課程名稱 : 微製造技術 Microfabrication Technology. 授課教師 : 王東安 Lecture 6 Etching

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1 課程名稱 : 微製造技術 Microfabrication Technology 授課教師 : 王東安 Lecture 6 Etching 1

2 Lecture Outline Reading Campbell: Chapter 11 Today s lecture Wet etching Chemical mechanical polishing Plasma etching Ion milling Reactive ion etching High-density plasma etching Liftoff 2

3 Prologue After resist formed, developed, next transfer image into substrate by etching. Wet etching: wafer is immersed in a solution that reacts with the exposed film to form soluable by-products. Wet etching is prone to defect Solution particulate contamination Not for small feature Large volume of chemical waste 3

4 Etch rate Uniformity Etch rate variation over a wafer Etch rate variation from wafer to wafer Selectivity Undercut Etch rate anisotropy Figure of Merit A = 1 R L /R V R L : lateral etch rate R V : vertical etch rate A=1 perfectly anisotropic A=0 perfectly isotropic 4

5 Range of etch process Ion milling: energetic beam in very low pressure, mean free path of ions is much longer than chamber diameter therefore high degree of anisotropy Wet etching: low anisotropy, high selectivity 5

6 Drawback: Lack of anisotropy Poor process control Particle contamination Pro Wet etching Highly selective Does not damage substrate 6

7 Consists of 3 processes Wet etching Movement of etchant species to wafer surface Chemical reaction with exposed film that produces soluble by-products Movement of reaction products away from wafer surface Wet etch solution is often agitated to assist movement of etchant to surface and removal of etch product Some wet etch use a acid spray to supply fresh etchant Small geometry features may etch more slowly, due to difficulty in removing etch products 7

8 Wet etching Undected resist scumming: when exposed resist is not removed in develop process Scum: A filmy layer of extraneous or impure matter that forms on or rises to the surface of a liquid or body of water 8

9 HF Wet etching of SiO2 Wet etching of SiO2 in dilute solutions of hydrofluoric acid: etchants are 6:1, 10:1, 20:1, meaning 6, 10, 20 parts (by volume) of water to one part of HF. It is isotropic. HF solutions are extremely selective of oxide over silicon, some etching of Si occurs since water will oxidize Si surface and HF will etch this oxide. 9

10 HF Wet etching of SiO2 SiO2+6HF -> H2+SiF6+2H2O This reaction consumes HF, reaction rate decrease with time To avoid that, use HF with a buffering agent (BHF) such as ammonium fluoride (NH4F), which maintains a constant concentration of HF through dissolution reaction NH4F <-> NH3+HF where NH3 (ammonia) is a gas. Buffering also controls etchant PH, which minimizes resist attack. 10

11 Silicon nitride etching H3PO4 at :10 of 49% HF (in H2O) and 70% HNO3 at 70 Selectivities in phosphoric etch are 10:1 for nitride over oxide and 30:1 for nitride over Si. If nitride layer is exposed to a high-temp oxidizing ambient, a dip in BHF is done before nitride wet etch to strip surface oxide that have grown on top of nitride. 11

12 Pattern metal by wet etching Al etchant: (by volume) 20% acetic acid, 77% phosphoric acid, 3% nitric acid Impurities in Al, such as Si, Cu, are difficult to remove in standard Al wet etch. 12

13 Si wet etching Use a strong oxidant to oxidize Si and HF to etch etch oxide. Etchant: HF, HNO3 in water Si HNO 3 6HF H2SiF6 HNO 2 H 2 H 2O 13

14 Etch rate of Si in HF and HNO3 Acetic acid (CH3COOH or HC2H3O2) is used as a diluent rather than water. 14

15 Etchants for GaAs H2SO4-H2O2-H2O system 15

16 Directional wet etchants For Si: mixtures of KOH, isopropyl alcohol, water. 23.4:13.5:63 mixture etches 100 times faster in (100) than in (111). This etchant contains no HF, a thermal oxide can be used as masking layer. 16

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18 Wet etching Br2-CH3OH for GaAs directional wet etch Doing selective etch: 1:3:8 mixture of HF/HNO3/CH3COOH. Etch rate of either type of heavily doped (>10 19 cm -3 ) layer s of Si is 15 times larger than etch rate of lightly doped layers. Ethylene-diamine-pyrocatechol-water (EDPwater) etches lightly doped Si, but does not attack heavily doped p-type layers. 18

19 Chemical Mechanical Polishing (CMP) Achieve global planarization 19

20 CMP Earliest process: a thick dielectric, commonly a spinon or CVD glass, is first applied. Wafer is mechanically abraded in alkaline slurry containing colloidal silica (a suspension of abrasive SiO2 particles) and an etching agent such as dilute HF. KOH and NH4OH are common matrix solutions for the suspension. ph, around 10, is maintained to keep silica particles negatively charged to avoid formation of a gel. ph buffering agent can be used to ensure stability of the process. 20

21 CMP Gross mechanical damage is prevented by the fact that SiP2 particles being used in the slurry are not harder than the film that is being polished Typical removal rates of SiO2 are several thousand angstroms per min Increase pad pressure increases removal rate, at cost of step height, residual oxide damage, metal contamination 21

22 CMP Metal planarization: copper, tungsten Acidic (ph<3) slurries are used. Theses slurries do not dorm colloidal suspensions and so agitation must be used to maintain uniformity Alumina is the commonly used abrasive for tungsten CMP because it is closer in hardness to tungsten than most other abrasives. Cu is polished in aqueous solution containing particles several hundred nanometers in diameter. Typical slurries include ammonium hydroxide, nitric acid, hydrogen peroxide. 22

23 CMP Trade off polish goals (uniformity, planarity, throughput) against cleaning goals (particles scratches, residual ionic and metallic contaminants) Postpolish cleaning: megasonic agitation can be used in combination with a soft pad scrubber or a cleaning solution to assist in removal of colloidal suspension from wafer. Wafer transferred to second pad reserved for cleaning. Transfer be timed to prevent drying of suspension on wafer surface. Scratches left behind after CMP may collect metal. Metal filled scratches are called rails. Dilute (100:1) HF may be included to lift off metal particles. 23

24 Basic regimes of plasma etching Advantages of plasma etching Easier to start and stop Less sensitive to changes of wafer temperature High anisotroy Fewer particles than liquid media Less chemical waste than wet etching 24

25 Plasma etch process A feed gas be broken down into chemically reactive species by plasma. These species diffuse to wafer surface. They move about until react with the exposed film. Reaction products be desorbed, diffused away from wafer, transported by gas stream out of etch chamber 25

26 Plasma etching Film surface is subjected to incident flux of ions, radicals, electrons, neutrals. Neutral flux is the largest, physical damage is related to ion flux. Chemical attack depends on ion flux and radical flux 26

27 High pressure plasma etching In CF4 plasma, small ion concentration, etch is not directly due to ions in plasma. Rather ion bombardment on surface creates unsatisfied bond that are exposed to reactive radicals. Form volatile products and are pumped away. At wafer surface, Si atom is bonded to 2 F atoms. Both SiF2 and SiF4 are volatile species, but SiF2 will not readily desorb since it is chemically bonded to wafer. 27

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29 Plasma etching Possible to use F2 as feed gas, but high toxicity Preferred species are CF4, C2F6, SF6, produce large concentrations of free F. Addition of small concentrations of O2 to a CF4 feed gas increase etch rate of Si and SiO2. O2 reacts with C to produce CO2. this removes C from plasma, increase F concentration. 29

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31 Anisotropic etch: fluorocarbons deposit on all surfaces. Ion velocity, which follows electric field, is vertical. Little ion bombardment on sidewall, fluorocarbon accumulates Process of producing non-volatile species that reduce etch rate is polymerization The film is said to passivate sidewall, prevent lateral etching 31

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33 Ion milling Pure ion milling/ion beam etching: no chemical reactions, since it uses noble gases such as argon. A strictly mechanical process Advantages: Directionality: ions are accelerated by a strong electric field, chamber pressure is low that atomic collision is not likely. Anisotropic etching is possible for any material since it is chemistry independent Applicability: it can be used to pattern a wide variety of materials including compounds and alloys 33

34 Kaufman source Most popular source of ion milling Electron filament heated using supply V F. Filament is held at a voltage Va below anode potential. Electron boil off of filament and are accelerated toward anode. Electrons impact neutral the neutral gas atoms to ionize them. To maintain plasma, source is held at torr. 34

35 Kaufman source A grid held at Vg accelerates ejected ions toward target Ejected ions enter target chamber To maintain directional etch, target chamber is pumped to low pressure, minimize collisions between ions and residual gas molecules 35

36 Ion beam milling Ions are positively charged, a voltage will build up on the surface unless wafer and film being etched are conductive To avoid charging effect, electron flood gun is used 36

37 Problems of ion beam milling Process erode mask layer, any taper in masking layer be transferred to pattern Eroded material from target is not volatile. Some will redeposit on wafer surface, lead to uneven etch and organic residue from resist mask. Trenching: mask erosion causes sidewalls of the pattern to be tapered at a steep angle. Some of low angle ions will reflect off of tapered surface toward the pattern edge 37

38 Ion beam milling An inert species such as argon can be mixed with small amounts of oxygen to reduce erosion rate of many metals in an ion mill. Introducing reactive species tend to attack source components. In particular, hot filament is attacked. 38

39 Reactive ion etching Need for anisotropic etch with higher selectivity than ion milling 39

40 Two common systems for RIE Chlorine plasma for anisotropically Si, GaAs, Al etch 40

41 Undoped Si etch slowly in Cl/Cl2 ambient without addition of ion bombardment Heavily n-type doped Si etches without bombardment in Cl, but not in Cl2. Doping implies that Cl etching involves electron transfer from substrate Chlorine RIE 41

42 Chlorine RIE model Atomic Cl chemisorbs on Si. Once surface Cl becomes negatively charged, it can bond ionically with the substrate. This frees additional chemisorption sites and increases probability that Cl atoms penetrate surface and produce volatile silicon chlorides. 42

43 Cl RIE Cl penetration is increased by ion bombardment Charge transfer produces isotropic etch in heavily doped layers such as polysi gates and Al metallizations. In these structures, obtain anisotropic etch by sidewall polymerization. Sidewall polymerization is done by adjusting relative concentrations of Cl2 and BCl3, CCl4 or SiCl4. 43

44 Damage in RIE Residual damage after RIE: physical damage chemical damage 44

45 Physical Damage in RIE Etch of SiO2 down to Si: surface is damaged with an extensive concentration of Si-O, Si-C bonds. H2 penetration can deactivate dopants in substrate. 45

46 Chemical Damage in RIE a concern in polymerization etches, leave behind residual films Gas particle deposition Metallic impurities due to sputtering of electrodes, chamber, fixture in contact with plasms 46

47 High-Density Plasma (HDP) etching High-density sources use crossed magnetic and electric fields to increase distance the free electrons in plasma travel. This, in turn, increase rate of dissociation and ionization. High density of ions and radicals can be used to increase etch rate. 47

48 High-Density Plasma (HDP) etching A HDP system: inductively coupled reactor, use RF current through coils to produce an oscillating magnetic field that creates an electric field. This induced magnetic field alters path of electrons in plasmas, increase plasma density. 48

49 liftoff An alternative to ion milling for patterning difficult to etch materials 49

50 Procedure of liftoff Resist is spun and patterned 50

51 Procedure of liftoff A thin layer of meal is deposited using evaporation. Evaporation: difficult in covering high aspect ratio features. If a reentrant profile is obtained in resist, a break in metal is assured. 51

52 Procedure of liftoff Wafer is immersed n a solution to dissolve resist. 52

53 Procedure of liftoff To form a reentrant angle by hardening resist surface. Soak DQN resist after softbake in chlorobenzene to reduce dissolution rate of upper surface of resist. After developing, a ledge appears. 53

54 Shortcoming of liftoff Surface topology must be smooth, since metal deposition step has poor step coverage. Metal lifted off remains solid and floats in bath. Pieces of it are likely to redeposit on wafer surface. 54