H. J. Yang

Highly Conformal Deposition of Pure Co Films by MOCVD Using Co2 ( CO ) 8 as a Precursor

Highly conformal Co thin films were deposited on SiO 2 trenches with an aspect ratio of 13 by metallorganic chemical vapor deposition (MOCVD) using Co 2 (CO) 8 as a precursor in a low-temperature regime of 50-70°C where the growth rate was 3.5-7.0 nm/min. Lowering the pressure of the process reduces the number of collisions in the gas phase and, thus, widens the temperature regime in which the surface reaction controls the growth rate. A processing pressure of 26.7 Pa (0.2 Torr) allows for conformal deposition only at 50°C, whereas deposition at a reduced pressure of 4.0 Pa (0.03 Torr) widens the temperature regime (50-70°C) in which excellent conformality can be obtained. The confonnal Co thin film, produced at 50°C and 4.0 Pa, showed a resistivity of 10-12 μΩ cm and contained 1.0 atom % oxygen and less than 1.0 atom % carbon. After annealing this film at 600°C, its resistivity was reduced to 6 μΩ cm, which is close to the bulk resistivity (5.7 μΩ cm) of Co. Therefore, this low-temperature process, which allows for the excellent conformal deposition of pure Co films, can be utilized to produce silicided contacts for advanced devices which require a low contact resistance and good electrical performance.

Effects of molybdenum, silver dopants and a titanium substrate layer on copper film metallization

Annealing of 100 nm-thick Cu, Cu(Mo) and Cu(Ag) films was carried out to investigate the effect of dopant atoms on the films. Molybdenum (Mo) and silver (Ag) were selected as immiscible dopants for out-diffusion studies. A thermally grown SiO2 layer and a sputtered Ti layer were used as substrates. The dopant and substrate effects were characterized in terms of surface morphology, resistivity, preferred orientation, and diffusional characteristics. The lowest observed resistivity was 2.32 μΩ · cm in the Cu(Ag) film, which was lower than that in a pure Cu film of the same thickness. Ag addition enhanced the surface morphology and thermal stability of the Cu(Ag) films. The highest thermal stability was obtained in the case of a Cu(Mo)/Ti film which maintained film integrity to 800°C. A Ti substrate enhanced Cu(111) texture growth. A highly oriented Cu(111)-texture was obtained in the Cu(Mo)/Ti films. Cu diffusion through the Ti layer was limited in the (111)-textured Cu(Mo)/Ti films, which showed good potential as a diffusion barrier.

Fabrication of Cu/Co bilayer gate electrodes using selective chemical vapor deposition and soft lithographic patterning

A templated Cu/Co bilayer gate electrode was fabricated using the combined method of consecutive and selective chemical vapor deposition (CVD), and octadecyltrichlorosilane (OTS) microcontact printing techniques. Soft lithographically patterned self-assembled monolayers (SAMs) can direct the growth of Co occurring at the low temperatures 50–90 °C and serve as a template for the consecutive and selective growth of Cu, thereby forming stable and high quality Cu/Co bilayer gate electrodes on a glass substrate. This simple process provides fewer process steps and higher performance than other conventional processes, and can be applied to the fabrication of large area and high resolution thin film transistor liquid crystal displays.

Thickness effect on grain growth and precipitate coarsening of a copper-silver thin film in an advanced metallization process

Annealing of Cu(Ag)/SiO2/Si films was carried out to investigate the relationship between grain growth and precipitate coarsening. Ag in copper was selected as an immiscible dopant for diffusion and precipitation. Annealing effects were characterized in terms of resistivity, morphology, preferred orientation, grain growth, precipitation, and diffusional characteristics. The lowest observed resistivity was 2.06 μΩ cm in a 900 nm thick film annealed at 500 °C. The resistivity decrease was attributed to Ag precipitation and grain growth. A transition thickness exists at below 220 nm in the grain growth and the precipitate coarsening of the Cu(Ag) films. A relationship was established between grain growth and precipitate coarsening above the transition thickness that fits well to the experimental data. The grain size in the Cu(Ag) film can thus be predicted from the Ag precipitate size by using this relationship.

Thermal Stability Enhancement of Cu Interconnects by Employing a Self-aligned MgO Layer Obtained From a Cu(Mg) Alloy Film

Self-aligned surface and interfacial MgO layers were formed by pre-annealing Cu(Mg)/SiO2/Si and Cu(Mg)/TiN/Si multilayer films at 500°C in an oxygen ambient, resulting in the structures of MgO/Cu/MgO/SiO2/Si and MgO/Cu/MgO/TiN/Si, respectively. During pre-annealing, Mg segregates preferentially to the Cu surface until a dense, uniform MgO layer of 150 A thickness is formed. Substantial Mg segregation to the SiO2 or TiN surface also takes place to form an interfacial MgO layer. Diffusion barrier characteristics of the surface MgO layer were investigated by vacuum-annealing the Si/MgO/Cu(Mg)/MgO/SiO2/Si multilayer structure. It was shown that self-aligned surface MgO produced by the annealing process prevents interdiffusion of Cu and Si up to 700°C. Furthermore, interfacial MgO between Cu and SiO2 or TiN reduces diffusion of Cu into the Si substrate at temperatures up to 700–800°C, indicating that self-aligned interfacial MgO plays an important role in suppressing interdiffusion between the Cu and TiN or SiO2. Consequently, the thermal stability of Cu/SiO2/Si and Cu/TiN/Si multilayer systems is significantly enhanced by introducing self-aligned surface and interfacial layers of MgO from Cu(Mg) alloy films.

Effects of the dissolved oxygen in Ti films on Ti reactions in Cu/Ti/SiO2/Si system upon annealing

The reactions of Cu/Ti/SiO2 structures at temperatures ranging from 200 to 700 °C have been studied for various Ti thicknesses. X-ray and Rutherford backscattering spectroscopy (RBS) analyses were used to identify the reaction products resulting from Ti reactions in Cu/Ti/SiO2 systems and the oxygen composition in the unreacted Ti, and revealed a correlation between the oxygen concentration in Ti films and the sequences of the Ti reactions. The reaction products initially formed, at around 300 °C, were a series of Cu–Ti intermetallics (Cu3Ti/CuTi) at the Cu–Ti interface with the oxygen dissolved in the Ti moving from the compounds into the remaining unreacted Ti. At 500 °C, the Cu3Ti was converted into Cu-rich intermetallics, Cu4Ti, which grew at the expense of the CuTi due to the increased oxygen content in the Ti. In addition, the outdiffusion of Ti, to the Cu surface, and the Ti–SiO2 reactions caused an abrupt increase in the oxygen content in the Ti layer, which placed thermodynamic restraints on further Ti reactions. Furthermore, thinner Ti layers showed a higher increased rate of oxygen accumulation for the same consumption of Ti, which led to significantly reduced Ti consumption. The diffusion barrier properties of SiO2 for Cu metallization decreased with an increasing Ti thickness.

Additive vapour effect on the conformal coverage of a high aspect ratio trench using MOCVD copper metallization from (hfac)Cu(DMB) precursor

A low pressure chemical vapour deposition technique was used to deposit MOCVD copper films on submicron-level trenches. Conformal coverage of the trenches with the MOCVD copper films has been investigated. MOCVD TiN (1000 A) was initially deposited on a Si wafer with trenches employing tetrakis-ethylmethyl-amido titanium precursor. The Cu film was then deposited on the TiN film employing (hfac)Cu(DMB) precursor. A high aspect ratio (7:1) trench was used to investigate conformal coverage of the Cu films. C2H5I vapour was added to the Ar carrier gas stream to alter the deposition characteristics and enhance the conformal coverage of the Cu films. The additive vapour affected the film growth rate, activation energy, preferred orientation, surface morphology/roughness and conformal coverage of the deposited Cu films. The conformal coverage of the Cu films on the trenches was enhanced with the addition of C2H5I and by decreasing the substrate temperature. Optimum conformal coverage was obtained at 140 °C with C2H5I as an additive.

Enhanced adhesion and performance of the source/drain electrode using a single-layered Ag(Cu) film for an amorphous silicon thin-film transistor

The feasibility of using a Ag(Cu) alloy film as a source/drain electrode for thin-film transistor (TFT) liquid-crystal displays has been investigated. The annealing of a Ag(Cu)/Si structure, for 30 min at 200 °C, produced a uniform Cu3Si layer at the Ag(Cu)–Si interface, as a result of the reaction of the segregated Cu with Si. This lowered the resistivity from 5.3 to 3.2 μΩ cm, which also led to improved adhesion properties. A hydrogenated amorphous silicon (a-Si:H) TFT was fabricated using a single layer of Ag (19 at. % Cu) alloy film as the source/drain metal. The subthreshold slope, mobility, and threshold voltage obtained from the fabricated a-Si:H TFT were 0.78 V/dec, 0.79 cm2/V s, and 2 V, respectively, revealing a reduction in the process steps, with excellent performance.

Adhesion, passivation, and resistivity of a Ag(Mg) gate electrode for an amorphous silicon thin-film transistor

The effect of Mg in Ag(Mg)/SiO 2 /Si multilayers on the adhesion, passivation, and resistivity following vacuum annealing at 200-500 °C has been investigated. The annealing of Ag(Mg)/SiO 2 /Si multilayers produced surface and interfacial MgO layers, resulting in a MgO/Ag/MgO/SiO 2 /Si structure. The formation of a surface MgO/Ag bilayer structure provided excellent passivation against air and CF 4 plasma chemistry. In addition, the adhesion of Ag to SiO 2 was improved due to the formation of an interfacial MgO layer resulting from the reaction of segregated Mg with SiO 2 . However, the negligible solubility of Si in Ag prevented the dissolution of free silicon into the Ag(Mg) film produced from the reaction Mg + SiO 2 = MgO + free Si, which in turn limited the reaction between Mg and SiO 2 , which led to a decrease in the adhesion of Ag to SiO 2 at the higher temperature. The use of an O 2 plasma prior to Ag(Mg) alloy deposition on SiO 2 produced an oxygen-rich surface on the SiO 2 which allowed for the enhanced reaction of the segregated Mg and SiO 2 at the surface, thus resulting in markedly increased adhesion properties.

A Study on the Operating Control of a Heat Pump System with Screw Compressors

A preliminary performance test of a 30RT 2-stage screw heat pump was carried out in order to develop a high performance large-scale unutilized energy source heat pump system, which will be used for district heating and cooling. In this study, two issues of the system operating control were investigated. The first issue is the mode switching control from 1-stage to 2-stage. A stable 2-stage heating operation is guaranteed, only if the load-side water inlet temperature is over a certain value, where the 1-stage heating operation should be done first from a cold start. The second issue is oil level control. An oil shortage problem in the low stage compressor, which depends on the degree of suction superheat, was solved by a proper oil level control scheme.