This lecture, Part 2 of a two-part series on sputtering deposition, delves into critical aspects of thin film deposition for micro and nanofabrication. It begins by explaining how step coverage is achieved and controlled in sputtering, highlighting that while good step coverage is generally desirable, specific applications like via filling often require poor step coverage to prevent void formation. Methods for controlling step coverage, including the use of collimators and the ionization of sputtered vapor, are discussed. The lecture then introduces sputter cleaning for in situ wafer preparation and bias sputtering, a technique that leverages concurrent deposition and etching/redeposition to improve feature filling. Finally, it uses aluminum alloy deposition as a practical example to illustrate the importance of process parameters, discusses the issue of film stress and its measurement, and provides a comprehensive overview of various film quality metrics, encompassing compositional, electrical, mechanical, defect, thickness, step coverage, and optical properties.
Sputtering Deposition: Control, Applications, and Film Quality
0:00 Step Coverage in Sputter Deposition: Sputter deposition typically offers good step coverage due to the scattering of deposited material, ensuring a wide range of angles and effective coverage of features, though some shadowing persists.
1:00 Improving Step Coverage Through Mobility: Step coverage can be further enhanced by increasing the mobility of deposited material on the wafer's surface, often achieved by heating the wafer to promote surface diffusion.
2:09 Challenges of Overheating: Excessive wafer heating can lead to the formation of metal grains, resulting in surface roughness, and potential reliability and conductivity issues.
2:44 Monitoring Surface Roughness: Film roughness is monitored using specular reflectivity; a decrease in reflectivity indicates increased scattering and a rougher film surface.
4:05 Via Fill Application: In filling deep features like vias (contact holes) with metal, processes with good step coverage can lead to problematic voids, as the top edges close off before the hole is completely filled.
5:50 Consequences of Voids: Voids pose significant reliability problems and defects, complicating subsequent planarization steps like Chemical Mechanical Polishing (CMP).
6:34 Preventing Voids with Poor Step Coverage: To avoid voids in via filling, deposition processes with intentionally poor step coverage are used, allowing the material to fill the feature from the bottom up.
Lowering Pressure: Reducing chamber pressure increases the mean-free path of sputtered atoms, making them hit the wafer more vertically but severely reducing deposition rates.
Using a Collimator: A collimator, a screen with tall, narrow openings placed above the wafer, physically directs sputtered atoms almost vertically, with wafer scanning ensuring uniform film thickness.
11:25 Ionizing Sputtered Vapor: An alternative for reducing step coverage is to ionize the sputtered atoms and use an electric field to direct them vertically to the wafer.
11:55 Sputter Cleaning: A slight negative bias applied to the wafer can cause argon ions from the plasma to sputter off surface impurities or native oxides (e.g., on silicon) in situ before deposition, but this risks damaging sensitive substrates.
14:05 Bias Sputtering for Enhanced Fill: During deposition, maintaining a negative bias on the wafer causes directional etching by argon ions. The sputtered material redeposits on sidewalls, effectively improving fill characteristics (leading to better step coverage in terms of filling a hole) and achieving a more uniform thickness at the bottom of features, provided the deposition rate exceeds the etch rate.
16:28 Aluminum Alloy Deposition: Aluminum, often alloyed with small percentages of silicon and copper, was a dominant metallization material. Sputtering allows for easy deposition of alloys by using a target of the desired composition.
17:47 Planar DC Magnetron System: This system type is commonly used for aluminum deposition, generating a high-density plasma for efficient, high-rate deposition.
18:11 Importance of Low Contaminants: Keeping oxygen and nitrogen levels extremely low in the chamber is vital to prevent adverse chemical reactions with the aluminum.
18:20 Film Stress and Reliability: Stress in deposited metal films (often due to thermal expansion mismatch during cooling) can severely impact reliability, particularly concerning electromigration under current, and is monitored by measuring wafer bow.
19:56 Key Aspects of Film Quality: A high-quality film is characterized by several factors, including:
Composition: Correct stoichiometry, low impurities, and accurate alloy composition.
Electrical Properties: Desired resistivity (metals) and high breakdown voltage (dielectrics).
Mechanical Properties: Controlled stress levels.
Defects: Absence of pinholes, particles, and good adhesion to the substrate.
Thickness Control: Accuracy and uniformity across the wafer, from wafer-to-wafer, and lot-to-lot.
Step Coverage: Achieving the specific step coverage (e.g., conformal or planarizing) required for the application.
Optical Properties: Desired reflectivity (related to roughness) and refractive index (important for subsequent lithography steps).