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Dry Etching Techniques Lecture 21 EE 441 Spring 2009: Tadigadapa Dry Etching Techniques • Anisotropy in dry etching is not a result of single crystal anisotropy , rather is controlled by plasma conditions. • At low pressures , 10-3 – 10-4 Torr range, physical ion etching is dominant with high anisotropy and poor selectivity. • At higher pressures 1 Torr chemical etching dominates leading better selectivity and poor anisotropy. • Often overetching is required to ensure uniformityyg of etching over whole wafer & from wafer to wafer. a: Barrel Reactor, b: Downstream Reactor, c: Parallel Plate Reactor Lecture 21 EE 441 Spring 2009: Tadigadapa Dry Chemical Etching • RtitlhilReactive neutral chemical speci es such as c hlihlorine and dfli fluorine at oms and molecular species generated in the plasma diffuse to the substrate where they form volatile products with the layer to be removed. • Chemical etching occurs at low bias and in principle no highly energetic ions bombard the surface, i.e. radiation damage to the surface is reduced. Lecture 21 EE 441 Spring 2009: Tadigadapa Nonuniformities in Dry Chemical Etching • Loading effect occurs as a result of gas phase etchant being depleted by reaction with the substrate material. As the etch rate depends on wafer loading, uniformity is jeopardized – supply of reactants limits the etch rate i.e. small variations in gas flow rate or gas distribution lead to large nonu niformities in the etch rate . Utilization factor (U) is the ratio of formation of etch products to the etch gas flow and needs to be U > 0.1 for uniform etching. • Bulls Eye Effect: Arises from the relative selectivity of the wafer surface with respect to the cathode material. In this case circular interference patterns result due to the lower or higher consumption of the reactants between the regions above the wafer & cathode material. • WfWafers must a lso be p lace d away from t he e dges o fhf the e lectro des to re duce edge effects arising from the changing sheath thickness as well as varying angles of incidence. • Local temperature variations can cause large nonuniformities during chemical etching processes – good thermal contact between the cathode and wafer must be established. • Grass consists of pillars of silicon (called black silicon) arises due to the sharpening of the ion angular distribution with increasing aspect ratio of the trench during etching. Lecture 21 EE 441 Spring 2009: Tadigadapa Anisotropy in Dry Etching Energy-DriAiiven Anisotropy – During ion assisted etching, bombardment by ions (<1000 eV) disruppgggts an unreactive substrate and causes damage such as dangling bonds and dislocations resulting in a reactive substrate towards etchant species.At low pressures and high energies, the mean free path of the reacting molecules is typically larger than the depth to be etched, resulting in horizontal surfaces being hit and etched by the reactive species a lot more than the sidewalls. In hibitor-DiDriven An iso tropy – In this case etching leads to the production of a surface-covering agent. Ion-bombardment clears the passivation from horizontal surfaces and reactions with neutrals proceeds on these cleared surfaces only. Protective films may originate from involatile etching products or from film-forminggp precursors that adsorb durin g etching. Passivating gases such as BCl3, CCl4, C4F8 etc. are sources of inhibitor forming species. Lecture 21 EE 441 Spring 2009: Tadigadapa Trench Profiles Dependence on Anisotropy Mask If Vx denotes etch rate under no bias and Vz the etch rate under bias, then we can write, Substrate V = 0, Occurs for Si in CF + H x Vx x 4 2 = plasma z Vz x Etch Rate of Si z Bias = 150V No Bias Vx ≠ 0, Occurs for Si in CF4 10 30 plasma, for SiO2 in CF4 +O2 % H in CF Plasma 2 4 Lecture 21 EE 441 Spring 2009: Tadigadapa Isotropppic & Anisotropic Etch Profiles Mask Layer 1 Substrate Isotropic Etch Isotropic Etch with Anisotropic Etch Mask Erosion T3>T2>T1 Over Etch Over Etch with Mask Erosion Lecture 21 EE 441 Spring 2009: Tadigadapa Gas Comppygosition for Dry Etching • Oxidizing agents such as Oxygen are added to the plasma to increase etchant concentration and to suppress polymerization. • Radical scavengers such as Hydrogen increase the concentration of inhibitor former and reduce the etchant concentration of fluorine in the etchant. • Inert gases such as Ar/He help stabilize the pp,lasma, enhance anisotropy, improve uniformity or reduce etch rate by dilution. Lecture 21 EE 441 Spring 2009: Tadigadapa New Gas Chemistries eliminate CFC’s Lecture 21 EE 441 Spring 2009: Tadigadapa Freqqyuently used Etch g ases & Masking Materials Lecture 21 EE 441 Spring 2009: Tadigadapa Deep Reactive Ion Etching Lecture 21 EE 441 Spring 2009: Tadigadapa Deep Reactive Ion Etching Sequential Deposition of a Polymer Layer using Octofluorocyclobutane (C4F8) – a cyclic fluorocarbon that breaks open to form CF2 and longer chain radicals And Etching of Silicon using Sulphur Hexafluoride (SF 6). Lecture 21 EE 441 Spring 2009: Tadigadapa DRIE Chamber • High Density Plasma Source RF Power Supply •Hihigh Th roug hput H ihigh DC Power Vacuum Pump Supply • Diffusion Chamber • Cooled Substrate with variable position • Separate Substrate Bias Lecture 21 EE 441 Spring 2009: Tadigadapa Aspect Ratio Dependent Etching • Narrow features etch slowly compared to wider features due to: – Decrease in the radicals availa bility d own th e hi gh aspect ratio feature – Reduction in ions due to sidewall scattering, electrostatic scattering. • Typical solution is to use a buried passivation layer • Over etching has to be carefully timed or else notching occurs Lecture 21 EE 441 Spring 2009: Tadigadapa 1 Some of the Non- Idealities in DRIE Bottling Effect: Typically arises due to the use of high energy ions which tend to erode the side wall passi vati on causi ng th e b ottl e sh ape (1) . A better way to achieve higher anisotropy is to use longggger etch times however, these resulting larger sidewall roughness or scalloping (2). 2 3 Notching: Occurs due to the charging of the oxide buried layer which causes the ions to deflect towards th e co nve x co rne r the reby b re aking dow n the passivation and results in notching (3). Lecture 21 EE 441 Spring 2009: Tadigadapa Cryogenic Etching • Uses SF6 for etching using fluorine radicals • This process relies in forming a blocking layer of oxide/fluoride (SiOxFy) on the sidewalls around 10-20nm thick along with cryogenic temperature for inhibiting the sidewall attack. • The low temperature operation also reduces the etch rate of the mask material since the attack by free-radicals is chemical in nature & therefore reduces with temperature . • Process pressure is typically around 10 mTorr. The ion energy controlled by the substrate bias (RF) and ion density controlled by the ICP power are primary factors affecting the mask erosion. Measured DC bias values of less than 20V can give selectivity between silicon and oxide masks of over 750:1. Lecture 21 EE 441 Spring 2009: Tadigadapa Gas Phase Etching •A non-plasma, isotropic dry etch process for silicon is possible using Xenon Difluoride (XeF2) and provides very high selectivity agg,ainst silicon dioxide, stoichiometric silicon nitride ,p, photoresist and aluminum. •XeF2 is a white solid at room temperature and sublimates at 1 Torr. The etch reaction proceeds as: 2XeF2 + Si → 2Xe + SiF4 where only silicon is in the solid phase. The reaction proceeds by non-dissociation adsorption of XeF2 at the silicon surface, dissociation of fluorine, and the reaction to form adsorbed SiF4 product and finally the desorption of the product and residual Xe. •Etch rates as high as 1-3 μm/min have been reported. However, the etched surfaces tend to be very rough and have a granular structure ~10 mm. Formation of silicon fluoride ppyolymer can dramaticall y slow or stop the etching process. Dehydration of the samples by baking can be used to avoid this problem..
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