Fluorocarbon chemistries are explicitly selected because they offer an easily tunable carbon-to-fluorine (C/F) ratio, which governs the fundamental balance between oxide etching and polymer deposition .
In depth
PMD 2 Etch is a critical intermediate process step in the multi-step contact opening sequence
within the 40nm BSI CMOS image sensor flow . Following photolithography and the initial PMD 4 and PMD 3 etch steps, this step continues the high-aspect-ratio anisotropic removal of the thick pre-metal dielectric (PMD) stack to form vertical pathways to the active device regions . The incremental multi-step approach is utilized to tightly control the evolving etch profile and minimize cumulative micro-loading effects across varying pattern densities . Ultimately, this sequential dielectric etching prepares the contact holes for the subsequent PMD 1 and CESL (Contact Etch Stop Layer) removal, which will safely expose the underlying silicide or epitaxial source/drain regions for metallization without damaging them . The PMD 2 Etch relies on an ion-chemical synergistic reaction mechanism within a high-density inductively coupled plasma (ICP) or reactive ion etch (RIE) system . The process utilizes fluorocarbon-based gas chemistries (such as CF4, CHF3, or C2F6) where fluorine radicals chemically react with the Si-O bonds of the dielectric to form volatile byproducts like SiFx and COx . Simultaneously, ion bombardment provides the necessary activation energy and directionality to achieve a highly anisotropic, downward vertical profile [P1, A2]. A critical competitive physical mechanism occurs during this etching: while active fluorine atoms etch the oxide, carbon-rich fluorocarbon radicals (CFx) continuously deposit a thin polymeric passivating film on the newly formed trench sidewalls . This polymer layer suppresses lateral chemical etching (undercutting) and maintains the verticality of the contact trench [P2, P3]. Fluorocarbon chemistries are explicitly selected because they offer an easily tunable carbon-to-fluorine (C/F) ratio, which governs the fundamental balance between oxide etching and polymer deposition . To optimize this balance, additive gases such as CH4 or H2 can be introduced to consume excess fluorine and enhance the polymerization rate, thereby improving the overall etch profile and structural fidelity . Key process parameters include RF bias power, which controls the ion bombardment energy, and chamber pressure alongside gas residence time, which dictates radical concentration and dissociation rates [P1, P3]. For instance, higher RF power increases the etch rate by raising ion energy, but it may induce physical damage or degrade selectivity if not carefully controlled . Controlling the residence time of reactive species is also essential to manage the competition between etching and polymerization, ensuring uniform deep etching without causing excessive polymer accumulation . In nanoscale technologies, the contact holes exhibit very high aspect ratios, increasing the risk of incomplete etching or excessive tapering that restricts the bottom contact area . Tapered contact profiles with inward-sloping sidewalls are inherently formed during anisotropic RIE, but managing the degree of this taper is critical to ensure an adequate landing area on the underlying structures while preventing upper-level short circuits between adjacent contacts . As device dimensions shrink, the physical contact resistance scales inversely with the effective contact area, meaning that dimensional fidelity during the PMD etching sequence is directly tied to the final drive current and parasitic resistance of the MOSFET [A2, T2].
Risks & Challenges
[High] Etch Stop or Pinch-off: Excessive polymer deposition from high C/F ratio fluorocarbon plasmas can prematurely close the upper portion of the contact opening (pinch-off) or completely halt the downward etching process at the trench bottom . This is especially prevalent in high-aspect-ratio structures where the clearing ion flux reaching the bottom is severely attenuated (Engineering Practice).
[Medium] Profile Bowing or Severe Tapering: Inadequate sidewall passivation or excessive lateral radical attack can cause mid-trench bowing, while insufficient ion energy combined with excessive polymer buildup leads to severe inward tapering [P3, A2]. This structural distortion reduces the effective contact area at the bottom, which sharply increases the specific contact resistivity of the device .
[Low] Micro-loading and Depth Non-uniformity: Variations in local layout pattern density cause localized differences in reactant depletion and polymer formation rates across the die (Engineering Practice). This results in uneven etch depths, potentially causing under-etching in dense contact arrays while over-etching in isolated regions, threatening the structural integrity of the underlying layers prior to the final CESL etch .