Introduction
Tapered profile etch, often referred to as slope etch or tapered etch, is a specialized plasma etching technique designed to fabricate features with controlled, non-vertical sidewall angles [P1, T1]. While vertical, highly anisotropic profiles are typically the gold standard for maintaining tight pitch density in integrated circuits, certain structures require a positive slope to facilitate subsequent processing [T1, P3]. In semiconductor manufacturing, creating a tapered slope is critical for ensuring reliable conformal film deposition over three-dimensional features [P3, T1].
Without this sloped profile, downstream deposition steps like physical vapor deposition (PVD), which are inherently line-of-sight, cannot adequately coat deep or narrow features, resulting in voids, pinch-off, or thin spots . For example, highly conformal techniques like atomic layer deposition (ALD) can handle steeper structures, but many thicker metallization layers still rely on sloped profiles for reliable coverage . Therefore, tapered etching plays an essential role across several manufacturing modules, including through-silicon via (TSV) fabrication, contact hole engineering, and shallow trench isolation (STI) gap-filling [P2, P3, A3]. To achieve this profile, engineers utilize specialized dry etching chemistries and parameter modulation to balance chemical reactions with directional physical forces [T1, P4].
Physics & Mechanism
Achieving a controlled sloped profile during plasma etching involves a dynamic balance between isotropic chemical reactions, anisotropic physical sputtering, and protective polymer deposition . The core mechanism rests on manipulating the polymer deposition-to-etch ratio along the feature's sidewalls . During dry plasma etching, reactive ions are accelerated vertically toward the substrate by an electric field, while neutral radicals diffuse isotropically to react with exposed surfaces .
The Passivation-Inhibitor Model
The most common physical model for generating a tapered profile is the passivation-inhibitor model . In this process, the etchant gas chemistry is formulated with polymerizing precursors that continuously deposit a protective inhibitor layer over all exposed surfaces of the trench . High-energy ions strike the bottom of the trench vertically, physically sputtering away the inhibitor and allowing the chemical etchants to react with the substrate . Because the sidewalls are parallel to the incoming ion flux, they do not receive direct ion bombardment; hence, the protective polymer layer remains intact . As the etch progresses deeper, this passivation layer continuously builds up on the upper sidewalls, narrowing the effective mask opening and causing the trench width to taper inward as a function of depth [T1, P4].
For example, in the dry etching of silicon or polysilicon, adding hydrocarbon gases like ethane (C2H6) to chlorine (Cl2) chemistries increases polymer buildup on the sidewalls, resulting in a more sloped profile . Similarly, when dry etching titanium nitride (TiN) hard masks, the addition of boron trichloride (BCl3) to a Cl2 plasma generates non-volatile boron-oxygen-nitrogen (BOxNy) passivating byproducts that deposit on the sidewalls, lowering the taper angle .
Time-Multiplexed Alternating Etch Mechanisms
An alternative method for generating sloped profiles, particularly in deep reactive ion etching (DRIE), is the time-multiplexed alternating process . Instead of relying on continuous, simultaneous deposition and etching, this method—often a modified Bosch process—alternates between anisotropic etching phases and isotropic chemical etching phases within the same chamber .
During the isotropic phase, bias-free sulfur hexafluoride (SF6) plasma is introduced to chemically undercut the sidewalls slightly via neutral radical reactions . By precisely adjusting the ratio between the anisotropic vertical etch depth and the isotropic lateral etch depth within each composite cycle, engineers can geometrically define the overall taper angle . Shortening the individual cycle times distributes the lateral undercut into tiny increments, preventing large lateral steps and ensuring a smooth, continuous taper .
Mask Erosion Method
Tapered profiles can also be achieved through controlled mask erosion . In this configuration, the initial photoresist or hard mask is patterned with a pre-existing sloped profile . During the subsequent plasma etch, the chemistry is chosen to have moderate selectivity between the mask and the substrate . As the etch proceeds, the thin edges of the sloped mask are gradually eroded laterally, exposing new substrate material at the outer margins . This lateral mask pullback transfers the sloped geometry of the mask directly into the underlying substrate .
Process Principles & Parameter Interactions
Controlling the sidewall slope angle requires precise, directional adjustment of plasma processing parameters to balance physical and chemical etch components [T1, P1].
- Gas Chemistry Ratio: The primary driver of the taper angle is the passivant-to-etchant gas ratio [T1, P4]. Increasing the flow of polymerizing gases (such as fluorocarbons or hydrocarbons) or passivating agents (such as oxygen in SF6 plasmas or BCl3 in chlorine plasmas) relative to highly active halogens decreases the sidewall taper angle, making the profile more sloped [T1, P4]. Conversely, increasing the chemical etchant concentration yields a more vertical profile [T1, P4].
- Radio Frequency (RF) Bias Power: RF bias power controls the platen power, which directly dictates the kinetic energy of the ions accelerating toward the wafer [P1, T1]. Decreasing the platen power reduces ion bombardment energy, which slows the physical removal of the passivation layer at the bottom corners of the trench [P1, T1]. This allows the polymer to encroach further toward the center of the trench bottom, leading to a shallower, more tapered slope [P1, T1].
- Substrate Temperature: Temperature strongly influences the adsorption and desorption kinetics of reactive radicals and passivation polymers . Lower substrate temperatures stabilize the polymer passivation layer on the sidewalls, preventing volatile desorption and facilitating a controlled, stable taper [P3, T1]. Higher temperatures increase the volatility of both byproducts and passivants, encouraging isotropic chemical lateral etching and reducing polymer thickness [P3, T1].
- Chamber Pressure: Adjusting the operating pressure changes the mean free path of ions in the plasma (Engineering Practice). Higher pressures increase the frequency of gas-phase collisions, which scatters the ions and reduces their vertical directionality (Engineering Practice). This scattered ion flux bombards the upper sidewalls, eroding passivation layers near the top of the trench and contributing to a more flared or tapered profile .
Challenges & Failure Modes
Executing a tapered profile etch presents several distinct physical challenges and failure modes that can compromise device yield if left unoptimized .
Taper Scalloping
In time-multiplexed alternating etch processes, the switching between anisotropic physical etching and isotropic chemical etching can leave pronounced ripples, known as taper scallops, on the sloped sidewall . If these scallops are too deep, they create localized physical barriers that disrupt the continuity of subsequent thin barrier or seed layers, leading to localized thinning or step-coverage failures [P1, P3].
Profile Pinch-Off and Etch Stop
If the passivation-to-etch ratio is too high, or if the chamber temperature is too low, the polymer deposition can become self-reinforcing [T1, P1]. The passivating material accumulates excessively near the upper neck of the trench, restricting the flux of incoming ions and neutral radicals . This causes the trench to narrow rapidly as it deepens, eventually resulting in a premature etch stop or "pinch-off" where the feature fails to reach its target depth [T1, P1].
Re-Entrant Profiles
Conversely, if the passivation layer is too weak or if the ion energy is too high, the process may transition from a positive taper to a re-entrant (negative) profile, where the trench is wider at the bottom than at the top [P1, T1]. Re-entrant profiles are catastrophic for subsequent deposition steps, as they lead to severe gap-fill voids during isolation or metallization steps .
Critical Dimension (CD) Loss and Mask Erosion
Using the mask erosion method to produce sloped sidewalls inherently increases the lateral footprint of the etched feature . If the mask erodes too quickly, the top critical dimension (CD) of the feature will expand significantly beyond its design limits, leading to potential electrical shorts with adjacent features or erosion of adjacent protective spacer caps .
Technology Node Evolution
The implementation and physical targets of tapered profile etching have evolved significantly across technology nodes to keep pace with structural scaling and new device architectures [P2, A3].
[28nm Planar Node]
│
▼ (STI Reshaping)
L-E-G Strategy / Downstream NF3/NH3
│
▼ (FinFET Era)
[14nm & 7nm Nodes]
│
├─► V-Shaped S/D Contacts
└─► High-k Metal Gate hard masks
│
▼ (3D Scaling)
[Advanced 3D Architectures]
│
├─► Sloped Memory/Periphery Trenches
└─► Asymmetric Flared Vias (BEOL)
At the 28nm Planar Flow node, the increasingly high aspect ratios of STI trenches made conventional gap-filling with oxide films highly prone to void formation . To solve this, engineers integrated a "Liner-Etch-Gap-fill" (L-E-G) sequence . This approach utilized a downstream chemical dry etch with nitrogen trifluoride (NF3) and ammonia (NH3) gases to selectively etch and reshape the top of the oxide liner, creating a gentle tapered slope that allowed subsequent sub-atmospheric chemical vapor deposition (SACVD) to fill the trench without voids .
With the introduction of the 14nm FinFET and 7nm FinFET nodes, controlling fin field effect transistor source/drain contact resistance became a major performance hurdle . To maximize the contact area within a constrained lateral footprint, engineers developed tapered contact holes with non-planar, V-shaped or U-shaped sloped bottoms . This non-planar bottom profile significantly expanded the physical contact interface area between the silicide and the source/drain epitaxial layer . This process required highly selective reactive ion etching (RIE) to carve the sloped contact without eroding adjacent gate caps, utilizing ultra-thin hard masks like hafnium dioxide (HfO2) as etch stops .
Furthermore, during the patterning of high-k metal gate (HKMG) stacks, precise slope control of TiN hard masks became necessary to prevent shadowing during subsequent implants . In 3D memory integration, massive height differences between the dense array stack and the peripheral circuit regions generate significant stress . Engineers introduced isolation trenches with sloped, tapered sidewalls in the boundary regions to smooth the geometric transition, thereby reducing stress-induced delamination and preventing film cracking .
Related Processes
A tapered profile etch is highly integrated with adjacent process steps, acting as an enabler for subsequent thin-film deposition and planarization modules .
- Photolithography and Hard Masking: The initial taper, roughness, and thickness of the photoresist or hard mask directly influence the final etched slope [T1, A1]. Any line-edge roughness or asymmetry in the mask profile can be transferred and amplified during the tapered dry etch process .
- Physical Vapor Deposition (PVD): The primary integration reason for inducing a tapered slope is to enable line-of-sight PVD metallization . For instance, in TSV fabrication, a positive sidewall taper (such as 85 degrees) is necessary to ensure that sputtered barrier layers (e .g., Ti, Ta, or TiN) and copper seed layers have continuous, void-free coverage along the deep sidewalls before electrochemical plating [P3, A2].
- Chemical Mechanical Planarization (CMP): After the tapered features are etched and filled with metals or dielectric materials, chemical mechanical planarization is used to polish away the excess overfill, ensuring a flat, coplanar surface for the next patterning level [A1, A2].
Future Outlook
As device dimensions scale toward the sub-2nm regime, tapered profile etching continues to evolve with novel architectures and advanced packaging requirements .
One emerging trend is the fabrication of interconnect vias with induced asymmetric profiles . In advanced back-end-of-line (BEOL) routing, creating a via that has a tapered flare along only one horizontal axis allows designers to significantly increase the top contact area—lowering contact resistance—while maintaining a narrow, symmetric profile at the bottom to prevent electrical shorting with tightly pitched adjacent lines .
Additionally, the semiconductor industry is increasingly integrating atomic layer etching (ALE) with traditional RIE to achieve monolayer-level precision on tapered sidewalls [A1, A3]. ALE's self-limiting reaction steps enable atomic-scale control over the sidewall slope, minimizing the formation of taper scallops and roughness that plague conventional deep etching processes . In high-aspect-ratio 3D architectures, such as nanosheet transistors and next-generation memory structures, dynamic profile tuning will remain a vital tool to keep vertical channels open while managing physical stress and resistance limits .