1. Field of the Invention
The present invention relates to an electronic device, a method of manufacturing the electronic device, a display, and a sensor.
2. Description of the Related Art
In general, electronic devices (for example, thin film transistors (TFTs)) are manufactured in accordance with a method including a process of forming an electrode by patterning (pattern-processing), through photolithography and etching, a metal film formed on a semiconductor layer.
Along with the recent progress in reducing the size, weight, and power consumption of electronic apparatuses, oxides (IGZO) including indium (In), gallium (Ga), and zinc (Zn) have been attracting attention as a semiconductor layer material particularly in the field of displays.
Since IGZO (an IGZO film) can be formed on a resin substrate at low temperatures, IGZO is expected to be applied to a variety of applications including flexible displays.
IGZO films as described above (particularly, amorphous IGZO films) are easily etched by an acid. Examples of the acid include a mixed etching liquid containing hydrochloric acid and nitric acid (which is known as an etching liquid for polycrystalline ITO (indium tin oxide)) and an oxalic acid-containing etching liquid (which is known as an etching liquid for an amorphous ITO or for IZO (indium zinc oxide)).
Further, it has been reported that the IGZO film is etched also by an alkaline etching liquid (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2008-141113).
Since etching of IGZO films is easily performed using an etching liquid of any kind, it is thought that there are no significant problems in etchability of IGZO films themselves.
However, when an IGZO film is used in an electronic device such as a TFT, a metal film is formed directly on the IGZO film or on the IGZO film with another film interposed therebetween, and then patterning of the formed metal film is performed by photolithography and etching. When performing etching of the metal film in this process, the underlying IGZO film has a tendency to be eroded. The thickness reduction of the IGZO film causes deterioration in device characteristics of the electronic device (for example, transistor characteristics of the TFT) in some cases.
Here, the metal selected may be a film of a metal having a high melting point such as molybdenum (Mo) or tungsten (W) (see, for example, JP-A No. 2007-134496).
As an example of a study on the thickness reduction of the IGZO film, production of a bottom-gate TFT as a TFT for use in a liquid crystal display (LCD) using an IGZO film for a semiconductor layer and using Mo for a source electrode and a drain electrode has been reported (see, for example, SID 08 DIGEST, pages 625 to 628). In the report, a Mo film is formed on an IGZO film, and the Mo film is subjected to patterning by wet etching or dry etching.
According the above report, the underlying IGZO film is etched regardless of whether the Mo film is subjected to patterning by wet etching or dry etching. However, patterning by dry etching resulted in less thickness reduction of the IGZO film.
Specifically, the above report describes that, in wet etching, the etching rate (etching speed) of Mo is from 100 Å/sec to 150 Å/sec, the etching rate of IGZO is from 10 Å/sec to 30 Å/sec, and the etching selection ratio between Mo and IGZO (etching rate of Mo/etching rate of IGZO) is from 5 to 10. The above report further describes that, in dry etching, the etching rate of Mo is from 30 Å/sec to 40 Å/sec, the etching rate of IGZO is from 2 Å/sec to 3 Å/sec, and the etching selection ratio between Mo and IGZO (etching rate of Mo/etching rate of IGZO) is from 10 to 20.
However, when the Mo film on the IGZO film is subjected to patterning by dry etching, there are problems in that dry etching apparatuses which process large-area substrates such as those for use in flat panel displays (FPDs) are expensive, and that it is extremely difficult to perform uniform control over the entire region of the large area due to irregularities of plasma densities.
Further, in dry etching, since the IGZO film is exposed to high density plasma, electric resistance of the IGZO film tends to decrease (the number of carriers tends to increase), as a result of which there are cases where device characteristics (for example, transistor characteristics) are deteriorated. For example, it has been reported that, when Ar (argon) plasma is experimentally irradiated on an IGZO film, electric resistance of the IGZO film decreases as the plasma irradiation time increases (see, for example, Applied Physics Letters 90, 262106 (2007)). It is thought that Ar+ ions generated in the Ar plasma increase the number of oxygen defects in IGZO, as a result of which the electric resistance is decreased (the number of carriers is increased). In general, most of plasma damages are incidents of this kind. Further, when the plasma energy is increased, the content of oxygen, In, Ga, or Zn changes, which may cause deterioration in device characteristics.
For the reasons described above, the method of patterning a metal on an IGZO film by dry etching has many problems from the viewpoints of costs and production suitability.
Further, it is also possible to perform a lift-off method that includes performing resist pattern formation on the IGZO film, Mo film formation, and resist pattern separation in this order as a method which does not involve dry etching and with which damage to the IGZO film is suppressed.
However, the lift-off method has problems in manufacturing, such as the following: (1) wastes easily occur due to the principle thereof, which is likely to cause a decrease in yield; (2) the cross-section of the resist pattern should have a reverse taper shape, which poses difficulty in the control of the resist pattern shape, and (3) there are cases where the Mo film is formed on a part of a reverse taper portion, at which a film should not be formed, of the resist pattern, so that a pattern edge, which should be formed in a straight line, often curves (in other words, cannot be formed in a straight line).
For the reasons described above, wet etching is preferred as a method of patterning a Mo film provided on an IGZO film, from the viewpoints of costs and production suitability.
A mixed etching liquid containing hydrochloric acid and nitric acid is generally used as an etching liquid for a Mo film. The mixed etching liquid containing hydrochloric acid and nitric acid is commercially available, for example, as “MO ETCHANT (trade name:)” from Kanto Chemical Co., Ltd.
Further, it has been reported that a Mo film is etched by an aqueous solution of hydrogen peroxide (see, for example, Osaka University nano-foundry H19 result report, “KIKI RIYO, F HANDAI H19-009”, searched on 20 Apr. 2009, internet <http://foundry.osaka-u.ac.jp/isir/theme/report/H19/H19-009.pdf>, and Journal of the Korean Physical Society, vol. 53, No. 5, November 2008, pages 2603 to 2606).
Furthermore, it has been reported that a W film is also etched by an aqueous solution of hydrogen peroxide (see, for example, Journal of Microelectromechanical Systems, vol. 12, No. 6, December 2003, pages 761 to 778).