Titanium nitride earns its place in the metal-etch stack for one blunt reason: photoresist alone often cannot survive the ion energy and chemistry needed to cut a clean profile through a modern metal or contact layer, and a thin, hard, chemically robust TiN film can. Sitting between the resist and the metal it is meant to protect, the TiN hardmask absorbs the etch's most aggressive plasma exposure, transfers the resist's pattern with a controlled bias, and is stripped away once its job is done, leaving a metal feature whose critical dimension was set by the hardmask rather than by resist that would have eroded before the etch finished. As metal and contact etch chemistries have grown more aggressive with each generation, the margin between what a resist mask can survive and what an etch step actually demands has narrowed to the point where a hardmask is no longer optional on the most demanding levels. TiN earns that role over other candidate hardmask materials because it combines a dense, etch-resistant film with a deposition and removal process that integrates cleanly into an existing metal-etch flow without introducing new contamination risk.

TiN hardmask films are deposited by PVD, CVD, or ALD at a thickness of roughly 20 nm to 60 nm, thin enough to keep the mask stack's aspect ratio manageable while still providing enough etch budget to outlast the underlying metal etch step. PVD TiN, sputtered from a titanium target in a nitrogen ambient, is typically laid down at a substrate temperature near 350 °C to 450 °C and offers a dense, low-defect film well suited to blanket coverage over planar topography. ALD TiN, by contrast, grows in a self-limiting cycle that adds roughly 0.1 nm per cycle, giving far tighter thickness control and better step coverage into the high-aspect-ratio features that PVD conformality alone cannot reliably fill. A production PVD chamber commonly holds thickness uniformity within 2% across a wafer, and run-to-run thickness drift is tracked continuously so that the hardmask etch budget does not silently shrink over hundreds of wafers.

Stoichiometry control matters as much as thickness, since a TiN film with excess titanium etches faster and offers less selectivity than a film held close to a 1:1 titanium-to-nitrogen ratio. Nitrogen flow during PVD deposition is tuned so the resulting film sits within a few % of stoichiometric TiN, because a nitrogen-deficient film measurably increases the etch rate seen by the downstream metal etch chemistry and erodes the very selectivity the hardmask exists to provide. XPS composition scans quantify the titanium-to-nitrogen ratio directly at the film surface and through a sputter depth profile, giving a compositional check that a thickness measurement alone cannot provide. A film that drifts more than roughly 5% off stoichiometric composition typically shows a measurable jump in etch rate during the subsequent hardmask open step, which is why composition is checked as routinely as thickness on a qualified process. Nitrogen partial pressure during sputter deposition is typically controlled to within a few % of its setpoint across a full production shift, since a slow drift in that parameter is otherwise the most common root cause of a gradual, hard-to-diagnose selectivity decline over hundreds of wafers. Target erosion on a PVD chamber can itself shift the effective nitrogen-to-titanium arrival ratio at the wafer over the life of a target, so composition is periodically rechecked rather than assumed stable from initial qualification alone.

Etch selectivity is the entire value proposition of a TiN hardmask, and it has to hold in two directions at once: high selectivity to the photoresist above it during hardmask open, and high selectivity to the metal below it during the main etch. TiN-to-resist selectivity typically peaks near 5x at a moderate bias power, a window chosen because pushing bias higher erodes the thinning resist faster than it improves TiN etch rate, while TiN-to-metal selectivity continues climbing toward roughly 20x as bias power increases, since the metal etch chemistry is comparatively insensitive to the harder TiN surface. Because these two selectivity curves move in different directions across the same bias sweep, the practical process window sits close to the TiN-to-resist peak rather than at the highest bias available, trading a small amount of metal selectivity for a hardmask open step that does not punch through the thinning resist early. A selectivity below roughly 3x at either interface is generally treated as a red flag during process qualification, since it leaves too little margin for the ordinary thickness and composition variation seen across a full wafer.

Pattern transfer through the TiN hardmask introduces its own etch bias, and controlling that bias is what ultimately sets the final critical dimension delivered to the metal below. A typical hardmask open step narrows or widens the resist-defined opening by roughly 5 nm to 15 nm as it cuts through the TiN, and because the subsequent metal etch inherits whatever profile the hardmask leaves behind, any drift in that bias propagates directly into the finished line width. Sidewall angle through the TiN is held close to vertical, typically within a few ° of 90°, since a sloped hardmask sidewall telegraphs directly into a sloped metal sidewall that degrades downstream gap-fill and reliability. Final CD is commonly held within ±2 nm of target across the hardmask-defined features, a tolerance that depends on the hardmask etch bias being reproducible from wafer to wafer rather than merely being on-target on average.

Removing the TiN hardmask after the metal etch has to be selective enough to leave the freshly etched metal profile untouched, which usually rules out reusing the same plasma chemistry that opened the hardmask in the first place. A dedicated ash or wet-strip step, run near 250 °C for roughly 60 s in an oxygen-based plasma, clears residual TiN without attacking the exposed metal sidewall, and any TiN left behind after strip shows up immediately as an electrical short or a downstream contamination source. Because a thin native oxide can form on exposed TiN between the etch chamber and the strip chamber, queue time between steps is controlled tightly enough that it does not measurably change strip performance. SIMS depth profiling after strip confirms that titanium and nitrogen signal has dropped to background levels rather than merely appearing visually clear, which is a distinction that matters when residual TiN is only a few nm thick. Strip endpoint is typically confirmed once the SIMS titanium signal falls more than 90% from its as-etched level, and a wafer that fails to clear that threshold within the qualified 60 s window is routed back for an extended strip rather than passed forward. Queue time between the metal etch chamber and the strip chamber is commonly held under a plant-specific limit measured in tens of minutes to keep native-oxide growth on exposed TiN from measurably changing strip rate.

Film stress in the TiN hardmask has to stay low enough that the mask does not lift, crack, or distort the pattern it is meant to protect, particularly as the underlying feature pitch shrinks toward advanced nodes. Compressive PVD TiN stress is generally kept under a few hundred MPa-equivalent by tuning sputter pressure and bias, and a film that drifts outside its qualified stress window tends to show pattern-dependent CD shifts that are difficult to distinguish from an etch-chemistry problem without a dedicated stress measurement. Sheet resistance, measured with a four-point probe, is tracked as a fast proxy for both thickness and film quality, since a properly stoichiometric, well-deposited TiN film in the tens-of-nm range typically lands in a narrow, repeatable sheet-resistance band, while a drifting deposition process shows up as sheet resistance moving outside that band before a visible defect ever appears. A shift of more than roughly 10% in four-point-probe sheet resistance between qualification and a production lot is usually enough to trigger a hold and a composition recheck. Deposition tools are typically qualified so that a 40 nm target film stays within about 1.5 nm across a lot of 25 wafers, since a thickness excursion much larger than that shows up directly as a selectivity shift downstream. Bias power for the hardmask open step commonly runs in a 100 W to 400 W range on a production etcher, and the 570-unit working point noted in the selectivity curve corresponds to roughly the upper third of that range on a typical chamber.

Deposit PVD/CVD/ALD TiN hardmask at 20-60 nm → Coat and pattern photoresist above TiN → Open TiN hardmask with resist-selective etch and controlled bias → Etch target metal using TiN-selective chemistry → Verify CD, sidewall angle, and residual resist → Strip remaining TiN in oxygen ash near 250 C → Confirm clean removal by SIMS and inspect profile by AFM

Viewed through a hardmask-selective etch engineering lens, TiN's job is narrow but essential: hold a stable stoichiometry, present a repeatable thickness, and deliver enough selectivity in both directions at once that a metal etch step too aggressive for resist alone can still land on target, wafer after wafer, without the mask itself becoming the source of CD or profile variation.