FR2518807A1 - Prodn. of a diode in amorphous silicon - suitable for integrated mounting on a liquid crystal visualisation screen - Google Patents

Abstract

The diode in amorphous silicon has at least three layers of different doping levels deposited on a substrate. During fabrication there is a stage of deposit by the thermal decomposition of a gaseous mixture and a thermal treatment stage in an atmosphere with a density greater than that of atomic hydrogen, both stages taking place at a temp. less than the crystallisation temp. of silicon. The thermal decomposition stage to produce a p-i-n type diode includes a first stage of depositing a p+ type layer from a gaseous mixture of molecular hydrogen, and silane or a mixture of polysilanes and diborane, a second stage of depositing an undoped layer from a gaseous mixture containing molecular hydrogen and silane or a mixture of polysilanes and a third stage of depositing a layer type n+ from a gaseous mixture of molecular hydrogen and silane or a mixture of polysilanes and phosphine. The thermal treatment stage either takes place after the deposit of the last layer of amorphous silicon or after one of the intermediary layers of amorphous silicon. In thermoelectric displays using liquid crystals. The diodes have electrical characteristics and physical dimensions allowing them to be directly joined onto the substrate of the screen of visualisation.

Description

PROCESS FOR PRODUCING AN AMORPHOUS SILICON DIODE,
EQUIPMENT FOR IMPLEMENTING SUCH
METHOD AND APPLICATION TO A DEVICE
LIQUID CRYSTAL DISPLAY
The present invention relates to a method for producing an amorphous silicon diode and the application of such a diode to a liquid crystal display device. The invention also relates to equipment allowing the implementation of the method.

 The diode obtained by the method according to the invention applies more particularly to thermoelectric display devices with smectic liquid crystals using the technique of multiplexing for the sequential heating of the rows or columns. Such a multiplexing system for display screen was described in French patent application No. 80 26 544 entitled "Device for controlling a display screen and display screen controlled by this device" and filed on December 15, 1980. The display device which is the subject of the aforementioned patent application comprises in series with each heating line a switching diode. A great simplification of the display system is obtained if the diodes can be placed directly on the display screen. The manufacture of the display device is greatly facilitated and its reliability greatly improved if, in addition, said diodes can be directly integrated on the substrate of the display screen proper. The screens of the current generation have a high resolution and the density of the rows and columns is such that there are no diodes having both the switching power and the small dimensions required for direct integration on said substrate. screen. In the prior art, hybrid circuits, comprising the diodes, are reported on the display screen. It is the essential object of the present invention to propose a method for producing diodes directly integrable on the substrate of display screens of the present generation.

 The methods used to date, in particular the decomposition of silane in an ionized medium (glow discharge) do not make it possible to obtain integrable diodes for display screens having both the desired current density (approximately 50 A / cm2) and sufficient reverse voltage (greater than 20 V).

 The present invention therefore relates to a method for producing an amorphous silicon diode comprising at least three differently doped layers deposited on a substrate introduced into a reactor characterized in that it comprises at least one phase of deposition by thermal decomposition. 'A gas mixture at a temperature below the crystallization temperature of silicon and at atmospheric pressure and a thermal annealing phase in an atmosphere containing a high density of atomic hydrogen at a temperature also below the crystallization temperature of silicon.

 The invention also relates to equipment allowing the implementation of the method.

 The invention finally relates to the application of such a diode to a liquid crystal display screen.

The invention will be better understood and other characteristics and advantages will appear on reading the description below and the appended figures among which:
FIG. 1 diagrammatically illustrates a diode of the pin type,
FIG. 2 schematically represents an apparatus allowing the deposition of the layers of amorphous silicon according to a phase of the manufacturing process,
FIG. 3 is a sectional view of the apparatus allowing the thermal treatment of the device under an atmosphere consisting of atomic hydrogen according to another phase of the process,
- Figure 4 shows a display screen in which the diodes manufactured according to the method of the invention can be implemented.

 FIG. 1 schematically illustrates a diode of the p-i-n type.

 On a substrate 40 of crystalline silicon is deposited a first layer p + 41 of amorphous silicon doped with boron by decomposition of diborane. A second layer of undoped amorphous silicon 42, the thickness of which, greater, fixes the value of the reverse voltage, is deposited on layer 41. Finally a third and last layer. n + 43 of amorphous silicon doped with phosphorus by decomposition of phosphine completes the device. The order of the layers can be reversed to make an n-i-p type diode.

 The main object of the invention is a process for manufacturing diodes of this p-i-n or n-i-p type from amorphous silicon.

 In the preferred application to display screens which will be described later in detail, these diodes are integrated in the extension of a filiform heating resistor which leads to the production of diodes whose typical dimensions are 4 mm in length by 0.25 mm wide.

 The method according to the invention comprises two phases; a phase called phase I in which the amorphous silicon is deposited by thermal decomposition of a gaseous mixture and another step called "phase II" consisting in carrying out a thermal annealing in an atmosphere consisting of atomic hydrogen. The apparatuses necessary to carry out these two stages of the process are the subject of a separate description. The production of a p-i-n type diode on crystalline silicon substrate was chosen, by way of example, to illustrate the description of the process according to the invention.

Phase I of the process includes the following steps:
a) Installation in a chamber with a controlled atmosphere of a crystalline silicon substrate (Figure 2)
b) Circulation in this enclosure of a gaseous mixture under atmospheric pressure comprising:
- a carrier gas consisting of hydrogen H2, the flow rate of which is approximately thirty two liters per minute,
- silane Si H4 at the rate of one liter per minute, the decomposition of which causes the deposition of a first layer of amorphous silicon on the substrate,
- Diborane B2 H6 at the rate of one molecule per thousand molecules of silane, the role of which is to give the layer of amorphous silicon p-type doping.

of type p
c) Raising the temperature of the substrate substantially below five hundred degrees celsius for approximately one minute to obtain a deposit of p + doped amorphous silicon of approximately one thousand angstroms in thickness.

 d) removing the silane and diborane for approximately fifteen minutes, the other conditions being maintained elsewhere.

 e) Recirculating the silane Si H4 for thirty minutes to obtain the deposition of a second layer of undoped a- Si amorphous silicon having a thickness of approximately five thousand angstroms.

 f) removal of the silane Si H4 for fifteen minutes while maintaining the other conditions.

 g) Recirculation of the silane Si H4 and introduction of phosphine PH3 at the rate of one molecule per thousand molecules of silane whose role is to give the third layer of amorphous silicon a-Si n + type doping. Maintaining these conditions for approximately six minutes to obtain a layer of n-doped a-Si silicon a thousand angstroms thick.

 In this phase I of the process, the silane can be replaced by a polysilane or a mixture of polysilanes in order to reduce the deposition temperature.

Phase ll of the process comprises the following steps: - h) Installation of the device obtained during phase I in another enclosure (FIG. 3)
i) Circulation in the atomic hydrogen enclosure under a pressure of approximately thirteen pascals with a flow rate of a few cm 3 per hour.

 j) Raising the temperature of the device to be treated to approximately four hundred degrees Celcius and maintaining these conditions for approximately one hour.

 The method according to the invention is not limited to the sole production of a diode of the p-i-n type on a crystalline silicon substrate or to the digital data indicated. The order of the layers can be modified to make diodes of the n-i-p type. The intrinsic layer i can be lightly doped, for example at the rate of 10-6 phosphine molecule for a silane molecule, then leading to diodes of the p- v -n or rl-v -p types.

Other substrates than crystalline silicon capable of withstanding 600 degrees celsius such as quartz or molybdenum can be used. Substrates with a lower temperature # like glass can be used by using extrapolations of the known techniques of deposition at very high temperatures such as for example the deposit: CVD (Chemical Vapor
Deposition) of polysilanes or the HOMOCVD technique (HOMOgeneous
Chemical Vapor Deposition) in which the substrate is kept at a temperature lower than that of gases. Phase II of the process can also be applied after the deposition of any of the layers carried out during phase I.

The stopping times corresponding to stages d and f of phase I which make it possible to obtain abrupt junctions can be modified to change the profile of the junctions and in particular not exist to obtain junctions with gradual profiles.
FIG. 2 schematically represents an apparatus allowing the deposition of the layers of amorphous silicon according to a phase of the manufacturing process.

He understands:
- a circular susceptor 1 in graphite coated with carbide
of silicon resting on a quartz pedestal (not shown
felt in the figure). The susceptor and pedestal assembly is
rotated, typically adjustable
between 0 and 30 revolutions per minute;
- a winding comprising a spiral coil 2, located in a
plane parallel to susceptor 1 and inductively coupled to
this one. The choke is connected to a high frequency generator
6 operating at 400 KHz and power 25 KW.

- a quartz skirt 3 isolating the choke from the
reaction;
- a quartz bell 4 placed on a base also in
quartz 5.

The assembly defines the volume of the reaction chamber.

 an injector 7 located in the center of the susceptor allows the introduction of the gases G to be decomposed in the reaction chamber, these gases being discharged from the bottom of the reactor through the orifices 8.

 The gas flow rates are conventionally measured using ball flowmeter tubes fitted with stainless steel bellows valves. On the susceptor 1 are shown, under the reference 30, devices in the process of being produced, that is to say devices comprising at least one substrate on which one or more layers will be deposited according to the method of the invention. The bell 4 may also include sight windows allowing the temperature of these devices 30 to be measured using infrared measuring devices. All these arrangements are well known to those skilled in the art.

 - - Figure 3 is a sectional view of the device for heat treatment of the device in an atmosphere consisting of atomic hydrogen according to another phase of the process.

 Heat treatment under atomic hydrogen is known, it is described in French patent application no. 77 17 245 entitled "Process for manufacturing electronic devices which comprise a thin layer of amorphous silicon and electronic device obtained by such a process'1 and deposited June 6, 1977. The process consists in obtaining atomic hydrogen by ionization of molecular hydrogen.The ionized medium constitutes a plasma in which, according to known art, is placed the device to be treated.

The disadvantage of this technique is that the plasma contains ions and electrons which attack the device being treated. It is one of the essential aspects of the invention to provide an apparatus, described below, in which the device, although placed in an atmosphere of atomic hydrogen, is not in an ionized medium, thereby eliminating the disadvantages inherent in prior art.

 A cylindrical quartz enclosure 200 inside which the device 30 to be treated is placed has an inlet orifice 203 allowing the introduction of molecular hydrogen and an outlet orifice 204 used firstly to effect the vacuum and then to ensure very low gas circulation during treatment. A central cavity 205, located in the axis of the cylindrical enclosure, allows without sealing problem, to measure the temperature of the device 30 using a thermocouple 206. Resonant cavities 207 and 208 excited by a generator with 209 microsndes transform ionic hydrogen H2 into atomic hydrogen H. A typical value of the power absorbed by each resonant cavity is 30 watts and the working frequency, corresponding for example to one of the frequencies standardized by the regulations in force in France, is 2.45 GHz (S band). The power consumed not used in the chemical transformation and appearing in calorific form is evacuated by circulation of nitrogen through the resonant cavities; a nitrogen source 210 is connected by conduits 213 to inlet orifices 212 placed on the resonant cavities 207 and 208; outlet orifices 214 situated on the resonant cavities and diametrically opposite to the inlet orifices 212 allow the nitrogen to escape towards the outside.

Thirty non-contiguous 215 heating coils are wound on the quartz enclosure and distributed over a length of approximately 15 cm; by radiation and convection, they increase the temperature of the substrate 30. The substrate 30 to be treated, unlike the prior art, is located outside of the ionized regions located inside the cavities 207 and
208; therefore the gaseous mixture circulating in the vicinity of substrate 30 does not contain ions or electrons capable of generating an attack by bombardment of said substrate. According to another important characteristic of the invention, two sources of atomic hydrogen obtained through the cavities 207
and 208 are arranged on either side of the substrate under treatment; this arrangement makes the distribution of atomic hydrogen in contact with the substrate 30 more homogeneous. In a practical embodiment example, with a device of about 30 cm in length and 4 cm in diameter, it was possible to treat 2 substrates up to 4 cm2 in area.

 - Figure 4 shows an example of display screen in which the diodes manufactured according to the method of the invention can be implemented.

 For clarity of the drawing, the example is limited to 3 lines and 16 columns.

This smectic crystal device uses the thermoelectric effect for the display. The liquid crystal is placed between two insulating plates, one placed at the back being generally reflective, the other reading side being transparent. These blades are recalled for the record and are not shown in the figure. Conductive lines L1, L2, L3 are arranged on the rear plate. Conductive columns C1 to C16 made of transparent material behaving like heating resistors are placed on the front plate, on the reading side. Each column undergoes a sequential rise in temperature followed by cooling, the thermal cycle being short-lived. It is in the cooling phase where the crystal returns from the nematic phase to the smectic phase that the electric potentials, applied between the lines L1, L2, and L3 on the one hand and the columns C1 to C16 on the other hand, act to make the crystal opaque or transparent at the points of intersection of the lines and the single column in the cooling phase. In this case, the message is displayed and updated column by column, in a fixed and predetermined order (for example from left to right). The circulation of a current successively in each of the heating columns suggests the introduction of a transistor in series with each of these columns, these transistors being made sequentially conductive for a short time by an appropriate control circuit. of a screen with 1024 columns, it would be necessary to use 1024 transistors (active elements) and their control circuits. The multiplexing technique applied to column heating takes advantage of the fact that only one column is heated at a time to considerably reduce the number of active elements (transistors) and their associated control circuits, largely replacing them with diodes (passive elements).

 It becomes possible by placing N diodes D1, D2 ..... DN respectively in series with the N columns C1, C2 .... CN as shown in Figure 4 to reduce the number of transistors to 2 N instead of N .

In the case of 1024 columns, 64 transistors are sufficient. In Figure 4 where N = -16, 2 16 transistors are required. These 8 transistors or more generally these 8 control devices apply to points Q19
Q2 'Q3, Q4, P1, P2, P3 and P4 either of the two polarities of a source so as to circulate a current in each of the 16 columns and in only one column at a time. It is the unidirectional conduction properties of the diodes that make this solution possible by making inactive all the positive combinations P associated with negative Q.

 The method according to the invention therefore has the main advantage of allowing the production of diodes having both the electrical characteristics and the physical dimensions allowing their direct integration on the substrate of display screens of the present generation, the known art consisting in report these diodes on hybrid circuits.

Claims (9)

 1. Method for producing an amorphous silicon diode comprising at least three differently doped layers deposited on a substrate introduced into a reactor characterized in that it comprises at least one phase of deposition by thermal decomposition of a gaseous mixture at a temperature below the crystallization temperature of silicon and at atmospheric pressure and a thermal annealing phase in an atmosphere containing a high density of atomic hydrogen at a temperature also lower than the crystallization temperature of silicon.  2. Method according to claim 1 characterized in that the diode being of p-i-n type the thermal decomposition phase comprises:  a first step consisting in the deposition of a p + doped layer by decomposition of a gaseous mixture comprising molecular hydrogen, silane or a mixture of polysilanes and diborane.  a second step consisting in the deposition of an undoped layer by decomposition of a gaseous mixture comprising molecular hydrogen and silane or a mixture of polysilanes.  a third step consisting in the deposition of an ns doped layer by decomposition of a gas mixture comprising molecular hydrogen, silane or a mixture of polysilanes and phosphine.  3. Method according to claim 1 characterized in that the diode being of p-v -n type the thermal decomposition phase comprises:  a first step consisting in the deposition of a p + doped layer by decomposition of a gaseous mixture comprising molecular hydrogen, silane or a mixture of polysilanes and diborane.  a second step consisting in the deposition of a lightly doped layer? by decomposition of a gaseous mixture comprising & BR <molecular hydrogen, silane or a mixture of polysilanes and phosphine at the rate of 10 6 molecule of phosphine for a silane or polysilane molecule.  - A third step consisting in the deposition of an n + doped layer by decomposition of a gas mixture comprising molecular hydrogen, silane or a mixture of polysilanes and phosphine.  4. Method according to any one of claims i to 3 characterized in that the thermal annealing phase under an atomic hydrogen atmosphere is carried out after the deposition of the last layer of amorphous silicon.  5. Method according to any one of claims 1 to 3 characterized in that the thermal annealing phase under atmosphere ~ of atomic hydrogen is carried out after the deposition of at least one of the interned layers ~ dlaires of amorphous silicon.  6. Method according to any one of claims 1 to 5 characterized in that the thermal decomposition phase comprises additional steps during the # stools the deposition process is stopped between the steps of depositing the layers for a time determined by sending in said pure hydrogen reactor in order to obtain abrupt profile junctions.  7. An apparatus for carrying out the annealing phase of the method according to any one of claims 1 to 6 comprising a cylindrical enclosure (200) with a controlled atmosphere and means (215) for raising the temperature of the substrate (30 ) characterized in that the enclosure comprises near each of its ends a zone equipped with means (207208) making it possible to ionize the gases only in this zone and a non-ionized central zone intended to receive the substrate (30).  8. Apparatus according to claim 7 characterized in that the means for ionizing the gases are annular resonant cavities (207, 208) placed on the cylindrical enclosure (200) and excited by an external generator (209) delivering an electrical signal located in the frequency band S.  9. Thermoelectrically controlled display screen constituted by an assembly of elementary cells defined by a blade of liquid crystal enclosed between two electrodes each connected to an electrical control connection and further comprising a network of resistive strips intended to selectively heat the liquid crystal of said cells characterized in that diodes obtained by the method according to any one of claims 1 to 6 are used for the control of the heating strips and in that these diodes are integrated directly on said substrate.