Direct multilevel thin-film transistors production method

Details Direct multilevel thin-film transistors production method Direct multilevel thin-film transistors production method

1996-02-22

1998-11-03

Niebling, John

Semiconductor device manufacturing: process

Making field effect device having pair of active regions...

On insulating substrate or layer

438 30, H01L 2100

Patent

active

058307850

DESCRIPTION:

BRIEF SUMMARY
This invention relates to a method for producing direct multilevel thin-film transistors (TFTs) with a small number of mask levels, to form a contact between a transistor gate and the source or drain of the same or another transistor, and for use in producing flat liquid crystal screens, particularly on screens having integral electronic control circuit.
A liquid crystal screen consists of a number of liquid crystal cells laid out in a matrix and connected by connection rows or columns to the electronic control circuitry. A first support plate consists of a substrate containing a first set of electrodes, the control components for these electrodes, and addressing rows and columns (active matrix). The liquid crystal is contained between this plate and a second support plate forming the counter-electrode. Each pixel (picture element) thus formed works as an optical valve. Local modification of the transmission or reflection of light is obtained by using the electronic control circuitry to apply a voltage between an access contact on the plate and a contact on the counter-plate. This voltage creates an electric field between facing electrodes and activates the volume of liquid crystal located between the two electrodes which more or less changes the characteristics of the light passing through it.
In the case of active matrix screens, an active element with two terminations (diode) or three terminations (transistor) is associated with each pixel and with each row-column intersection. This type of screen may be manufactured using methods for producing active matrices composed of thin-film transistors. These transistors may have a direct multilevel structure, in other words the gate is above the source and drain with respect to the substrate, or may be reverse multilevel, when the gate is below the source and drain.
A direct multilevel structure is described in European patent application 82 783 "Method of manufacturing thin-film silicon transistors on insulating substrate" (F. Morin et al), and in an article in JAPAN DISPLAY '86 "A 6" Diagonal Active Matrix Addressed LCD for Minitel Application" by the same authors. The technology described in these documents is very economic since it can be used to make thin-film transistors with two levels of masks only, by making the data columns at the same time as the electrodes. Furthermore, an article by Y. Ugai et al "A 7.23-in.-Diagonal Color LCD Addressed by a-SI TFTs" (SID 84 DIGEST, page 308) suggests the production of a direct multilevel transistor made with three levels of masks.
However, these technologies have a number of disadvantages, the most important of which is due to photoconductivity of silicon. These transistors are sensitive to light from above, in other words through their edges, and the "dark mask" technology (gate fully covering the semiconductor material) is impossible. These transistors are also sensitive to light from below, in other words through the channel of the transistor in direct contact with the substrate when it is transparent. Moreover, an additional passivation step has to be introduced in the production method for these transistors, to prevent the semiconductor material from being exposed on the surface at the end of the process. Finally, a size limitation is that no gate-source contact is possible.
Solutions have been suggested to solve the problem of photoconductivity from below, for instance as described in HOSIDEN EP 186036 and EP 179915 patents, and in an article by T. Wada et al "1280.times.800 Color Pixel 15 inch Full Color Active Matrix LCD" (EURODISPLAY'90 The Tenth International Display Research Conference", page 370). These solutions require at least four mask levels and do not allow any gate-source or gate-drain contact. Furthermore, the only way of solving the problem of photoconductivity from above with these solutions is to deposit and etch an opaque mask on the counter-electrode.
This invention can overcome these disadvantages by means of an economic production method with three or four levels of masks.
This invention

REFERENCES:
patent: 5032531 (1991-07-01), Tsutsui et al.
patent: 5238861 (1993-08-01), Morin et al.
patent: 5299289 (1994-03-01), Omae et al.
patent: 5320973 (1994-06-01), Kobayashi
patent: 5455182 (1995-10-01), Nishimoto et al.

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