Self-aligned Electroless Plating on Aluminum Gates
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Solution Overview
Problem
Electroless plating processes have not been developed for aluminum (Al) due to its oxidation in aqueous baths, preventing self-aligned growth of gate metal contacts, while Al offers a cost advantage over copper (Cu) with moderately higher resistivity.
Innovation Solution
The development of an electroless plating process for aluminum gates using sacrificial cobalt (Co) deposition, where Co is selectively deposited on source and drain contacts but not on the aluminum gates, utilizing boron and phosphorous in the plating bath to assist the process and prevent Co deposition on oxidized metal gates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If electroless plating is used for copper gates, then self-aligned growth of gate metal contacts is achieved, but additional metallization is formed on metal gates which is undesirable
Solution Approach 1:
The patent applies local quality by making the gate metal selectively non-reactive to electroless plating through oxidation. The metal gate surface is treated to have different chemical properties (oxidized state) compared to the source/drain regions, allowing plating to occur only where needed. This resolves the contradiction by enabling self-aligned growth while preventing unwanted metallization on the gate itself.
Solution Approach 2:
The patent converts the harmful oxidation of aluminum (which prevents conventional electroless plating) into a beneficial feature. The oxidized aluminum gate surface acts as a natural barrier that prevents copper deposition, while the oxidation byproducts are removed or managed. This transforms the previously problematic oxidation into a self-protective mechanism that enables selective plating.
2Ease of manufacture
If aluminum is used as gate metal, then cost advantage is achieved, but electroless plating process cannot be developed due to oxidation in aqueous baths
Solution Approach 1:
The patent changes the chemical state parameter of the aluminum gate surface by controlled oxidation. This parameter change creates a surface that is incompatible with electroless copper plating, thereby enabling the use of aluminum gates with electroless plating processes. The oxidation state transformation allows aluminum to maintain its cost advantage while becoming compatible with the manufacturing process.
Solution Approach 2:
The patent introduces an intermediary layer of oxidation products on the aluminum gate surface. This intermediary layer acts as a barrier between the electroless plating bath and the aluminum gate, preventing unwanted copper deposition. The intermediary oxidation layer mediates the interaction between the plating chemistry and the aluminum substrate, enabling process compatibility.
3Device complexity
If electroless plating is performed without selective inhibition, then simple process is maintained, but self-alignment cannot be achieved due to deposition on metal gates
Solution Approach 1:
The patent employs self-service by allowing the aluminum gate surface to automatically inhibit copper deposition through its inherent oxidation in the aqueous plating bath. The gate metal itself provides the inhibition mechanism without requiring external protective layers or complex process controls. This self-inhibiting property maintains process simplicity while achieving the desired self-alignment.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables self-aligned metal contacts on source and drain regions without shorting to exposed metal gates, maintaining the cost advantage of Al while avoiding additional metallization on metal gates, and allowing for further deposition of conductive materials like carbon nano-tubes or refractory metals.
Implementation Method 1
The development of an electroless plating process for aluminum gates using sacrificial cobalt (Co) deposition
Implementation Method 2
Al has a moderately higher resistivity than Cu, but a relative cost advantage. It is desirable to form self-aligned contacts on the source and drain regions while avoiding the formation of additional metallization on metal gates
Data Source
AI summary
A transistor structure includes a semiconductor substrate with a first surface, a diffusion region at the first surface of the substrate, a sacrificial gate formed on the diffusion region, and insulating side walls formed adjacent to the sacrificial gate. A metal gate is formed by etching out the sacrificial gate and filling in the space between the insulating side walls with gate metals. Silicided source and drain contacts are formed over the diffusion region between the side walls of two adjacent aluminum gates. One or more oxide layers are formed over the substrate. Vias are formed in the oxide layers by plasma etching to expose the silicided source and drain contacts, which simultaneously oxidizes the aluminum gate metal. A first metal is selectively formed over the silicided contact by electroless deposition, but does not deposit on the oxidized aluminum gate.