Role of Potassium Permanganate-Based Solutions in Controlling the Galvanic Corrosion at Al-Co Interface

Abstract

Al-Co metal gate has been recently proposed both for 25 and 11 nm devices as an alternative for Al-Ti used for 45 nm structures. The polishing dispersions used for Al-Ti gates when tested on Al-Co showed severe pitting and static etch rates upto ∼10 nm/min. The present work describes the development of the required chemistry to reduce the corrosion potential (E corr ) gap between Al and Co and minimize galvanic corrosion. The effect of pH, several oxidizers and additives on the open circuit potentials (E oc ) of Al and Co was investigated and it was discovered that solutions of KMnO 4 and saccharides help reduce the E corr gap in between Al and Co to ∼10 mV. A preliminary corrosion protection mechanism involving the formation of a protective manganese oxide coating on the surface of Al/Co is proposed.Reduction of SiO 2 gate oxide thickness and length has been crucial to the continued scaling down of CMOS devices and obtain performance gain. For example, the 90 nm node had a gate oxide thickness between 8-12 Å. 1,2 In 32 nm devices, a further reduction in the gate oxide thickness is needed but was not possible due to the associated increase in leakage current. 3-6 A combination of high-K dielectric gate film with an appropriate metal gate has effectively solved the problem, as was shown by several authors. [7][8][9] For integrating the metal gate as well as the high-K gate oxide, the gate last or replacement metal gate (RMG) scheme is widely used. 10-13 During this process, the gate is initially fabricated with a dummy material (generally poly-Si), which is later etched off and filled with the appropriate gate metal. 14 Scaling to a gate length (L gate ) of ∼30 nm has been successfully achieved using Al as the gate material. 15 A suitable wetting layer, usually Ti, is needed to flow the Al into and fill the trench vacated by the dummy gate during the RMG process. A typical structure is shown in Fig. 1. However, for L gate < 25 nm, after the poly open CMP, Ti deposited using physical vapor deposition (PVD) is reported 15 to exhibit overhangs at the top of the gate which would retard the flow of Al into the open space. Kwon et al., 16 at IBM, have very recently developed a novel cobalt-aluminum-based metal fill scheme in the replacement metal gate (RMG) process that can be scaled down to 11 nm. Using Co instead of Ti improved the flow of Al into the high aspect ratio trench as well as maintained the desired electrical properties of the gate.One of the challenges of this strategy is that Co and Al in contact with each other in an aqueous medium during a chemical mechanical planarization (CMP) step would form a galvanic couple due to their high (>1V) open circuit potential (E oc ) difference. Galvanic corrosion can occur when two metals with such a large E oc difference contact with each other within the device architecture and are exposed to charge conducting CMP slurries. Al acts as the anode and corrodes faster in the presence of Co as its standard electrode potential (E • = −1.66 V) is low compared to that of Co (...

An Aluminum Gate Chemical Mechanical Planarization Model for HKMG Process Incorporating Chemical and Mechanical Effects

“…It is found that KMnO 4 is a good oxidizer which can achieve oxidbillity preservation and be used to enable the formation of a protective MnO 2 film to reduce the corrosion potential gap and minimize galvanic corrosion. 8,9 According to the Pourbaix diagram of Al, 7 the oxide or hydroxide is not stable and can be dissolved into the slurry under the acidic environment and the rate constant k 32 :…”

“…During the first stage, the excess Al bulk is polished till the remaining thickness is just several hundreds of angstroms, and then the remaining Al, barrier and oxide layers are removed on the second polishing step. Because of the process control of the metal barriers, such as Al-Ti, or Al-Co and SiO 2 at very similar rates after removing the overburden alloy films, 8 the dishing value is mainly coming from the oxide and aluminum removal process. Therefore, the present model just focuses on this process especially after the clear removal of the barrier above oxide.…”

“…This corrosion protection is likely due to the formation of a film consisting of some oxidized form of sucrose and MnO 2 which also inhibits the loss of electrons from sucrose, further lowering the currents. 8,9 Otherwise, the control of the metal gate height uniformity and defectivity, including microscratch and corrosion defect types in Al-CMP process is quite crucial to influence the device and yield performance of HKMG structures. In spite of extensive experimental researches of Al-CMP, 8-20 many fundamental effects of chemical reagents on the material removal are not well understood, especially for HKMG structure which has not been formulated systematic mathematical models to adequately describe the process mechanism.…”

“…8,9 Otherwise, the control of the metal gate height uniformity and defectivity, including microscratch and corrosion defect types in Al-CMP process is quite crucial to influence the device and yield performance of HKMG structures. In spite of extensive experimental researches of Al-CMP, [8][9][10][11][12][13][14][15][16][17][18][19][20] many fundamental effects of chemical reagents on the material removal are not well understood, especially for HKMG structure which has not been formulated systematic mathematical models to adequately describe the process mechanism. Therefore, in order to obtain a good HKMG Al gate surface planarity, chemical and P61 physics-based model is quite helpful in providing fundamental insight on the evolution of the gate profile, optimization and control guidance of the process, and performing sensitivity analyzes of operating parameters during Al-CMP.…”

PASSIVATION OXIDATION OF THE SURFACE OF AMA Al87Ni8Y5 IN THE PRESENCE OF OXYGEN-CONTAINING INHIBITORS

“…Відомий також спосіб покриття поверхні порівняно менш благородними металами, які легко піддаються окисненню, утворюючи на поверхні оксидні або гідроксидні захисні шари [6][7][8][9][10]. Однак такі самочинно утворені захисні шари бувають недостатньо однорідними, тому для їхнього ущільнення використовують збагачені на оксиген сполуки [11][12][13][14][15]. За теорією корозії [16] на поверхні сплаву утворюється захисний оксид легуючого елемента, який ускладнює дифузію та окиснення основного металу.…”