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Lecture 10 Microfabrication Pattern Transfer (III) Wet Etching - Isotropic Wet Chemical Etching - Selected Wet Etchants and Selectivity - Surface Micromachining Material Systems -

1 Lecture 10 Microfabrication Pattern Transfer (III) Wet Etching - Isotropic Wet Chemical Etching - Selected Wet Etchants and Selectivity - Surface Micromachining Material Systems - Design of Surface Micromachining Process - Etch Selectivity in Surface Micromachining - Stiction in Fabrication - Surface Micromachining Materials - Silicon Isotropic Wet Etching MEMS_Lect10_1

2 Wet Chemical Etching Wet Chemical Etching: Immersion of the patterned substrate in a suitable liquid chemical, and the etchant attacks the exposed region and leaves the protected region alone. The rate of etching and the shape of the resulting etched feature depend on many thing: 1. the type of substrate, 2. the specific chemistry of the etchant, 3. the choice of masking layer and the tightness of its adhesion to the substrate, 4. the temperature (which controls reaction rates), and 5. whether or not the solution is well stirred (which affects the rate of arrival of fresh reactance at the surface). Depending on the temperature and mixing conditions, the etching reaction canbeeither reaction-rate controlled, dominated by temperature, or mass-transfer limited, determined by the supply of reactants or the rate of removal of reaction products. MEMS_Lect10_2

Isotropic Wet Etching Most wet etching is isotropic, in that the rate of material removal dose not depend on the orientation of the substrate. Isotropic etching.

3 Isotropic Wet Etching Most wet etching is isotropic, in that the rate of material removal dose not depend on the orientation of the substrate. Isotropic etching. When etching single-crystal substrates with certain etchants, orientationdependent etching can occur. It is called anisotropic etching. Because the wet-etching is isotropic, the mask is undercut, the side wall is typically papered and curved. Adhesion of the mask to the thin film is also important. If the adhesion is weak, enhanced etching can actually occur at the film-mask interface, exaggerating the sloping of the sidewall. Such tapered sidewalls can be an advantage when attempting to cover the etched feature with an additional film. Perfectly vertical steps are harder to cover. MEMS_Lect10_3

4 Selected Wet Etchants and Selectivity Etch rate : Speed of etching for the desired materials. Selectivity : Discrimination between the etched materials and the un-etched materials. HF etches silicon dioxide, but also etches silicon nitride slowly. If silicon nitride is the mask material, one must be concerned with how long it it must remain exposed to the etchant. In surface micromachining, where long etch times may be required to remove all of the oxide beneath structural elements, the etch selectivity of protective layers is important. Table Selected wet etchants Material Thermal or CVD silicon dioxide Silicon nitride Polysilicon Aluminum Copper Gold Etchant Buffered hydrofluoric acid (5:1 NH 4 F: conc HF) Hot phosphoric acid KOH or ethylene diamine/pyrochatecol (EDP) PAN (phosphoric, acetic, nitric acids) Ferric chloride Ammonium iodide/iodine alcohol MEMS_Lect10_4

5 Stirring of Isotropic Wet Etching - 等方性식각은모든방향으로같은속도로이루어지나, 가늘고긴통로에서는반응의확산이제한되기때문에속도가느려진다. - 그런경우, 식각액의교반 (stirring) 이식각속도나식각된구조물의형상을좋게해준다. - 교반을잘해주면거의반구에가까운면을얻을수있는데, 이는교반이반응액이나생산물을잘수송해서식각속도를높여주고수송을균일하게해주기때문이다. Illustration of isotropic etch cross sections showing the effects of mask geometry and agitation. Note that this type of etchant is an option for silicon and is, in general, the only wet etch option for glasses. After Petersen(1982). MEMS_Lect10_5

6 Surface Micromachining Material Systems A sacrificial layer is deposited and patterned on a substrate. Then, a structural material is deposited and patterned so that part of it extends over the sacrificial layer and part of it provides an anchor to the substrate. Finally, the sacrificial layer is removed, leaving a cantilever beam of the structural-layer attached at one end to the substrate. Illustrating surface micromachining: the use of an isotropic wet etchant to remove a sacrificial layer beneath a cantilever beam. MEMS_Lect10_6

7 Design of Surface Micromachining Process Enormous flexibility in the design of surface micromachining process. One needs three or four different materials: (1) a substrate (or a suitable thin-film coating over a substrate to provide the anchoring surface), (2) a sacrificial material, (3) a structural material, (4) and an electrical insulation material (isolation the structural elements form the substrate). The etchant that is used to release the structure must etch the sacrificial layer quickly, and the remaining layers very slowly, if at all. For example, 5:1 buffered HF etches thermal oxide at about 100 nm/min, but etches silicon-rich silicon nitride at a rate of only 0.04 nm/min. Table Surface micromachining material systems Structural Sacrificial Release Etch Isolation Ref. Polysilicon SiO 2 Buffered HF Si 3 N 4 + SiO 2 [20] Polyimide Aluminum PAN Etch SiO 2 [21] LPCVD Si 3 N 4 + Al Polysilicon XeF 2 SiO 2 [22] Aluminum Photoresist Oxygen plasma SiO 2 [23] MEMS_Lect10_7

8 Etch Selectivity in Surface Micromachining During the release etch, when it is typically necessary to exploit the undercutting of the structural elements to remove thin films over distances large compared to the isolation-layer thickness, the etch selectivity becomes critical. In the case of silicon dioxide as a sacrificial material with silicon nitride as the isolation material, if the stoichiometry is adjusted to make the nitride silicon rich, not only does the etch rate in HF decrease significantly, but its residual stress decreases, allowing thicker layers to be deposited without cracking or other deleterious mechanical effects. MEMS_Lect10_8

9 Stiction in Fabrication The unintended adhesion of released mechanical elements to the substrate. When using a wet release etch, the surface tension during drying can pull compliant beams into contact with the substrate, and during the final drying, they can adhere firmly together. Methods of avoiding stiction include (1) the use of self-assembled molecular monolayers (SAM s) to coat the surfaces during the final rinse with a thin hydrophobic layer, reducing the attractive force, (2) the use of vapor or dry-etching release methods, such as XeF 2, (3) various drying methods (freeze drying and drying with supercritical CO 2 ) that remove the liquid without permitting surface tension to act, (4) temporary mechanical support of the movable structure during release using posts of photoresist or some other easily removed material. MEMS_Lect10_9

10 Surface Micromachining Materials The most widespread material system used to date is silicon-rich silicon nitride as a coating on an oxidized silicon substrate to provide electrical isolation and an anchor, silicon dioxide as the sacrificial layer, polysilicon as the structural material, and buffered HF as the etchant. This combination is used in Analog Devices surface micromachined accelerometers. The use of LPCVD Si 3 N 4 with aluminum electrodes as the structural material, polysilicon as the sacrificial layer, and the vapor etchant XeF 2 as the release etch is used in the Silicon Light Machine optical projection display. Texas Instruments uses aluminum as the structural material in their projection display, with photoresist as the sacrificial material removed by plasma etching. MEMS_Lect10_10

11 - HNA : a mixture of HF, HNO 3,CH 3 COOH. -HNO 3 : 실리콘을산화. HF/HNO 3 /Acetic Acid ( HNA ) -HF:HF 의 fluoride ion 은용해가능한 silicon compound 인 H 2 SiF 6 를생성. - Acetic acid :HNO 3 가 NO - 3 나 NO - 2 로분해하는것을막고, 실리콘의산화에직접적으로관여하는성분을만들어준다. N2O4 2NO2 - 초산이없더라도 NO 2 가없어질때까지잠시식각이된다. - 식각화학식은복잡하나, 식각속도는용액의조성과 silicon doping에영향받는다. 18 HF + 4HNO3 + 3Si 2H 2SiF6 + 4NO( g) + 8H 2O - 이식각액의단점은 SiO 2 를비교적빠른속도 (30~70 nm/min) 로상하게한다는것이다. - Light doping ( <10 17 cm -3 n- or p- type ) 된영역은 heavily doped region 보다 150 배정도더디게식각된다는점에유의해야한다. (continued) MEMS_Lect10_11

12 HF/HNO 3 /Acetic Acid ( HNA ) - 단결정실리콘습식식각의기본원리. (1) Si 에 hole 이주입되어 Si 2+ 나 Si + 를생성. (2) Si 2+ 에 OH - 가붙어서 Si(OH) 2 2+ 를생성. (3) hydrated Si (Silica) 가용액의복잡한용제와반응. (4) 반응생성물이용액에용해. - 이런식각에서는 hole 공급원, OH -, 복잡한용제가필요하다. Hole을만드는화학반응은화학식에서전자를다른변으로옮기는것에지나지않는다. e + HNO + HNO + H O 2HNO + 2OH 다른표현으로 HNO + HNO + H O HNO + 2OH + 2h 이것은전기화학반응이고, 이식각이기본적으로전하이송에의한공정이므로 dopant type/concentration 그리고외부에서가해주는전위가식각공정에영향을주게된다 h MEMS_Lect10_12

13 Table of HNA Etchant Formulations Table of HNA etchant formulations, assuming standard acid concentrations (note that water or acetic acid can be used) and their properties. From Petersen (1982). MEMS_Lect10_13

14 HNA Isotropic Etching 날카롭다. 요철모양대로식각이진행한다. 둥글게된다. 둥글게된다 ( 평탄화 ) 고농도층의선택식각 HF-HNO 3 계 etchant 에의한실리콘의등방성식각 MEMS_Lect10_14

PowerPoint Presentation

PowerPoint Presentation Dry etch 1. Wet etch and dry etch 2. Wet etch and dry etch의장. 단점 3. Dry etch의종류 4. Plasma etch의특성 5. Dry etch에서고려하여야할점 6. Film etch 6.1 Si etch 6.2 SiO 2 etch 6.3 Si 3 N 4 etch 6.4 Al etch 6.5 Silicide