What Process Window Governs Ru Liner Removal in Sub‑7 nm Dual Damascene Interconnects?

Ruthenium liners are integrated into copper dual damascene flows at linewidths where conventional TaN/Ta barrier resistivity consumes significant cross-sectional copper volume. Atomic layer deposition produces a Ru layer of 3–6 nm on low‑k dielectrics after barrier metal deposition. The ruthenium slurry is applied after bulk copper removal and must clear the Ru films while stopping on carbon-doped oxide or porous low‑k dielectrics. On 300 mm rotary CMP tools such as the Applied Materials Reflexion LK or Ebara FREX 300, typical process parameters are downforce 1.2–2.5 psi, platen speed 70–100 rpm, head speed 60–90 rpm, and slurry flow 150–300 mL/min. Pad choice is often a hard polyurethane pad with in-situ diamond disk conditioning to maintain surface texture. The transition from copper to ruthenium to low‑k generates a distinct motor current signature because the coefficient of friction changes at each interface. Closed-loop endpoint algorithms using Fourier transform analysis of motor current are implemented to stop the polish before excessive dielectric erosion occurs.

The slurry system uses colloidal silica with a mean particle size 30–60 nm measured by dynamic light scattering per ISO 22412:2017. The oxidizer package is usually periodate-based rather than hydrogen peroxide because periodate drives ruthenium oxidation at a controlled rate while limiting RuO4 volatilization at alkaline pH. The pH is buffered in the range 8–10 with KOH or with an organic alkaline buffer; this range shifts the ruthenium surface to a mixed RuO2/RuO4²⁻ state and reduces the violet RuO4 vapor pressure that appears in acidic formulations below pH 3. Removal rate and selectivity to low‑k are monitored with patterned wafer testing; final dielectric erosion is typically held below 2–3 nm per barrier clear. Post-cleaning uses dilute organic acid or TMAH-free formulations to remove silica and metal residues without increasing porous low‑k k-value shift. Slurry filtration through a 0.1 µm membrane is mandatory before dispense because oversized particles generate micro-scratches that compromise line yield at sub‑7 nm nodes.

When Ruthenium Hard Masks Are Polished on Patterned DRAM Capacitor Wafers

The planarization of ruthenium hard masks on DRAM capacitor modules is constrained by the need to preserve tight capacitor height and avoid dishing across high-density cylinder arrays. Ruthenium hard mask overlayers are deposited over dielectric or TiN/high‑k stacks, and CMP removes the hard mask after electrode etchback. The slurry requires high Ru removal rate but low removal of the sublayer, typically TiN or silicon dioxide, to prevent capacitor height collapse. Process conditions on 300 mm systems use low downforce 1.0–2.0 psi to limit delamination at the Ru/dielectric interface; platen speed is maintained between 60–90 rpm and the head is operated in a floating mode to manage wafer curvature generated by thick memory stack films. Endpoint detection uses motor current or eddy current signatures because the Ru hard mask layer has a distinct frictional response.

Formulation design for this segment diverges from logic slurries because the Ru surface area on patterned capacitor wafers is highly localized and the pattern density varies across the die. Ceria abrasives or mixed silica-ceria slurries are used to raise Ru removal rate without aggressive pad embossing. The oxidizer concentration is adjusted to give a Ru removal rate in the 30–70 nm/min range on blanket films, with a removal rate ratio of Ru:TiN above 5:1 in typical process qualification. Selectivity to silicon dioxide is tested by measuring oxide erosion on 100 µm line/space patterns with an atomic force profiler; oxide loss is commonly specified below 3 nm after endpoint. Batch-to-batch variation in abrasive aggregate size is controlled by large-particle counting and zeta potential measurement; slurry with aggregate counts above 100 counts/mL at ≥0.5 µm is rejected before dispense. The terminal capacitor device requires uniform electrode height because uneven planarization shifts capacitance distribution and degrades refresh time in high-density DRAM architectures.

In copper interconnect reliability builds, ruthenium capping layers of 2–5 nm are selectively deposited onto Cu lines to suppress electromigration and improve adhesion to the surrounding dielectric. A low-pressure touch CMP operation removes the ruthenium overburden from the adjacent dielectric while preserving the Ru cap over Cu. This operation uses a high-selectivity slurry that oxidizes ruthenium rapidly but passivates Cu through an inhibitor system. The touch CMP step is run at downforce 0.5–1.2 psi on soft or hybrid pads to avoid increasing line resistance by thinning the Cu cap. The dielectric loss limit is tighter than in standard barrier CMP; post-touch oxide erosion below 1.5 nm is required in many 300 mm process specifications.

Formulations for Ru cap touch polishing often replace standard silica with low-abrasive or abrasive-free oxidizer solutions to minimize scratches on Cu and low‑k surfaces. The oxidizer is typically an aqueous solution of periodate or persulfate at acidic-to-neutral pH, combined with a Cu corrosion inhibitor such as benzotriazole at 0.01–0.1 wt%. The endpoint is determined by monitoring Cu or Ru thickness through eddy current measurements on a post-CMP metrology stand. Slurry filtration through a 0.1 µm membrane is mandatory because large particles induce line micro-scratches in low‑k areas. The material is dispensed through an integrated slurry delivery system with low-shear diaphragm pumps and point-of-use ultrasonication to break down soft agglomerates without fracturing abrasive particles. In high-volume production, the touch polish is sequenced immediately after the main barrier CMP step to prevent air exposure of the copper surface from promoting corrosion under residual slurry films.

Galvanic Corrosion and Pitting Control During Ru Barrier Clearing

Because ruthenium possesses a more noble electrochemical potential than copper, an aqueous CMP environment can form a galvanic couple at the Cu/Ru interface when both metals are exposed during barrier clearing. This couple drives enhanced Cu dissolution or pitting if the slurry lacks an effective copper inhibitor. Formulations for Ru barrier clearing therefore incorporate azole derivatives, typically benzotriazole or tolyltriazole, at concentrations between 0.005 and 0.1 wt%, to form a Cu-benzotriazole film that reduces the corrosion current density. Electrochemical measurements using rotating disc electrodes in pH 8–10 periodate systems show that the Cu static etch rate must be held below 1 nm/min and the Ru static etch rate below 2 nm/min to maintain final metal line integrity. Patterned wafer open-circuit potential differences are controlled by reducing oxidizer concentration after endpoint and adding a conductivity-adjusting salt to keep slurry conductivity below 200 µS/cm.

Pitting on ruthenium films after CMP is often caused by local accumulation of RuO4 or by residual ceria or silica particles adhering to the surface. A post-CMP clean sequence with dilute oxalic acid or citric acid at pH 2–4 is applied on polishing platforms fitted with brush stations. Brush cleaning uses PVA brushes and a rinse of ultra-pure water with CO2 injection to prevent particle redeposition. The cleaned wafers are then dried in an isopropyl alcohol vapor dryer. Defect metrology on KLA Surfscan SP2 or SP3 instruments at capture size 0.16 µm typically monitors post-barrier CMP defect density; a release limit below 50 defects per wafer is used in many advanced logic flows. The ruthenium slurry itself is specified with anion tolerance because residual chloride or sulfate aggravates pitting and must be controlled below 10 ppm by ion chromatography per ISO 10304-1:2009.

Embedded MRAM flows that integrate perpendicular magnetic tunnel junction stacks introduce a ruthenium CMP challenge in which the Ru layer may act as a spacer or hard mask and must be polished without damaging thin MgO tunnel barriers beneath. The Ru film thickness in pMTJ hard mask stacks is typically in the 5–30 nm range, and the CMP process is used to planarize the top Ru layer before etch or to remove a sacrificial Ru mask after patterning. Because the underlying magnetic stack is sensitive to mechanical stress and metal ion diffusion, the slurry must be formulated with ultralow trace metal contamination and low downforce. Polishing parameters are limited to 0.5–1.5 psi downforce and platen speeds of 40–70 rpm, often on a soft pad to reduce shear stress.

The terminal devices require ruthenium remaining only on defined magnetic tunnel junction pillars; residual Ru metal ion contamination above 10¹⁰ atoms/cm² on the surrounding dielectric can shift tunneling magnetoresistance and degrade magnetic anisotropy. Slurry suppliers therefore control transition metal impurities by ICP-MS using an acid-hydrolyzed digest and a cleanroom bottling environment. The abrasive concentration is reduced to minimize scratching of the MgO interface; formulations may use colloidal silica at 0.5–2 wt% solids loading with a mean particle size below 40 nm. Process qualification includes patterned MRAM wafer tests measuring shorting between top and bottom electrodes and normalized TMR retention after annealing at 300 °C for 60 min. Batch release requires a particle size distribution uniformity where the D90/D50 ratio is less than 1.5 by ISO 22412:2017 dynamic light scattering.

Slurry Impurity Budgets and Metrology-Driven Release in 300 mm Fabs

EL-grade ruthenium metal polishing slurry for 300 mm fabrication facilities is released against a comprehensive impurity budget that aligns with front-end-of-line contamination requirements. The slurry is packaged in fluoropolymer-lined drums and dispensed through a filtration loop that maintains particle size stability over a 30-day shelf life. The release table below lists the main electronic-grade control points and the corresponding metrology used in high-volume CMP lines.

The dispense system uses low-shear diaphragm pumps and point-of-use 0.1 µm filters to prevent aggregate introduction. Slurry batches are qualified on a monitor wafer before lot release; the monitor wafer removal rate must fall within a defined control range, commonly centered at 30–70 nm/min for blanket Ru films. Viscosity is held between 1.0 and 3.0 mPa·s at 25 °C to ensure stable pad film thickness and uniform wafer-scale removal. Cleanroom bottling is conducted under ISO 14644-1 Class 5 conditions to limit airborne particle contamination. The terminal product in all described segments is a planarized 300 mm wafer with ruthenium remaining only in designed features, ready for post-CMP clean and subsequent dielectric or electrode deposition.