Additive Techniques. Thermal Oxidation of silicon

Transcription

1 Additive Techniques Elements required in MEMS/ IC context Si, Al, Au, Ti, W, Cu, Cr, O, N, Ni-Fe alloys}-mems and IC S. Zr, Ta, Ir, C, Pt, Pd, Ag, Zn, and Nb}- MEMS only. In sensors and biomedical devices as large number of diverse materials are usually required. Solids can be deposited from a liquid, a plasma, a gas, or a solid state. The process is usually followed by thermal processing for desired material properties and substrate adhesion. Indicators for Film quality Film composition Grain size Thickness (usually in Å) Uniformity Step- coverage Adhesion Corrosion resistance Thermal Oxidation of silicon Oxide is used as mask insulating layer sacrificial material Involves heating of a Si wafer in a stream of steam at 1 atm in wet or dry oxygen/n 2 mixtures at 600 C/ 1250 C. Si oxidizes even in room temperature. At high temperature the oxidant diffuses into Si + 2H 2 O SiO 2 +H 2 (wet) Si+O 2 SiO 2 (dry) Thickness is proportional to density, X s = 0. 46X ox ie 10,000Å oxide can be formed out of 4600Å of Si. Growth time is linear near the surface, parabolic at a depth. Oxidation rate depends on crystallographic ( (1 0 0 ) surface oxidizes 1.7 times more slowly than a (1 1 1 ) surface) orientation of Si. Si doping presence of impurities in the oxidizing gas, pressure of oxidizing gas use of plasma or photon flux, Ellipsometer is used to measure oxide thickness. Color table to determine oxide thickness. Oxidation furnaces from Thermcraft ( ). Tystar ( ). 27

2 Properties of thermal SiO 2 Major Deposition Schemes e.g., PVD, CVD Physical vapor Deposition PVD reactors can use solid, liquid, or gas as raw material. e.g, Thermal Evaporation. Sputtering. Molecular beam epitaxy. LASER sputter deposition. Ablation deposition Ion plating Cluster beam technology. Thermal Evaporation An evaporation system consists of a vacuum chamber, pump, wafer holder, crucible, and a shutter as shown in figure below. The source metal to be deposited is placed in an inert crucible, and the chamber is evacuated to a pressure of Torr. The crucible is heated using a tungsten filament or an electron beam to flash-evaporate the metal from the crucible and condense onto the cold substrate. The film thickness is determined by the length of time that the shutter is opened and can be measured using a QMB-based film thickness monitor. The evaporation rate is a function of the vapor pressure of the metal. Hence, metals that have a low melting point (e.g. 660 C for aluminum) are easily evaporated, whereas refractory metals require much higher temperatures (e.g. 3,422 C for tungsten) and can cause damage to polymeric or plastic samples. In general, evaporated films are highly disordered and have large residual stresses; thus, only thin layers of the metal can be evaporated. The chemical purity of the evaporated films depends on the level of impurities in the source; contaminations of the source from heater, crucible, 28

3 or support materials; and due to residual gases within the chamber. In addition, the deposition process is relatively slow at a few nanometers per second. In the deposition of metal alloys, the constituents are evaporated independently of one another. The deposited film has atoms that are less tightly bound than inorganic compounds. Sources of individual metals are often kept at different temperatures. Schematic Diagram of a thermal evaporation unit for depositing metals and other materials Based on the boiling off (sublimating ) of a heated material and transferring these onto a substrate in a vaccum, The depostion rate is proportional to the number of molecules leaving an unit area of evaporant per second. Deposition rate for Al, 0.5µm/min i.e fast process, no damage on substrate. Methods for heating: Resistive heating eg in lab set ups. Tungsten boat/ filament as containment structure. Filament life limits thickness.( for industrial use) Al is the most popular interconnect material. Resistivity: 2.65µΩcm. Good adherance to Si/SiO 2. Corrosion resistant, compared to Cu. Easy to deposit / etch. Ohmic contact is formed with Si at C Ebeam/RF induction: High intensity electron beam gun (3 to 20 kev) is focused on the target material that is placed in a copper hearth ( water cooled). The evaporant, which melts locally. Therefore, no contamination from crucible. High quality films. High deposition rate 50 to 500nm/min. Disadvantages: Process might induce x-ray damage and ion damage at the substrate. At high energy(> 10kev), the incident electron beam causes x-ray emission. Deposition equipment is more costly. 29

4 Sputtering Sputtering is a physical phenomenon involving the acceleration of ions via a potential gradient and the bombardment of a target or cathode. Through momentum transfer, atoms near the surface of the target metal become volatile and are transported as a vapor to a substrate. A film grows at the surface of the substrate via deposition. A typical sputtering system comprising of a vacuum chamber, a sputtering target of the desired film, a substrate holder, and a high-voltage DC or RF power supply. After evacuating the chamber down to a pressure of 10-6 to 10-8 Torr, an inert gas such as helium is introduced into the chamber at a few mtorr of pressure. Plasma of the inert gas is then formed. The energetic ions of the plasma bombard the surface of the target. The energy of the bombarding ions (~ kev) is sufficient to make some of the target atoms escape from the surface. Some of these atoms land on the sample surface and form a thin film. Sputtered films tend to have better uniformity than evaporated ones, and the high-energy plasma overcomes the temperature limitations of evaporation. Most elements from the periodic table can be sputtered, as well as inorganic and organic compounds. Refractory materials can be sputtered with ease. In addition, materials from more than one target can be sputtered at the same time. This process is referred to as cosputtering, and can be used to form compound thin films on the substrate. Sputtering process can however be used to deposit films with the same stoichiometric composition as the source, and hence allows the utilization of alloys as targets. Sputtered thin films have better adhesion to the substrate and many grain orientations than evaporated films. The structure of sputtered films is mainly amorphous, and its stress and mechanical properties are sensitive to specific sputtering conditions. Some atoms of the inert gas can be trapped in the film causing anomalies in its mechanical and structural characteristics. Therefore the exact properties of a thin film vary according to the precise conditions under which it was made. The deposition rate is proportional to the square of current density and inversely proportional to the spacing between the electrodes. Figure below shows a Two-electrode setup (diode) for RF ion sputtering or sputter deposition. For ion sputtering, the substrates are put on the cathode (target); for sputter deposition, the substrates to be coated on the anode. The target, at a high negative potential is bombarded with positive argon ions created in a (high density) plasma. Condensed on to substrate placed at the anode. Wider choice of materials. Better adhesion to substrate. Sputter yield= #of atoms removed per incident ion Sputter yields for various materials at 500ev Argon Al 1.05 Cr 1.18 Au 2.4 Ni 1.33 Pt 1.4 Ti 0.51 Deposition rate is proportional to yield for a given plasma energy Disadvantages: High cost of equipment. Substrate heating due to electron (secondary) bombardment. Slow deposition rate. (1 atomic layer/sec). Advantages of sputtering over evaporation: Complex stoichiometries possible. Films can be deposited over large wafer (process can be scaled) 30

5 Comparison of evaporation and sputtering technologies Inverted Cylindrical Magnetron (ICM) RF sputtering Figure below illustrates the ICM sputter gun set up. It consists of a water-cooled copper cathode which houses the hollow cylindrical BST (a ceramic material used in RF MEMS applications-phase shifters etc) target surrounded by a ring magnet concentric with the target. A stainless-steel thermal shield is mounted to shield the magnet from the thermal radiation coming from the heated table. The anode is recessed in the hollow-cathode space. It aids in 31

6 collecting electrons and negative ions minimizing re-sputtering the growing film. Outside the deposition chamber, a copper ground wire is attached between the anode and the stainless-steel chamber. A DC bias voltage could be applied to the anode to alter the plasma characteristics in the cathode/anode space. The sputter gas enters the cathode region through the space surrounding the table. Schematic diagram of ICM sputter gun Using the above set up, BST film could be deposited at temperatures ranging from 550 to 800 C. The substrate temperature was maintained by two quartz lamps, a type-k thermocouple and a temperature controller. The films were deposited at 135 W to a film thickness of 7000Å. The films were cooled to room temperature in 1 atm of oxygen before removing them from the deposition unit. This was then followed by annealing the films in 1 atm of flowing oxygen at a temperature of 780 C for 8 hours in a tube furnace. Laser Ablation Uses LASER radiation to erode a target, and deposit the eroded material onto a substrate. High energy focused laser beams avoids x-ray damage encountered to the substrate. The energy of the laser is absorbed by the upper surface of the target resulting in an extreme temperature flash, evaporating a small amount of material. Material displaced, is deposited onto the substrate without decomposition. The method is highly preferred when complex stoichiometries are required. Thin film keeps the same atomic ratio as the target material. Usually pulsed laser is used. Disadvantages Not useful for large scale coatings. Small source size requires rotating of sample. 32

7 Chemical Vapor Deposition Constituents of a vapor phase, often diluted with an inert carrier gas, react at the hot surface to deposit a solid film. In the reaction chamber, the reactants are adsorbed on the heated substrate surface and the adatoms undergo migration and film-forming reactions. Film-forming by To improve the film quality in CVD, Heterogeneous reactions 1. wafer chuck is heated to smooth out Occuring at or close to heated surface. the film surface Homogenous reactions 2. RF plasma ion bombardment to fill Occuring in gas phase. voids 3. fairly high temperatures used Result in stoichiometric correct film. Versatile method. Works at low/ atmospheric pressure. Epitaxial,uniaxially oriented poly crystalline layers can be deposited by high degree of purity, control and economy. Used for very thin Si deposition, copper, low dielectric insulators. Films deposited include: SiO2, Si3N4, borosilicate glass (BSG), phosphosilicate glass (PSG), BPSG, tungsten, copper, etc. Transport and reaction processes involving CVD 1. Mass transport of reactant (and diluent gases ) in the bulk gases flow region from the reactor inlet to the deposition zone. 2. Gas phase reactions leading to film precursors and by-products. 3. Mass transport of film pre-cursors and reactants to the growth surface. 4. Adsorption of film precursors and reactants on the growth surface. 5. Surface reactions of adatoms occurring selectively on the heated surface. 6. Surface migration of film formers to the growth sites. 7. Incorporation of film constituents into the growing film. 8. Desorption of by-products of the surface reaction. 9. Mass transport of by-products in the bulk gas flow region away from the deposition zone towards the reactor exit. Energy to drive these reactions by thermal, photons, electron. Plasma-enhanced CVD Low power densities. Higher pressures. Higher substrate temperature. - Less severe radiation damage than sputter deposition. Films are not stoichiometric, because Deposition reactions vary. Particle bombardment during growth of a multi-component system changes the composition according to the ratios of sputtering yields the component materials. Good adhesion. Low pin hole density. Good step coverage. Adequate electrical properties. Disadvantages: Possibility of wafer damage Can create voids in trenches Photoresists can not be used during PECVD can cause thermal flow, and start reactions leaving out volatile compound 33

8 Compatibility with fine line-width pattern transfer from oxides, nitrides and oxynitrides for small feature sizes and line widths for devices unable to withstand high temp. Reactor parameters 1. Total reactor pressure. Gas density raises with pressure. Mean free path is longer at lower pressures, therefore lower pressure ions can gain more energy better quality. Two types of PECVD reactors. (A) Applied materials plasma 1 cross-section. (B) Susceptor electrode (grounded electrode) is radiantly heated to provide rapid thermal processing. 2. Freq of the RF excitation (used for plasma) At low frequency (<3Mhz) ions experience the full amplitude of the RF voltage, therefore more energy. 3. RF power More power more intense bombardment, film deposition rate increases. 4. Growth temperature Influences structure of the film. Low temperature Surface diffusion is slow. Therefore adsorbed molecule reacts with incoming molecule -> amorphous film. High temperature Faster surface diffusion. Materials that can be deposited: Polysilicon, silicon nitride, oxide, copper,tungsten Example for deposition of Tungsten (from tungsten hexa fluoride) WF 6 +3H 2 W+6HF (gas) Atmospheric pressure APCVD (100pa to 10kpa) For growing epitaxial films of 0.2 to 1.5µ/min Si or GaAs to deposit SiO 2 ( at high rates). High temperature C. Reactor walls are cooled. Advantages: Simple reactor Fast deposition rate Low temperature Disadvantages Particle contamination (no vacuum) Low throughput Can be used ofr oxide, BSG, PSG, BPSG e.g., SiH 4 +O 2 SiO 2 +2H 2 Low pressure LPCVD ~10Pa Low deposition rate. High operating temperatures. For Poly Silicon(structural layer), SiO 2, Phospho silicate glass(psg) (sacrificial layer) Widely used because of Economy throughput uniformity. Advantages Excellent purity and uniformity Conformal step coverage 34

9 Larger wafer capacity Disadvantages High temperature Low deposition rate Require vacuum system A quartz tube is heated by a three-zone furnace, and gas is introduced at one end of the reactor and pumped out at the opposite end. The substrate wafers are held vertically in a slotted quartz boat. Usually reaction chamber LPCVD process parameters are in the following ranges: 1. Pressure between 0.2 and 2.0 Torr 2. Gas flow between 1 to 10 cm 3 /s 3. Temperatures between 300 and 900 C. CVD is used extensively in depositing SiO 2, silicon nitride (Si 3 N 4 ) and polysilicon. SiO 2 can be CVD deposited by several methods. It can be deposited from reacting silane and oxygen in LPCVD reactor at 300 to 500 C where 500 C 4 + O2 SiO2 2H2 SiH + SiO 2 can also be LPCVD deposited by decomposing tetraethyl orthosilicate (TEOS or, Si(OC 2 H 5 ) 4 ). TEOS is vaporized from a liquid source. Alternatively, dichlorosilane can be used as follows: 900 C 2 + SiCl H2 + 2H2O SiO2 + 2H2 2HCl Likewise, Si 3 N 4 can be LPCVD deposited by an intermediate-temperature process or a low-temperature PECVD process. In the LPCVD process, which is the more common process, dichlorosilane and ammonia react according to the reaction ~ 800 C 2H 2 + 4NH3 Si3N 4 + 6HCl 6H 2 3 SiCl + Polysilicon is also deposited by a similar technique A lowpressure reactor, operated at a temperature of between 600ºC and 650 C, is used to deposit polysilicon by pyrolizing silane according to the following reaction: 600 C SiH 4 Si + 2H 2 Most common low-pressure processes used for polysilicon deposition operate at pressures between 0.2 and 1.0 Torr using 100% silane. Very low pressure CVD ~ 1pa For growth of single crystalline Si at low temperature. Polysilicon is often used as a structural material in MEMS. This is also used in MEMS and microelectronics for electrode formation and as a conductor or high-value resistor, depending on its doping level (must be highly doped to increase conductivity). Polysilicon is commonly used for MOSFET Gate electrode: Poly can form ohmic contact with Si. When doped resistivity µΩcm Easy to pattern Polysilicon comprises of small crystallites of single crystal silicon, separated by grain boundaries. Metallorgaic MOCVD Also called organo-metallic vapour phase epitaxy Thickness control of ~1 atomic layer. Used for compound SC devices, opto electronic devices solar cells. Metallo-organic chemical vapor deposition (MOCVD) is a relatively low temperature ( C) process for epitaxial growth of metals on semiconductor substrates. Metallo-organics are compounds where each atom of the element is bound to one or many carbon atoms of hydrocarbon groups. For precise control of the deposition, high purity materials and most accurate controls are necessary. However due to the high cost, this approach is used only 35

10 where high quality metal films are required. A summary of MOCVD reaction parameters for depositing various metals are given below. Reaction conditions for MOCVD of various metals. Metal Reactants Conditions Al Trimethyl aluminum C, 1 atm Tryethyl aluminum Tri-isobutyl aluminum Demethyl aluminum hydride Au Dimethyl 1-2,4 pentadionate gold, NA Dimethyl-(1,1,1-trifluoro-2-4-pentadionate) gold, Dimethyl-(1,1,1-5,5,5 hexafluoro 2-4 pentadionate) gold Cd Dimethyl cadmium 10 Torr, Cr Dicumene chromium C Cu Copper acetylacetonate Copper hexafluoroacetylacetonate C 200 C Ni Nickel alkyl Nickel chelate 200 C in H C Pt Platinum hexafluoro-2,4-pentadionate C in H 2 Tetrakis-trifluorophosphine Rh Rhodium acetyl acetonate Rhodium trifluoro-acetyl acetonate 250 C,1 atm 400 C, 1 atm Sn Tetramethyl tin C Triethyl tin Ti Tris-(2,2 bipyridene) titanium <600 C 36

11 Review of CVD processes Comparison of additive processes important in microsensors and micromachining 37

12 IC and MEMS materials, deposition method, and typical applications 38

13 Wet Etching Reasons for etching Cleaning. Shaping. Removing surface damage. Polishing. Characterizing structural and compositional features. Materials that can be etched: Semiconductors. Conductors. Insulators. Wet etching of Si Has better selectivity than dry etching Faster few microns to tens of microns per minute for isotropic etchants. 1µm/min for anisotropic. (0.1µm/min in dry etching) Wet etching involves Transporting of the reactants by diffusion at the surface Chemical reaction at the surface Reaction products transported away from the surface, presumably, by diffusion (again) Some definitions Aspect ratio: Ratio of height to lateral dimensions of etched microstructures. Selectivity: ability of the process to choose between the layer to be removed and the interleaving layers (usually 40:1 is required) Etch rate: the speed with which the process progresses Etch profile: slope of the etch wall Etch uniformity: Undercut (bias) Isotropic etching Use acidic etchants. Rounded patterns formed. Used for Removal of work damaged surfaces. Rounding of sharp edges (formed by anisotropic etching) to avoid stress concentrations. Removing roughness after dry/anisotropic etching. Thinning For creating structures / planar surfaces on single crystal Silicon. Patterning single crystal, poly crystalline or amorphous films. Delineation of electrical junctions and defect evaluation. For Silicon, common etchants is HNA:- Mixture of HNO 3 and HF (Preferred acetic acid diluent to prevents dissociation of HNO 3 (oxidant). HNo 3 + H 2 O+(impurity)HNO 2 2HNO 2 +2OH - +2H + In this auto catalytic reaction; HNO 2 generated causes further production of holes in the system. Oxidant creates hole injection into the valence band of Si, resulting in oxidized Si fragments Si+2H + Si 2+ Hydroxyl ions reacts with silicon ions Si 2+ +2(OH) - Si(OH) 2 Silicon hydroxide dissociates Si(OH) 2 SiO 2 +H 2 SiO 2 dissolves in HF. SiO 2 +6HF H 2 SiF 6 +2H 2 O H 2 SiF 6 is soluble in water. Overall reaction Si+HNO 3 +6HF H 2 SiF 6 +HNO 2 +H 2 O+H 2. 39

14 Isotropic etching of Si with (A) and without (B) etchant solution agitation. Masking for isotropic etchants. SiO 2 Fast: speed ~ 50µ/min all planes of Si are attacked equally. In comparison, SiO 2 is etched only at the rate of 300 to 800Å/min in HF: HNO 3 System. Therefore thick layers can be used. so easy to form than other masks. Dopant can also be used to block the reaction rate. eg: HNA on n or p type Si (10 18 / cm 3 ) etch rate is only 1 to 3 µm/min. Difficulties in isotropic etching Masking. Etch rate is agitation sensitive and temp sensitive, difficult to control lateral and vertical etch rates. Etchants used for wet etching of various materials Si Mixture of HF, HNO3, acetic acid (HNA) SiO2 HF Glass HF Wet etchants for electronic materials Material Etchant Etch rate (Å/min) Si 3ml HF+5ml HNO 3 + 3ml CH 3 COOH 3.5x10 5 GaAs 8ml H 2 SO 4 +1ml H 2 O 2 +1ml H 2 O 0.8x10 5 SiO 2 28ml HF + 170ml H 2 O + 113g NH 4 F 15ml HF + 10ml HNO ml H 2 O Si 3 N 4 Buffered HF H 3 PO Al 1ml HNO 3 + 4ml CH 3 COOH + 4ml H 3 PO 4 + 1ml 350 H 2 O Au 1kg KI + 1g I ml H 2 O 1x10 5 Cu FeCl 3 40

15 Masking materials for acidic etchants Anisotropic etching Geometries bounded by slowest etching crystallographic planes. Slow etch rate 1µm/min. Rate is temperature sensitive not agitation sensitive. Etchants : Alkaline aqueous solutions of KOH, NaOH, LiOH,. NH 4 OH Alkaline organics, Ethylene di amine, choline, hydrazine. Performance of common Etchants KOH (Most popular) Use near saturated solution ( 1:1 in water) at 80 C. Selectivity with SiO 2 is not very good. KOH is incompatible with IC fabrication process, since it attacks Al bond pads. It can cause blindness if gets into contact with eyes. EDP Ethylene diamine pyrocatehol +water Masking SiO 2, Si 3 N 4, Au, Cr, Ag, Ta. Selectivity with SiO 2 : 5000:1( 400:1 For KOH ) Issues in (anisotropic) wet bulk micromachining Extensive real estate consumption Large area wasted between devices. Device becomes fragile. Two membranes formed in a <100>-oriented Silicon wafer. Solutions Etching from the front anisotropic etchants will undercut masking material. Using silicon fusion bonded wafers, 41

16 one bottom wafer for handling. With a cavity top sensing wafer with implanted resistors (thinned by electrochemical etching). Anisotropic Etching of Crystalline Silicon Etchant Temp ( C) Etch rate µm/hr <100> <110> <111> KOH: H 2 O KOH EDP N 2 H 4 :H 2 O NH 4 OH Isotropic and anisotropic etching of silicon Isotropic etch [100] Anisotropic etch [100] [100] Anisotropic etch [100] [110] Width of bottom surface: W= W 0-2h cot55 =W 0-1.4h. W 0 is the width of the window on the wafer surface. 42

17 Requirements for mask alignment with crystalline planes 43

18 Etch Stop Techniques Etch stop is a region at which the wet etching slows down. Dopant selective etching (DSE) Technique is useful for heavily doped layers, leaving behind lightly doped. Heavily doped Boron layer (which can be grown epitaxially, or can be formed by diffusion or implantation) on a lightly doped substrate This technique is useful with etchants such as KOH, NaOH, EDP, Hydrazine In KOH, with a B conc>10 20, the etch rate is reduced by a factor of 20 In EDP, with a B conc>2.7x10 19, the etch rate is reduced by a factor of 50 In heavily doped regions, the lattice constant of Si decreases slightly. This leads to highly strained membranes DSE can be used to undercut microstructures, defined by a mask of heavy boron diffusion in an n-type or p-type substrate. Advantages of DSE Independent of crystal orientation Smooth surface finish Offers possibilities for fabricating release structures with arbitrary lateral geometry Disadvantages High boron conc. introduces mechanical stress into the material. Electrochemical isotropic etching When simple photo resists are required to be used, strong reactants like HNA is not feasible. Therefore electrochemical acidic etching, electrical power to drive the chemical reaction. Electrochemical etching apparatus. Oxidation of silicon can be promoted by the voltage applied to the Si wafer ( electrochemical etching) This causes accumulation of holes in the Si/solution interface Holes are transported to the negative electrode as H+ ions and released there as Hydrogen bubbles. 5% HF solution used as NH 4 F(5 wt%) electrolytic cell is kept in the dark, at room temperature. Electrode separation : 1-5cm. Not used in micromachining. Used for polishing surfaces. 44

19 Dry Etching Techniques. Material removal for IC s, MEMS by physical by ion bombardment or chemical chemical reaction through a reactive species at the surface or combination. Plasma etch Generated within the chamber itself( in vaccum) (diode setup or glow discharge). Otherwise, plasma is generated in a separate chamber (triode setup or ion discharge). Grids are used to direct the ion beam towards the substrate. Steps involved in Plasma Etching 1. Reactive etching species are generated by electron/molecule collisions 2. Etchant species diffuse through stagnant region to the surface of the film to be etched 3. Etchant species adsorb onto surface 4. reaction takes place 5. etched product desorbs from the surface 6. etch products diffuse back into bulk gas and removed by vacuum Effects of various process parameters on the etch process power sheath potential, e- velocity, ions and radicals etch rate selectivity Pressure etch rate Area etch rate Electrode spacing ion energy, ion density Features of Plasma Etching Temperature Plasma temperature is usually 100 C plus room temperature, low enough for virtually any process. Chemistry = Selectivity. Radicals react with the wafer or film surface to form volatile etch products that are pumped away. Bombardment = Uniformity Accelerated ions strike the wafer surface and remove material by physical contact, a pressure regulated process. 45

20 Plasma etching is a balance between: o Selective removal (what is intended vs. what is protected) of material through chemical reactions. o Nonselective removal of material through ion bombardment (pressure and power related) o Deposition of sidewall polymers for passivation. These parameters must be balanced to maintain critical dimensions. Selective Etching o Etching that is done so that certain material is removed, but other materials or areas of the materials are not affected. o Selective etching is difficult to achieve when chemically different layers form similar etch products. Example: SiO2 and Si3N4 both form SiF4 during the etching process (reduced selectivity) Physical etching Ions of sufficient energy impinging vertically on a surface momentum transfer (sputtering) causes bond breakage ballistic material ejection throws the material around in vacuum. Ions should have energy ev to cause sputtering Volatility of etched material does not matter. Therefore etch rate is nearly same for most materials (ie non selective etching) Directional anisotropy. Slow etch rate ~few 100Å/min. High gas pressure required. Faceting due to angle-dependent sputter Corners are rounded. Plasma etching schemes High pressure RIE scheme. Reactive neutral chemical species such as Cl 2 or F 2 generated in plasma. These diffuse to the substrate and form volatile products in the layer to be removed. Plasma supplies gaseous reactive etchant species. Mechanisms for dry etching e.g., Si etching by Cl 2 plasma (i) Ion, radical and electron creation e+cl 2 Cl + +e (ii) Etchant formation e+cl 2 2Cl+e (iii) Adsorption of etchant on substrate Cl (on silicon wafer) Si-nCl (iv) Reaction to form product Si-nCl (ions) SiCl x (ads) (v) Product adsorption SiCl x (ads) SiCl x(gas) Gas Compositions used for Etching Various Electronic Materials Material Gases Si CF 4 -based, CF 3 Cl; SF 4 -based, Cl 2 /H 2, C 2 ClF 5 /O 2, NH 3, C 2 Cl 3 F 3, CCl 4 /He, HBr/Cl 2 /He SiO 2 CF 4 /H 2, C 2 F 6, C 3 F 8, CHF 3 Si 3 N 4 CF 4 /O 2, CF 4 /H 2, C 2 F 6, C 3 F 8, SF 6 /He Organic solids O 2, O 2 /CF 4, O 2 /CF 6 Al BCl 3, CCl 4, SiCl 4, BCl 3 /Cl 2, CCl 4 /Cl 2, SiCl 4 /Cl 2 Au C 2 Cl 2 F 4, Cl 2 46

21 Frequently used reactive plasma gases Frequently used reactive plasma gases, reported etch rates, and etch ratios Deep reactive ion etching: High plasma density > /cm 3. Vapor-phase etching without plasma XeF 2. SCREAM (Single crystal reactive etching and metallization) Cornell nano fab facilty. For laterally driven micromachines requires narrow, thick beams and other structures. Should move freely, and in parallel with the substrate. Increased beam thickness for greater power for a given voltage. 47

22 Increased stiffness structure perpendicular to plane of motion. Thin poly silicon microstructures are fabricated with a combination of dry and wet etching, but for thick beams ( >20µm) obtained by dry etching. Possible even on quartz, GaAs and Si. Independent of crystal orientation. Partial list of subtractive Processes important in micromachining 48