Chemical Vapor-deposited Copper Thin Films On Self-assembled Monolayers

Copper has lower resistivity and higher resistance to electromigration and stress migration compared with aluminum alloys. To fabricate the high-performance interconnects with low RC delay, the integration of low resistivity metal wiring and low-k inter-metal dielectric is crucial for next-generation ultra large-scale integrated circuit technology. To implement copper into a metal line via interconnects, the dual damascene process is required because copper is difficult to etch. In considering methods to deposit copper, metal-organic chemical vapor deposition processes have several advantages such as the ability to achieve good conformality and the possibility of selective deposition.Compared to traditional physical vapor deposition (PVD) to prepare metal thin films, chemical vapor deposition (CVD) become the first choice to deposit copper thin films in ultra large-scale integrated circuit because of its good step coverage and the good ability to fill higher aspect ratio trench and contact-via holes in the multilevel interconnection structure.This research was based on a MOCVD (metalloorganic chemical vapor deposition) system, which was designed, built previously. Cu^â…¡(hfac)₂ was used as precursor, and hydrogen as carrier gas and reactant. Chemical vapor deposited copper thin films on Si (100) substrate, SiO₂ substrate, 3-mercapto-propyltrimethoxysilane (MPTMS) and 3-amino-propyltrimethoxysilane (APTMS) self-assembled monolayers (SAMs) modified Si substrate. It is mainly composed of the following five parts:Chapter 1: Briefly introduce the background, the challenge, the ideas and the study content of chemical vapor deposition copper thin films. Chapter 2: Chemical vapor deposition copper thin films on Si (100) and SiO₂ substrate were studied. It shows: 1. Compared to N₂, using H₂ as carrier gas can deposit copper thin films at lower temperature, and there was no Cu₂O found in the deposited copper thin films. The copper thin films were polycrystalline when using H₂ as carrier gas. The Cu (111) was the preponderant face in the films deposition. 2. By measuring the thickness of the copper thin films, we got that the deposition rates of copper thin films on Si (100) substrate are 8～22 nm/min and 11～25 nm/min on SiO₂ substrate, respectively. We also get that the deposition activation energies are 92.35 k J/mol for Si (100) substrate and 81.3 kJ/mol for SiO₂ substrate by calculation.3. SEM shows that at the same deposition temperature more copper deposited on the hydroxylated SiO₂ substrate than on the Si substrate. It is easier for copper to deposit on the hydroxylated SiO₂ substrate.Chapter 3: Chemical vapor deposition copper thin films on MPTMS-SAMs modified substrate was studied. It was found: 1. MPTMS formed an ordered self-assembled monolayer with a thickness of 0.66±0.01 nm on the surface of SiO₂ substrate. 2. It is easier to deposit copper thin films on MPTMS-SAMs modified substrate. The coverage of copper on MPTMS-SAMs surface is also much lager than that on Si (100) and SiO₂ substrate surfaces. The deposition temperature can be effectively reduced and continuous copper thin films can be obtained at lower temperature when using MPTMS-SAMs as substrate. 3. The apparent activation energy of Cu CVD is reduced from 92.35 kJ/mol for the Si substrate to 69.2 kJ/mol for the MPTMS-modified substrate. The deposition rate also can be largely improved.4. The electromigration resistance of copper thin films at 350℃on SAMs modified substrate was similar to that on Si substrate at 400℃. 5. MPTMS-SAMs can effectively prevent copper from diffusing into substrate. When the voltage was±1 V, the leakage current was only 10^-6～10^-8 A/cm². 6. XPS shows: There was a strong Cu-S interaction between copper and the -SH terminal group of MPTMS-SAMs. That is the reason that it's easier to chemical vapor deposited copper thin films on the surface of MPTMS-SAM.Chapter 4: Chemical vapor deposition copper thin films on APTMS-SAMs modified substrate was studied. It can be concluded: 1. APTMS formed a self-assembled monolayer with a thickness of 1.77±0.06 nm on Si substrate at low concentration of APTMS, while gathering at the surface and forming disordered multilayers at high concentration of APTMS. 2. It is easier to deposit copper thin films on APTMS-SAMs modified substrate. The coverage of copper on APTMS-SAMs surface was also much lager than that on Si (100) and SiO₂ substrate surfaces. The deposition temperature can be effectively reduced and better copper thin films can be got at lower temperature when using APTMS-modified substrate. 3. The electromigration resistance of copper thin films at 350℃on APTMS-SAMs modified substrate was dramatically enhanced compared to that on Si substrate at 400℃. 4. The surface roughness of CVD copper thin films on APTMS-SAMs modified substrate is reduced as the deposition temperature increased, while it keeps constant when the temperature is higher than 330℃. 5. APTMS-SAMs can effectively prevent copper from diffusing into substrate. When the voltage was±1 V, the leakage current was 10^-7～10^-8 A/cm². 6. XPS shows: There was a strong Cu-N interaction between copper and the -NH₂ terminal group of APTMS-SAMs. That is the reason that it's easier to chemical vapor deposited copper thin films on the surface of APTMS-SAMs.Chapter 5: The terminal group of HS-SAMs which was self-assembled by MPTMS was modified. Copper was deposited on three SAMs with three different terminal groups using Cu^â…¡(hfac)₂ as precursor. By advancing and receding contact angle measurement, X-ray photoelectron spectroscopy, scanning electron microscopy, energy dispersive analysis of X-rays technology, it shows: 1. A thiol-terminated monolayer was formed on the MPTMS-SAMs modified SiO₂ substrate. 2. The thiol terminal group was oxidized to -SS- group when treating HS-SAMs with KHCO₃ and bromide at room temperature, while transformed to -SO₃H group when treating with 30% H₂O₂ and acetic acid at 40-50 centigrade. 3. Copper was selectively deposited on the three SAMs with different terminal groups at the lower deposition temperature and shorter deposition time. It is easiest for copper to deposit on HO₃S-SAMs, then on HS-SAMs, most difficult on -SS- terminal SAMs. 4. As temperature increased or time elapsed, the selective deposition phenomena of copper on the three various self-assembled monolayers disappeared. 5. Finally we can pattern CVD copper thin films by modifying terminal groups of SAMs to various functional groups. Cu thin film was deposited on the MPTMS-SAMs regions on a patterned SAMs site-selectively because of the interactions between the Cu and the thiol groups. A micropattern of Cu thin film on a PET substrate was thus successfully fabricated without using as etching process.