Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
1999-04-30
2003-09-23
Booth, Richard (Department: 2812)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S479000, C438S166000
Reexamination Certificate
active
06624009
ABSTRACT:
INTRODUCTION
The present invention relates generally to the field of semiconductor devices and in particular the invention provides an improved method for the formation of crystalline films on glass substrates.
BACKGROUND OF THE INVENTION
Considerable attention internationally is being directed towards developing technology for depositing polycrystalline silicon films on glass. This interest arises from the use of these films for forming electronic devices and circuits for active matrix liquid crystal displays and for use of these films in solar cells. Silicon film thickness could be in the 30 nm to 100 &mgr;m range depending on application. For solar cell use, silicon film thickness in the 0.5-100 &mgr;m range is of particular interest, with optimal designs likely to be in the 1-5 &mgr;m range.
As part of these efforts, considerable attention is being directed to the development of supporting glass substrates with specific properties, particularly in relation to the thermal expansion coefficient of the glass relative to silicon and the strain point of the glass (temperature at which viscosity reaches a value of 10
14.5
poise). For example, Corning Glassworks have developed a barium alumina silicate glass known as Corning 1737 of strain point above 660!C with a thermal expansion coefficient closely matched to silicon below this temperature for use in active matrix liquid crystal displays; the Max Planck Institute in Stuttgart has developed a glass of unspecified composition which has a strain point of 820!C and a thermal expansion coefficient even better matched to silicon below this temperature for use in solar cells.
A surprising conclusion from research of the present inventors is that this earlier work is heading in the wrong direction. Because amorphous silicon shrinks irreversibly when crystallised, use of a high-strain point glass as a substrate will result in a highly stressed film prone to cracking, even if it is perfectly expansion matched. The present invention arises from the realisation by the inventors that the normal soda lime glasses, developed over the centuries largely to produce durable glass which could be manufactured at low processing temperatures, and hence having low strain points, are ideal for this application. Throughout this specification, the term ‘amorphous silicon’ is used to describe silicon and silicon alloys containing a high proportion of amorphous silicon such that the material displays the shrinkage characteristic of amorphous silicon material upon its crystallisation. However, the material may include a proportion of crystalline silicon (eg Crystalitec) as well as alloying substances and impurities.
STATEMENT OF INVENTION
According to a first aspect, the present invention provides a method of forming a thin film of crystalline semiconductor material on a glass substrate, including the steps of:
depositing a film of the semiconductor material in amorphous form over the glass substrate;
processing the semiconductor material to form crystalline semiconductor material;
during or subsequently to the processing step, heating the substrate and semiconductor material to a temperature at or above the strain point temperature of the substrate;
subsequently to the heating step, cooling the substrate and semiconductor material to a temperature below the strain point of the substrate.
According to a second aspect, the present invention provides a method of forming a thin film of crystalline semiconductor material on a glass substrate, including the steps of:
a) heating the glass substrate to a temperature at which deposition of the crystalline semiconductor material may occur;
b) depositing a film of the crystalline semiconductor material over the glass substrate;
c) during or subsequently to the depositing step, heating the substrate and semiconductor material to a temperature at or above the strain point temperature of the substrate;
d) subsequently to the heating step, cooling the substrate and semiconductor material to a temperature below the strain point of the substrate.
According to a third aspect, the present invention provides a method of forming a thin film of crystalline semiconductor material on a glass substrate, including the steps of:
a) heating the substrate to a temperature at or above the strain point temperature of the substrate;
b) after the heating step and while the substrate is still at or above the strain point temperature depositing a film of the crystalline semiconductor material over the glass substrate;
c) subsequently to the deposition step, cooling the substrate and semiconductor material to a temperature below the strain point of the substrate.
According to a fourth aspect, the present invention provides a method of processing an amorphous semiconductor film on a glass substrate to crystallise the film, the method including the steps of:
a) processing the semiconductor material to form crystalline semiconductor material;
b) during or subsequently to the processing step, heating the substrate and semiconductor material to a temperature at or above the strain point temperature of the substrate;
c) subsequently to the heating step, cooling the substrate and semiconductor material to a temperature below the strain point of the substrate.
According to a fifth aspect, the present invention also provides a method of forming a thin film of crystalline semiconductor material supported by a glass substrate, including the steps of:
a) forming a low strain point temperature film over the substrate;
b) depositing a film of amorphous semiconductor material over the low strain point temperature film;
c) processing the semiconductor material film to form crystalline semiconductor material film.
According to a sixth aspect, the present invention also provides a device manufactured according to any one of the above methods.
According to a seventh aspect, the present invention further provides a semiconductor device including a film of crystalline semiconductor material formed on a glass substrate the substrate having a strain point temperature below the desired crystallisation temperature of the semiconductor material and a temperature co-efficient not less than that of the semiconductor material.
Preferably the substrate is a glass having a strain point temperature below the desired crystallisation temperature of the semiconductor material.
In this context desired crystallisation temperature is the temperature at which crystallisation occurs to achieve desired crystalline characteristics. In one method according to the invention, during or after the crystallisation step the substrate is heated to a temperature where it will deform under gravity against a planar form, within a predetermined processing period, to reverse buckling caused by differential stress between the substrate and the semiconductor film. This temperature will be somewhere between the strain point and the working point of the glass, the temperature used being dependent upon the speed with which the glass is required to flatten. Using this method the substrate can also be placed on a shape form to produce purpose shaped panels for applications such as vehicle sun roofs. In the case of soda lime glass the temperature used with good effect in one embodiment of the invention is 650° C.
In another method according to the invention, during or after the crystallisation step the substrate is heated to a temperature at or above the strain point temperature but below the sagging temperature. The sagging temperature is the temperature where it will deform under gravity within a predetermined processing period. Using this alternative method films can be successfully processed with a maximum temperature of 620° C. This method may be performed with the substrate clamped to a supporting form to reduce buckling.
In another method according to the invention the cooling step includes a step of rapidly cooling the surface of the substrate carrying the semiconductor film. Preferably also the cooling step includes a step of rapidly cooling the surface of the substrate opposite to
Basore Paul Alan
Green Martin Andrew
Ji JingJia
Shi Zhengrong
Booth Richard
Jordan and Hamburg LLP
Pacific Solar Pty Limited
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