This lecture, Part 2 of a two-part series on sputtering deposition in micro and nanofabrication, delves into advanced topics of film quality and process control. Professor Chris Mack begins by elaborating on step coverage, explaining how sputtering typically achieves good coverage but can be manipulated for specific applications like via filling, where intentionally poor step coverage is desired to prevent voids. He details methods to control step coverage, including pressure manipulation, the use of collimators, and ionization. The lecture then covers sputter cleaning for in situ wafer surface preparation and bias sputtering, which enhances step coverage by combining deposition with directional etching. An example of aluminum alloy deposition using a planar DC magnetron system highlights practical considerations like alloy composition and contaminant control. Finally, the lecture discusses film stress, its origins and measurement, and provides a comprehensive overview of various critical aspects of deposited film quality, including composition, electrical and mechanical properties, defect control, adhesion, thickness uniformity, and optical properties like reflectivity and refractive index.
Sputtering Deposition: Control and Film Quality
0:19 Step Coverage in Sputtering: Sputter deposition generally provides good step coverage due to significant scattering of deposited material, ensuring a wide range of arrival angles at the wafer. While better than line-of-sight methods like evaporation, shadowing still occurs.
0:22 Improving Step Coverage via Mobility: Step coverage can be improved by enhancing the mobility of deposited material on the wafer surface, often by heating the wafer to promote diffusion. However, excessive heat can lead to the formation of large metal grains, causing surface roughness (monitored by specular reflectivity) and potential reliability issues.
4:05 Via Fill Application: For filling small holes (vias), good step coverage can paradoxically lead to reliability-compromising voids, as the top opening can close before the hole is fully filled.
6:35 Achieving Void-Free Via Fills: To prevent voids in via fills, a process with poor step coverage is preferred. This ensures that material builds up faster at the bottom of the via, filling it completely before the top closes, leaving an indentation that can be removed by chemical mechanical polishing (CMP).
Lowering Pressure: Increasing the mean-free path of target atoms results in more vertical incidence, but drastically reduces deposition rates.
Collimators: Using a screen with tall, narrow openings (collimator) above the wafer directs sputtered atoms nearly vertically, even at moderate pressures, but requires scanning the wafer for uniformity.
Ionization and Electric Fields: Ionizing the sputtered vapor and using an electric field can precisely direct atoms vertically onto the wafer.
11:57 Sputter Cleaning: Applying a negative bias to the wafer in a sputtering system allows argon ions to directly sputter away unwanted surface contaminants (like native oxides) in situ before deposition, though this can potentially damage delicate substrates.
14:05 Bias Sputtering: Maintaining a negative bias on the wafer during deposition allows for simultaneous deposition and etching. The directional etching, particularly at the bottom of features, and subsequent redeposition of sputtered material onto sidewalls, can lead to better step coverage and more uniform film thickness.
16:30 Aluminum Alloy Deposition: Aluminum, often alloyed with silicon (2%) and copper (0.5%) for improved electrical and reliability properties, is a common metal deposited via sputtering. Sputtering allows for easy deposition of alloys by using a target with the desired composition.
17:47 Planar DC Magnetron: This common sputtering system uses a DC electric field and magnetic field to create a high-density plasma, resulting in high deposition rates. It's critical to maintain very low levels of contaminants (e.g., oxygen, nitrogen) to prevent unwanted chemical reactions.
18:24 Film Stress: Stress in deposited films, often caused by thermal expansion mismatches between the film and substrate during cooling from high deposition temperatures, can significantly impact device reliability (e.g., electromigration). Stress is typically measured by monitoring changes in wafer bow.
19:43 Key Aspects of Film Quality: Important film quality considerations include:
Composition: Purity, stoichiometry, and alloy accuracy.
Electrical Properties: Resistivity (metals) and breakdown voltage (dielectrics).
Mechanical Properties: Stress.
Defects: Absence of pinholes and incorporated particles.
Adhesion: Strong bonding to the substrate.
Thickness: Accuracy and uniformity (across wafer, wafer-to-wafer, lot-to-lot).
Step Coverage: Achieving the desired profile, whether conformal or planarizing.
Optical Properties: Specular reflectivity (indicates roughness) and refractive index (important for lithography).