formed is as thick as about 0.7-1.1 mm, part of the light beam
314
carrying video information is irregularly reflected inside the substrate
300
to cause stray light
317
and
318
.
If stray light reaches an active layer, the number of carriers increases through optical excitation. The increase in the number of carriers results in leak current (off-current) that flows when the TFT is in an off-state. A large leak current may cause a display failure.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an electronic apparatus which is so configured as to suppress leak current that is caused by stray light in the manner as described above (hereinafter called stray light leak current), to such a level that it causes no problems and high-quality image display can be realized.
According to a first aspect of the present invention, there is provided an electronic apparatus having an optical engine including three LCDs, wherein each of the three LCDs has a circuit including a plurality of TFTs; and wherein the TFTs of one of the three LCDs, that is illuminated with the shortest wavelength light among the three LCDs, have a smaller active layer thickness than the TFTs of the other two LCDs.
According to a second aspect of the present invention, there is provided an electronic apparatus having an optical engine including three LCDs, wherein each of the three LCDs has a circuit including a plurality of TFTs; and wherein the TFTs have a smaller active layer thickness as the wavelength of light to illuminate the associated one of the three LCDs becomes shorter.
According to a third aspect of the present invention, there is provided an electronic apparatus having an optical engine including three LCDs for red display, green display, and blue display, respectively, wherein each of the three LCDs has a circuit including a plurality of TFTs; and wherein the TFTs of the LCD for the blue display have a smaller active layer thickness than the TFTs of the LCDs for the red display and the green display.
In the above electronic apparatuses, it is preferable that the TFTs of the LCD for the blue display have an active layer thickness of 30 nm or less, the TFTs of the LCD for the green display have an active layer thickness of 50 nm or less, and the TFTs of the LCD for the red display have an active layer thickness of 100 nm or less.
As described above, one aspect of the present invention is intended to decrease stray light leak current and obtain superior image quality in which display failures such as crosstalk are in such levels as to be not problematic, by optimizing the active layer thickness in accordance with the wavelength of the light for illuminating a LCD.
By optimizing the active layer thickness in accordance with the wavelength of illumination the light, the present invention can also greatly increase the yield.
Crosstalk may be prevented by making every active layer thickness small. However, if the active layers are too thin, there occurs a problem in that the process margins are decreased.
For example, in a step of etching an insulating film on the active layers, the active layers are etched little by little during over-etching. Therefore, if the active layers are too thin, there is a possibility that even the active layers may disappear. Hence, the thin active layers make the process control difficult, possibly resulting in a reduction in yield.
In contrast, in the present invention, the active layers of at least the LCDs for the green display and the red display can be made thick, and hence the reduction in yield can be decreased for those LCDs. Therefore, the reduction in the total yield of a three-panel optical engine can be prevented by thinning the active layers of only the LCD that needs thin active layers.
On the other hand, if an optical engine can be constructed by using three LCDs having the same structure, high productivity is surely obtained because all of the LCDs can be manufactured by a single process. In this case, it is necessary to set the active layer thickness so that an off-current value of the TFTs measured when the same LCD is illuminated with green light or blue light is smaller than or equal to two times an off-current value of the TFTs measured when the LCD is illuminated with red light, where the red light, the green light, and the blue light corrected so as to have the same light intensity.


REFERENCES:
patent: 5499126 (1996-03-01), Abileah et al.
patent: 5656825 (1997-08-01), Kusumoto et al.
patent: 5681756 (1997-10-01), Norman et al.
patent: 5743611 (1998-04-01), Yamaguchi et al.
patent: 5825052 (1998-10-01), Shakuda
patent: 5963276 (2000-01-01), Inbar
patent: 6013930 (2000-01-01), Yamazaki et al.
patent: 8-78329 (1996-03-01), None
patent: 9-312260 (1997-12-01), None

Affiliated with

Also associated with

Rating

Electronic apparatus having thin film transistors does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Electronic apparatus having thin film transistors, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Electronic apparatus having thin film transistors will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2535695