LAMP
Laboratory for Advanced Molecular Processing

Chemical Vapor Deposition/Atomic Layer Deposition

Metal gate electrodes

In 1968, at R&D Laboratories of Fairchild Semiconductor in Palo Alto, Faggin demonstrated a metal oxide semiconductor field effect transistor (MOSFET) that used poly-Si as a gate electrode. Poly-Si gate electrode has a lot of benefits like tunable work function, effective cost, and reliability, so it replaced the traditional Al gate electrode. As the circuit size decreased and the dielectric layer became thinner, the poly-Si gate electrode caused problems like gate depletion, high resistivity, and boron penetration. Thus the metal gate electrode concept has been revived.
Metal gate electrodes
In 2008, Intel produced a microprocessor that had a high-k dielectric layer and metal gate electrode. The metal gate electrode requires low resistivity, tunable work function, and thermal stability. Thus a TaCxNy material has been spotlighted as a candidate for gate electrodes. In particular, the TaC material has a very low resistivity and suitable work function (4.2eV) that are appropriate for NMOSFET gate electrode.

Process

TaCxNy materials can be deposited using various techniques, including sputtering, chemical vapor deposition (CVD), and atomic layer deposition (ALD). The ALD process uses a precursor and reactive gas to make a film. The Precursor and reactive gas were injected separately into a chamber, producing a surface reaction only (self limiting reaction). Nowadays step coverage and thickness control have become more important for 3D structure concepts like dual gate, finFET, and vertical gate. ALD has been highlighted due to its excellent step coverage, low process temperature, and excellent thickness control.
The ALD process is very slow, however, sometimes a manufacturer wants a faster but rough process. In the CVD process, the precursor and reactive gas are injected into a chamber at the same time, producing both a surface reaction and gas phase reaction. Due to this gas phase reaction, CVD has a better throughput than ALD.
Siyrce A ubhectuib - Purge - Source B Injection - Purge Gas-Solid

Objectives

Our goal is developing ALD/CVD processes for CMOS gate electrodes. We control the film phase using various process conditions. The film phase affects electrical properties (resistivity, work function, etc) and physical properties (thermal stability, hardness, etc). We can get the appropriate film for CMOS gate electrode by optimum process conditions.
Objectives

Transparent conducting oxides

Transparent conductive oxides (TCOs) are doped metal oxides used in optoelectronic devices such as flat panel displays and photovoltaics. Most of these films are fabricated with polycrystalline or amorphous microstructures. On average, these applications use electrode materials that have greater than 80% transmittance of incident light as well as conductivities higher than 10^3 S/cm for efficient carrier transport. In general, TCOs for use as thin-film electrodes in solar cells should have a minimum carrier concentration on the order of 10^20 cm^-3 for low resistivity and a band gap less than 380 nm to avoid absorption of light over most of the solar spectra. Mobility in these films is limited by ionized impurity scattering and is on the order of 40 cm^2Vs. Current TCOs used in industry are primarily n-type conductors, meaning their primary conduction is from the flow of electrons. Suitable p-type transparent conducting oxides are still being researched.
For the purpose of obtaining lower resistivities, various TCO semiconductor materials have been developed; n-type TCO semiconductors now available for thin-film transparent electrodes are listed in table 1, grouped by compound type. One advantage of using binary compounds as TCO materials is the relative ease of controlling the chemical composition in film depositions compared to using ternary compounds and multicomponent oxides. Up to now, various TCO thin films consisting of binary compounds such as SnO2, In2O3, ZnO, and CdO have been developed, with impurity doped SnO2 (SnO2:Sb and SnO2:F), impurity-doped In2O3 (In2O3:Sn, or ITO) and impurity-doped ZnO (ZnO:Al and ZnO:Ga) films in practical use. In addition, it is well known that highly transparent and conducting thin films can also be prepared using metal oxides without intentional impurity doping. The resulting films are n-type degenerated semiconductors with free electron concentrations of the order of 10^20 cm^-3 provided by native donors such as oxygen vacancies and/or interstitial metal atoms. However, since undoped oxide films were found to be unstable when used at a high temperature, binary compounds without impurity doping have proved unusable as practical transparent electrodes.

Zinc oxides (ZnO)

Recently, the demand for ITO thin-film transparent electrodes has dramatically increased in the field of ptoelectronic devices. If the increase in usage of ITO films for flat panel displays and solar cells continues, not only will the price of ITO continue to rise but also the availability of In may be jeopardized in the near future. The development of alternative TCO materials is necessary to resolve this serious problem. For obtaining a resistivity of the order of 10^-5 cm, impurity-doped binary compounds such as Al-doped ZnO (AZO) and In-doped CdO have been proposed as alternative materials. With consideration for the environment, it appear sthat impurity-doped ZnO, particularly AZO, is a promising alternative because of its use of ZnO or Zn, both being inexpensive, abundant and non-toxic materials, and its ability to exhibit resistivity comparable to ITO, as described above.
ZnO is a key technological material. The lack of a centre of symmetry in wurtzite, combined with large electromechanical coupling, results in strong piezoelectric and pyroelectric properties and the consequent use of ZnO in mechanical actuators and piezoelectric sensors. In addition, ZnO is a wide band-gap (3.37 eV) compound semiconductor that is suitable for short wavelength optoelectronic applications. The high exciton binding energy (60 meV) in ZnO crystal can ensure efficient excitonic emission at room temperature and room temperature ultraviolet (UV) luminescence has been reported in disordered nanoparticles and thin films. ZnO is transparent to visible light and can be made highly conductive by doping.
Zinc oxides (ZnO)
In the 1980s, ZnO films with a resistivity of the order of 10^-4 ohm cm were prepared by impurity doping. However, a low resistivity below approximately 2 * 10^-4 ohm cm has only been obtained in AZO and GZO films, as shown in table 2 summarizes the minimum resistivity and the maximum carrier concentration obtained for typical impurity-doped ZnO films prepared with optimal doping content for various dopants and deposition methods. In general, the obtainable electrical properties of impurity doped ZnO films are strongly dependent on the deposition methods and conditions. Impurity-doped ZnO films with a resistivity of the order of 10^-4 ohm cm have been prepared by vacuum arc plasma evaporation (VAPE), metal organic molecular beam deposition (MOMBD) and metal organic chemical vapour deposition (MOCVD) as well as by MSP and PLD. Recently, AZO and GZO films with a resistivity of the order of 10^-4 cm were prepared using PLD and VAPE. Although VAPE using an Uramoto gun as the arc plasma source, similar to activated reactive evaporation (ARE), has attracted much attention as a new deposition technique with a high deposition rate on large area substrates, vacuum evaporation of AZO is difficult to achieve because the vapour pressure of Al2O3 is too low in comparison with that of ZnO. AZO films with a resistivity of the order of 10^-5 ohm cm have been prepared by PLD, but preparing films on large substrates with a high deposition rate is also very difficult to achieve. When preparing highly conductive and transparent impurity-doped ZnO films, controlling the oxidation of Zn is much more difficult than with other binary compounds such as SnO2 and In2O3, because Zn is more chemically active in an oxidizing atmosphere than either Sn or In. Because of this binding energy of Zn and O, the activity and amount of oxygen must be precisely controlled during the deposition. As a result, ZnO films with low resistivity are attainable only by depositions in atmospheres that are less oxidizing than in depositions of In2O3 and SnO2 films.

Niobium Oxide (NbO)

Stoichiometric niobium oxides mainly exist in the form of NbO, Nb2O3, NbO2 and alpha, beta Nb2O5. The first two compounds have been obtained by melting the richest niobium oxide, Nb2O5, with Nb at high temperature. Non-stoichiometric suboxide phases have also been observed. Nb2O5, also known as niobia or niobic acid anhydride, is however the most studied material. It has been prepared by different methods such as oxidation of metallic niobium in air, by hydrolyzing alkali-metal niobates, niobium alkoxides and niobium pentachloride or by precipitation from solution in hydrouoric acid with alkali-metal hydroxide or ammonia. Stoichiometric Nb2O5 is an insulator with, for example, a conductivity of 3 * 10^-6 S/cm for a H-type Nb2O5 single crystal. However, it becomes an n-type semiconductor at lower oxygen content and Nb2O4.978 has a conductivity of 3 * 10^3 S/cm. The conduction band is built from the 3d orbitals of Nb atoms and the valence band from the 2p orbitals of oxygen. The band gap observed from optical measurements varies between 3.2 and 4 eV.

Direct liquid injection chemical vapor deposition (DLI-CVD)

Direct-Liquid injection(DLI)-MOCVD can be used for liquid precursors or a solution containing all the precursors required. The solution is transported , most often via syringe, to a vaporization chamber adjacent to the CVD reactor from where it is swept, using a carrier gas, in to reactor.
Direct liquid injection chemical vapor deposition (DLI-CVD)
In general, the MOCVD process involves the following steps.
1. Generation of active gases reactant species.
2. Transport of the gaseous species into the reaction chamber.
3. Gaseous reactants undergo gas phase reactions forming intermediate species.
4. Absorption of gaseous reactants on to the heated substrate, and the heterogeneous reaction occurs at the gas-solid interface which produced the deposit and by-product.
5. The deposits will diffuse along the heated substrate surface forming the crystallization centre and growth of the film.
6. Gaseous by-products are removed from the boundary layer though diffusion or convection.
7. The unreacted gaseous precursors and by-products will be transported away from the deposition chamber.

Sputtering

Sputtering is a mechanism by which atoms are dislodged from the surface of a material as a result of collision with high-energy particles. Thus, PVD by Sputtering is a term used to refer to a physical vapor deposition (PVD) technique wherein atoms or molecules are ejected from a target material by high-energy particle bombardment so that the ejected atoms or molecules can condense on a substrate as a thin film. Sputtering as a deposition technique may be described as a sequence of these steps:
1. Ions are generated and directed at a target material
2. Ions sputter atoms from the target
3. The sputtered atoms get transported to the substrate through a region of reduced pressure
4. The sputtered atoms condense on the substrate, forming a thin film
Sputtering
Sputtering offers the following advantages:
1. Sputtering can be achieved from large-size targets, simplifying the deposition of thins with uniform thickness over large wafers
2. Film thickness is easily controlled by fixing the operating parameters and simply adjusting the deposition time
3. Control of the alloy composition, as well as other film properties such as step coverage and grain structure, is more easily accomplished than by deposition through evaporation
4. Sputter-cleaning of the substrate in vacuum prior to film deposition can be done
5. Device damage from X-rays generated by electron beam evaporation is avoided