Sputtering in IC Fabrication

Key Takeaways

• Sputter deposition involves ejecting material from a target to a substrate through plasma generated by inert gas, forming uniform thin films.

• Sputtering is crucial for IC fabrication, enabling the deposition of contact metals, barrier layers, and conductive components, ensuring chip performance and stability.

• Sputtering target material quality directly impacts semiconductor uniformity, performance, and cost-effectiveness, which are vital for nanoscale components in evolving technology.

Sputtering in ic fabrication diagram.

Sputter deposition is a type of physical vapor deposition (PVD) method of thin film deposition, used many times as part of the IC fabrication process and in ceramic PCB manufacturing.  Sputtering involves ejecting material from a "target" that is a source onto a "substrate," creating a thin film.  Sputtered thin films have excellent uniformity, density, and adhesion. Read on as we discuss sputtering in IC fabrication and how it is accomplished.

Sputtering Uses in IC Fabrication

Explaining the Sputtering Process

Placed within a vacuum chamber, the target material (acting as the source) and the substrate (the intended destination) are subjected to a voltage differential. This voltage setup designates the target as the cathode while the substrate is linked to the anode, as shown in the figure above.

Through ionization of a sputtering gas—commonly an inert gas like argon or xenon—a plasma is generated. Inert gases are favored as sputtering gases due to their minimal reactivity with the target material and process gases and their capacity to yield heightened sputtering and deposition rates owing to their substantial molecular weight.

Thin Film Formation Step-by-Step

The sputtering process commences as the sputtering gas bombards the target material. This collision results in an energy transfer that propels target particles, leading them to escape, traverse, and eventually settle onto the substrate as a film. As the gas enters the chamber for sputtering, the following steps occur:

1. Material is eroded from the target under the bombardment (cathode) of the gas. This phenomenon corresponds to the sputtering mechanism elucidated in this context.

2. Ions become integrated into the target material, possibly even forming a chemical compound. This phenomenon takes on the name of (reactive) ion implantation.

3. The ions amass on the impacted substrate, culminating in the creation of a layer. This specific scenario is known as ion beam deposition.

Sputtering Specifics

As atoms are expelled from the target, they exhibit a broad energy spectrum, often reaching up to tens of electronvolts (equivalent to 100,000 Kelvin). Among these expelled particles, sputtered ions—typically a minor portion, around 1 percent—are ionized. These ions can travel in direct trajectories from the target, impacting the substrates or the vacuum chamber with substantial energy, thereby inducing resputtering. Alternatively, in scenarios of heightened gas pressures, the ions interact with gas atoms, which act as moderators. This interaction causes the ions to move diffusively, leading them to eventually reach the substrates or the walls of the vacuum chamber. They undergo condensation after undergoing a random walk through this diffusive process.

The range of behavior spans from high-energy ballistic collisions to low-energy thermalized motion, which can be manipulated by adjusting the background gas pressure. Typically, an inert gas like argon serves as the sputtering gas. For effective momentum transfer, the atomic weight of the sputtering gas should closely match that of the target material. Consequently, for sputtering lighter elements, neon proves advantageous, whereas heavier elements like krypton or xenon find application. Reactive gases also find utility for sputtering compounds. Depending on the process parameters, compounds can form on the target surface, during flight, or on the substrate. The multitude of parameters governing sputter deposition contributes to its complexity. However, this complexity grants experts significant control over film growth and microstructure.

Sputtering for IC Fabrication

Sputtering is used extensively in the semiconductor industry to deposit thin films of various materials in integrated circuit processing. Because of the low substrate temperatures used, sputtering is an ideal method to deposit contact metals for thin-film transistors. Semiconductor chips have high technical requirements and high prices for sputtered targets. Their requirements for sputtered targets' purity and technology are higher than flat-panel displays, solar cells, and other applications. 

• The sputtering target is used to fabricate the barrier layer and the packaging metal wiring layer. For the barrier layer, the principal metal targets encompass the tantalum and titanium sputtering targets. The barrier layer assumes a dual role: firstly, it acts as a deterrent and insulator, safeguarding against the diffusion of conductive layer metals into the wafer's main silicon material. Secondly, it serves as an adhesion point for bonding metal silicon materials. 
• In the wafer manufacturing process, the target material is mainly used to make the wafer’s conductive layer, barrier layer, and metal grid. These targets predominantly involve metals like aluminum, titanium, copper, and tantalum. The conductive layers facilitate the controlled flow of electric current, enabling transistor, capacitor, and interconnect functionality. 
• In the chip packaging process, the sputtering target material is used to generate the metal layer under the bump, wiring layer, and other metal materials. The metal target materials mirror those used in wafer fabrication, encompassing copper, aluminum, and titanium. Among these, the primary metal targets employed for constructing the conducting layer encompass the aluminum sputtering target and copper sputtering target. 

Although the amount of target materials used in wafer manufacturing and chip packaging is small,  the cost of target materials in wafer manufacturing and packaging process accounts for about 3%. However, the quality of sputtering target materials directly affects the uniformity and performance of the conductive layer and barrier layer and further affects chip transmission speed and stability, so sputtering target materials are the core raw materials for semiconductor production one of the materials.

The dimensions of modern transistors and other components have reached nanoscale levels, demanding precise layering of materials to ensure functionality and reliability. Sputtering, with its capability to deposit materials with sub-nanometer precision, is vital in enabling the creation of these highly miniaturized components. 

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