Physical Vapor Deposition (PVD) of Indium Bumps

Flip chip, also known as controlled collapse chip connection (or C4 for short), is a method to interconnect semiconductor devices, such as integrated circuit (IC) chips, microelectromechanical systems (MEMS) and photodiode arrays (PDA), to external circuitry with indium bumps that have been deposited on the devices such as focal plane arrays.

The synthesis of indium bump bonds with a small diameter and corresponding high aspect ratio is imperative for applications with ultra-fine pixel pitch because small diameter features minimize the possibility of electrically shorting adjacent bumps. Thermocompression bonding of indium bumps to join chips is used in 3D superconducting circuitry as the basis to fabricate quantum computers.

Indium metal has become the primary interconnect material because of its cryogenic stability, good thermal and electrical conductivity and highly ductile nature. Photolithography is typically employed to define the via trenches in which indium bumps will be deposited. After deposition, a lift-off process will remove any excess indium and photoresist, making the bumps ready to be connected, by compressive pressure.

Kurt J. Lesker has been manufacturing PVD tools for indium bumps for many years. Because of this, our team of dedicated applications, design and vacuum experts have developed unrivalled know-how about the creation and application of this important class of thin films.

Important thin film properties such as the surface roughness, aspect ratio and crystalline quality of indium bumps can be optimized by carefully tuning the growth temperature, using cooled (often cryogenic) substrate holders and the indium flux.

Other factors, including the overall chamber pressure during the deposition, the throw distance between the source and the wafer, the photoresist used for photolithography and the source temperature. These all contribute to the quality of the deposited indium bumps.

To ensure the fill of trenches, direct deposition should be realized. The deviation angle from the normal direction should be small (~5°- 7°).

The substrate temperature determines both the in-plane growth rate and the crystalline quality of the deposited materials. To achieve a high aspect ratio and optimal crystalline structure, the substrate should be cooled, often cryogenically.

The evaporation rate can result in different growth modes for indium. As a result, the aspect ratio and materials quality are both dependent on the indium flux.

We understand that tool personalization that addresses a researcher's unique requirements is critically important and we have therefore developed and support an extensive capabilities portfolio including:

• HV and UHV base pressure process chamber options for very low film impurities
• Optimized process chamber geometries to enable the filling of trenches with indium and deposition of a highly collimated film
• Multi-site wafer holders (4 to 8 inches) and variable throw distance capability to ensure collimated, low-angle direct deposition
• Ion sources for substrate cleaning prior to deposition
• Resistive thermal heating sources for controlled evaporation of indium metal
• Easily removeable deposition shielding to facilitate easy cleaning of chambers
• In-situ film measurement to accurately monitor real-time deposition parameters
• Process chamber shielding to ensure minimal crosstalk between deposition sources and quartz crystal monitors
• Cooling substrate platens to affect indium film morphology and crystallinity
• Load lock chambers to reduce takt time and increase productivity
• Fully integrated film recipe and system control using our Lesker eKLipse™ process control platform for precise, repeatable deposition conditions, facilitating the growth of high-quality indium bumps

We want to hear from you. Our thin film experts and service support team are eager to help enable your important research.