Patent Application
20150380561
1. Field of the Invention
Embodiments described herein generally relate to reducing hydrogen in dielectric and passivation layers. More specifically, embodiments described herein generally relate to reducing hydrogen in silicon-containing layers for use in metal oxide thin film transistors (TFTs).
2. Description of the Related Art
Metal oxide semiconductors, such as zinc oxide (ZnO) and indium gallium zinc oxide (IGZO) are attractive for device fabrication due to their high carrier mobility, low processing temperatures, and optical transparency. TFTs made from metal oxide semiconductors (MO-TFTs) are particularly useful in active-matrix addressing schemes for optical displays. The low processing temperature of metal oxide semiconductors allows the formation of display backplanes on inexpensive plastic substrates such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN). The transparency of oxide semiconductor TFTs leads to improved pixel apertures and brighter displays.
The MO-TFT's stability and performance is very sensitive to hydrogen content, both as incorporated into the MO-TFT itself and incorporated into contacting layers. The contacting layers can include a channel interface layer or a bulk layer. The contacting layers include CVD deposited films, such as silicon oxide (SiO), silicon oxynitride (SiON), silicon nitride (SiN), etc. In many semiconductors, interstitial hydrogen (hydrogen between layers) has been found to act as an amphoteric impurity (an impurity that can act as a donor or an acceptor depending on the semiconductor material it is added to). Thus, in p-type materials, hydrogen generally acts as a donor, and, in n-type materials, hydrogen generally as an acceptor. In MO-TFTs, however, hydrogen can be deleterious. Conventional plasma enhanced chemical vapor deposition (PECVD) films create a very high hydrogen content in the film. For example, SiO deposited by conventional PECVD contains approximately 4% hydrogen and SiN deposited by conventional PECVD contains approximately 35% hydrogen. The hydrogen content of conventional PECVD films induces a high threshold voltage shift (Vth shift) under voltage/light bias conditions.
Therefore, there is a need in the art for lower hydrogen content in films for use with MO-TFTs.
The embodiments described herein generally relate to substantially hydrogen free films for use with MO-TFTs and methods for making the same. In one embodiment, a thin film transistor can include a substrate; a metal oxide semiconductor layer formed over a portion of the surface of the substrate; a channel interface layer comprising silicon oxyfluoride (SiOF) in contact with the amorphous metal oxide layer, wherein the channel interface layer is substantially free of hydrogen; and a cap layer comprising silicon formed over the interface layer.
In another embodiment, a method for making a thin film transistor includes positioning a substrate in a processing chamber; depositing a metal oxide semiconductor layer over a portion of the surface of the substrate, the metal oxide semiconductor layer comprising a zinc oxide; activating a deposition gas comprising SiF4 using MW-PECVD to create an activated deposition gas, wherein the deposition gas does not include hydrogen; delivering the activated deposition gas to the substrate to deposit a channel interface layer comprising SiOF over the metal oxide thin film transistor layer; and depositing a cap layer over the channel interface layer and the metal oxide thin film transistor layer.
So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings.
It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation.
Higher stability MO-TFT structures and methods for making the same are disclosed herein. Due to hydrogen's donor activity in MO-TFT structures, hydrogen concentration in both the MO-TFT layer and the encapsulation layer, which can include the channel interface layer and cap layer, should be limited. To accomplish this, a passivation layer can be deposited using a deposition gas which is activated by microwave PECVD (MW-PECVD). In embodiments herein, gases activated by MW-PECVD can include gases that are ignited directly by the MW-PECVD or activated indirectly, such as activation of the deposition gas by delivering a remote plasma formed from an inert gas or a constituent gas of the deposition gas. In one or more embodiments, the passivation layer can be a multilayer structure comprising at least a channel interface layer and a cap layer. The channel interface layer is the lowermost layer and forms the interface between the passivation layer and the metal oxide semiconductor. Typical channel interface layers can include highly porous silicon containing dielectric layers, such as silicon oxyfluoride (SiOF). The cap layer is formed over the channel interface layer and serves to seal the porous channel interface layer. Typical cap layers can include dense silicon containing dielectric layers, such as silicon oxide (SiOx), silicon oxynitride (SiON) and silicon nitride (SiN). Not only does the deposition of the overlying layers occur at lower temperatures when using MW-PECVD than an equivalent deposition using standard PECVD but hydrogen incorporation into the resulting layer is also reduced. The embodiments disclosed herein are more clearly described with reference to the figures below.
The embodiments described below can be practiced in a PECVD chamber available from AKT America, Inc., a subsidiary of Applied Materials, Inc., Santa Clara, Calif. It is to be understood that the invention has applicability in other chambers as well, including apparatus available from other manufacturers.
The substrate 102 may be, among others, a thin sheet of metal, plastic, organic material, silicon, glass, quartz, or polymer materials. In one embodiment, the substrate 102 is a glass substrate upon which a silicon-containing layer will be deposited. In other embodiments, the substrate 102 may be doped or otherwise modified glass substrate, such as a glass substrate with a MO-TFT layer formed thereon.
The process chamber 100 generally includes chamber walls 104, a chamber bottom 106 and a chamber lid 108 which define a process volume 199 therein. The process volume 199 is coupled to a vacuum system 109 and has a substrate susceptor 110 disposed therein. The process volume 199 is accessed through a slit valve opening 112 such that the substrate 102 may be transferred in and out of the process chamber 100. The chamber walls 104, chamber bottom 106, and the chamber lid 108 may be fabricated from a unitary block of aluminum or other material compatible for plasma processing. The chamber lid 108 is supported by the chamber walls 104 and can be removed to service the process chamber 100. The substrate susceptor 110 may be coupled to an actuator 114 to raise and lower the substrate susceptor 110.
The substrate susceptor 110 may optionally include heating and/or cooling elements to maintain the substrate susceptor 110 at a desired temperature, such as a resistive heater 198 and/or cooling fluid conduits 196. Lift pins 116 are moveably disposed through the substrate susceptor 110 to controllably support the substrate 102 prior to placement onto the substrate susceptor 110 and after removal from the substrate susceptor 110.
The major components of the process chamber 100 in accordance with the present invention may include, among others, a gas feeding source 120 and a microwave source 126. The microwave source 126 may include one or more microwave antennas 128 that are configured to be parallel to the longitudinal direction of the gas feeding source 120. The gas feeding source 120 may be located between the microwave source 126 and the substrate 102.
The gas feeding source 120 may include an array of gas feeding lines 121 that are configured to receive one or more precursor gases and/or carrier gases from a gas source 122A and/or gas source 122B. The microwave source 126 may be located between the gas feeding source 120 and the top (e.g., the chamber lid 108) of the process chamber 100. The microwave source 126 generally includes the microwave antennas 128 and a coupling mechanism 130 connected to the microwave antennas 128. The microwave source 126 may be coupled to ground. While only one microwave antenna 128 is shown, it is contemplated that the number of the microwave antennas 128 may be increased depending upon the size of the substrate.
A microwave supply 132 is connected to the microwave source 126 and can deliver microwave power to the antennas 128. In operation during a process, such as a deposition process, the microwaves travel along the microwave antennas 128 and go through a high attenuation by converting electromagnetic energy into plasma energy which ignites a plasma within the process volume. Radical species generated by the plasma disassociates the reactive precursors (e.g., SiH4, SiF4, N2O, O2, N2 or combinations thereof) coming from the gas feeding lines 121, which are directed toward the substrate 102 (as indicated by arrows 124) and uniformly distributed across the substrate surface to form a film (e.g. SiOx, silicon oxynitride (SiON), SiN or SiOF) on the substrate 102 that is held by the substrate susceptor 110. Pressure within the chamber during deposition is controlled by a vacuum system 109.
In
As shown in
As shown in
As shown in
In
In
When silicon oxide is used as the cap layer 220, the silicon oxide can be deposited either by MW-PECVD, PECVD or PVD. The plasma damage associated with PVD and the hydrogen incorporating from PECVD can be reduced or avoided using MW-PECVD. In one embodiment, MW-PECVD is used to deposit a SiO2 cap layer. MW-PECVD deposition provides highly conformal deposition results, less plasma damage to the deposited films and reduction of hydrogen concentration in the deposited layer. MW-PECVD silicon oxide is normally deposited with SiH4+O2 or SiH4+N2O as the source gases, where the former provides better film quality than the latter.
A metal oxide semiconductor layer is deposited over a portion of the surface of the substrate, as in step 304. The metal oxide semiconductor layer can be deposited as described with reference to
A deposition gas comprising SiF4 is then activated using PECVD or MW-PECVD to create an activated deposition gas, as in step 306. When depositing SiOF by either MW-PECVD or standard PECVD, deposition gases can include SiF4, SiH4, N2O, O2 or combinations thereof. In one embodiment, SiOF is deposited by RF PECVD using a deposition gas including SiF4, SiH4 and O2. In this embodiment, the SiH4 is believed to compensate for the relatively low electron density of the RF plasma as compared to the MW plasma, thus allowing for formation of the SiOF layer.
The microwave power used in embodiments herein can be a relatively high power, such as a microwave power between 3000 W and 5000 W, for example a microwave power of 4000 W. The microwave power may be directed by one or more antennas, such as six antennas. The antennas can be positioned so as to maintain the plasma until it reaches the substrate.
The activated deposition gas is then delivered to the substrate to deposit a channel interface layer comprising SiOF over the metal oxide semiconductor layer, as in step 308. The activated deposition gas can be delivered to the substrate to deposit a channel interface layer over the metal oxide semiconductor layer. The channel interface layer will be deposited conformally over the active channel and the source and drain electrodes, creating a hydrogen free channel interface layer. The channel interface layer comprising SiOF is highly porous, so the deposited layer should be maintained in hydrogen free conditions prior to deposition of any subsequent layers. The channel interface layer can have a thickness of from 20 Å to 3000 Å. When depositing the channel interface layer using MW-PECVD, the temperature for deposition can be from 200° C. to 350° C., such as 230° C. to 330° C. The channel interface layer can comprise more than one layer, such as a channel interface layer with three layers.
A cap layer is then deposited over the channel interface layer and the metal oxide semiconductor layer, as in step 310. The cap layer can be a layer composed of SiOx, SiON, SiN or combinations thereof. The cap layer can have a thickness of from 50 Å to 3000 Å. The cap layer, like the channel interface layer, can comprise more than one layer. Further, each layer of the cap layer can be of a different composition than any other layer of the cap layer. In one embodiment, the cap layer includes a SiO layer formed over the channel interface layer, a SiN layer formed over the SiO layer and an SiO layer formed over the SiN layer. Further, each of the layers of the cap layer may have a different thickness than other layers in the cap layer.
The cap layer is deposited using a silicon containing precursor and an oxidizing precursor. Silicon-containing precursors can include silicon hydrides, such as SiH4. The silicon-containing precursor can be flowed into a processing chamber for deposition of a SiOx film. In an exemplary chamber, flow rates for silicon hydrides, such as SiH4, can be from 100 sccm to 500 sccm, for example flow rates from 150 sccm to 450 sccm, such as a flow rate of 350 sccm. When the cap layer is deposited using MW-PECVD, the deposition temperature can be between 100° C. and 350° C., such as temperatures between 130° C. and 200° C., for example 130° C.
The oxidizing precursor can include diatomic oxygen (O2), ozone (O3), nitrous oxide (N2O) or other oxidizing gases. The oxidizing precursor can be flowed into a processing chamber alongside silicon hydrides and silicon halides. In an exemplary chamber, such as the one described above, flow rates for O2, O3 or N2O when deposited with silicon hydrides can be from 2000-5000 sccm, such as a flow rate of 3500 sccm. In another embodiment, flow rates for O2 or O3 when deposited with silicon halides can be from 5000 to 7000 sccm, such as a flow rate of 5500 sccm. In another embodiment, flow rates for N2O when deposited with silicon halides can be from 3000 to 7000 sccm, such as a flow rate of 4000 sccm.
By using lower temperatures, such as temperatures between 100° C. and 350° C., such as between 130° C. and 200° C., a largely hydrogen-free and pinhole-free layer can be deposited from microwave activated precursors while avoiding the some of the deleterious effects of using silane (SiH4) and some oxidizing precursors. Temperature will preferably be higher when using silicon tetrafluoride (SiF4), as a higher quality and higher deposition rate SiOF can be deposited at temperatures between 200° C. and 350° C., such as from 230° C. to 330° C.
MW-PECVD creates a lower hydrogen concentration in the deposited layer than equivalent layers deposited by PECVD using RF plasma. Without intending to be bound by theory, MW plasma induces a higher electron density than RF plasma. The higher electron density of the MW plasma contributes to break weak Si—H, N—H, O—H bonding in the deposition gases. By breaking these bonds, hydrogen deposition in the SiOx, SiOF, or SiN film during film formation is reduced as compared to the RF plasma deposition of the same layers. In standard embodiments, the RF PECVD films have high hydrogen content such as approximately 4% in SiO film and approximately 35% in SiN film, whereas MW-PECVD films have very low comparative hydrogen content, such as approximately 1% in SiO film and approximately 16% in SiN film.
Deposited silicon oxide SiOx can include SiO2, SiO, or combinations thereof. The formation of the SiOx layer can be controlled by deposition factors such as temperature, pressure, flow rate of reactant gas and amount of microwave power applied among other factors. Pin-hole-free SiOx layers help maintain MO-TFT integrity. Pin hole density is strongly correlated with RF power and weakly with pressure.
The cap layer acts to prevent hydrogen containing species from penetrating the porous SiOF layer. Without intending to be bound by theory, reduction in hydrogen concentration is important in the creation of various features on a substrate, such as gates. Hydrogen is a ubiquitous impurity in SiOx and is believed to be responsible for fixed charge in the oxide. Release of hydrogen during operation is believed to be responsible for the creation of defects such as trap generation which can lead to intrinsic dielectric breakdown. Further, hydrogen incorporation into the MO-TFT layer is believed to create a high threshold voltage shift. As such, reduced hydrogen concentration is believed to be important to avoidance of such defects.
Embodiments described herein relate to the formation of a MO-TFT with reduced hydrogen in the dielectric and passivation layers. Metal oxides such as IGZO and zinc oxides are sensitive to the presence of hydrogen. As hydrogen is a ubiquitous impurity in many dielectric layers, reduction of hydrogen is important to MO-TFT stability and consistency. By employing microwave plasma, silicon containing layers with significantly reduced hydrogen concentration, such as SiOF, SiOx and SiN, can be deposited at various stages of MO-TFT formation. The channel interface layer can be substantially composed of SiOF. Subsequent high density layers, such as SiOx, can be deposited as a cap layer to prevent hydrogen diffusion into the channel interface layer.
While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2014/014951 | 2/5/2014 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61771257 | Mar 2013 | US |
