Microfabrication by electrochemical metal removalWord文件下载.docx

1、Microfabrication by electrochemical metal removal#Microfabrication by electrochemical metal removal#Datta, M #Recent advances in the development of electrochemical metal-removal processes for microfabrication are reviewed in this paper. After a brief description of the process, several important par

2、ameters are identified that determine the material-removal rate, shape control, surface finishing, and uniformity. The influence of surface film properties, mass transport, and current distribution on microfabrication performance are discussed. Several examples of microelectronic component fabricati

3、on are presented. These examples demonstrate the challenges and opportunities offered by electrochemical metal removal in microfabrication. Introduction #Material-removal techniques are among the key processing technologies that are used in the fabrication of microelectronic components 1. These meth

4、ods are popularly known as etching techniques. Dry vacuum processes for thin-film etching are based on plasmaassisted processes and include ion etching, plasma etching, and reactive ion etching 1. Ion etching is a physical process, whereas plasma etching involves a chemical reaction. Reactive ion et

5、ching is a combination of both physical and chemical effects contributing to material removal. In ion etching, ions are extracted from a gaseous plasma and accelerated to the substrate, where the surface is eroded by momentum transfer. Plasma etching employs a glow discharge to generate active speci

6、es such as atoms or free radicals. The active species diffuse to the substrate, where they react with the surface to produce volatile products. Addition of reactive gases to the ion source （reactive ion etching） enhances the physical etch rate and introduces chemical etching as well. These processes

7、 are particularly employed in the semiconductor industry for ultralarge-scale integration （ULSI） because of their ability to remove material with precision. However, some of the disadvantages that are inherent in dry-etching techniques include high equipment cost, lack of selectivity, and problems a

8、rising from redeposition on the sample and deposition on the vacuum chamber. Dry-etching techniques are used for precision etching of thin films involving very small amounts of material removal. Lately, concern has intensified about the safety, environmental impact, and disposal of the toxic gases u

9、sed in plasmaassisted dry etching. #Wet chemical etching involves removal of unwanted material by the exposure of the workpiece to an etchant. The exposed material is oxidized by the reactivity of the etchant to yield reaction products that are transported from the surface by the medium. Chemical et

10、ching converts a solid insoluble material to a soluble form by dissolving the extended lattice of metal atoms so that these atoms can enter the solution as soluble compounds. This is accompanied by removal of electrons from the metal （oxidation）. These electrons are accepted by the etchant, which ac

11、ts as an oxidizing agent in chemical etching. The metal-removal reaction typically involves several sequential steps, the dissolution kinetics being controlled either by the chemical reactivity of the species involved （activation-controlled） or by the speed at which the reaction product is removed f

12、rom the surface and the fresh reactant is supplied to the surface （diffusion- or mass-transport-controlled）. Temperature variations also profoundly influence the kinetics of metal-removal reaction. #Wet chemical etching baths contain chemicals that are generally aggressive and toxic 2, thus posing s

13、afety and disposal problems. In many wet-etching manufacturing processes, waste treatment and disposal costs often surpass actual etching processing costs. The everincreasing cost of incineration and the imposition of landfilling restrictions are the main reasons behind the need for developing alter

14、native processes. #Electrochemical metal removal is an alternative wetetching process where the workpiece is made an anode in an electrolytic cell in which a salt solution is used as an electrolyte and controlled metal removal takes place by application of an external current. Several nonconventiona

15、l machining processes such as electrochemical machining （ECM） and electropolishing are based on the principle of electrochemical metal removal 3, 4. The ECM process has been used predominantly in the production of turbine engine parts and for other aerospace applications, but applications of ECM als

16、o exist in the production of automotive components, medical implants, appliance parts, artillery projectiles, gun-barrel rifling, etc. because of its ability to machine complex features and complicated contours without machining marks, burrs, or surface stresses. ECM is used to perform machining ope

17、rations analogous to broaching, turning, and die sinking, while static machining with a stationary tool is used to deburr, polish, and/or radius components. The rapid metalremoval rate along with the advantage of nonconsumed tooling makes it an attractive process for metal shaping and finishing. How

18、ever, electrochemical metal removal has received little attention so far in the microelectronics industry. Recent investigations of the development of advanced electrochemical metal-removal processes demonstrate that the ECM concepts can be effectively used in the removal and patterning of conductin

19、g films that are of interest in the electronics industry 5, 6. #Electrochemical micromachining （EMM） #Application of ECM in the processing of thin films and in the fabrication of microstructures is referred to as electrochemical micromachining （EMM）. Compared to predominantly used chemical etching,

20、the EMM process offers better control and flexibility, requires very little monitoring and control, and has minimum safety and environmental concerns 5, 6. A variety of metals and alloys can be machined by EMM. EMM is now receiving considerable attention in the electronics and other hightechnology i

21、ndustries, particularly as an alternate, #greener# method of processing metallic parts 5, 6. #Most of the thin films of metals and alloys, including conducting ceramics and highly corrosion-resistant alloys, that are of interest in the microelectronics industry can be anodically dissolved in neutral

22、 salt electrolytes such as sodium nitrate, sulfate, or chloride. In these electrolytes, the dissolved metal ions form hydroxide precipitates which remain in suspended form in solution and can be easily filtered, thus significantly minimizing problems of safety and waste disposal. Hydrogen evolution

23、is generally the main cathodic reaction. The cathode, therefore, remains unaltered. Accumulation of reaction products in solution and depletion of bath components are of little concern in EMM, thus making it a simpler and more environmentally friendly manufacturing process. #Microfabrication by EMM

24、may involve maskless or through-mask material removal. Thin-film patterning by maskless EMM requires highly localized material removal induced by the impingement of a fine electrolytic jet 5, 7. Investigation of jet and laser-jet EMM demonstrated that neutral salt solutions can be effectively used f

25、or highspeed micromachining of many metals and alloys. An example of such an investigation is shown in Figure 1, which demonstrates the feasibility of employing an electrolytic jet for generating complicated patterns in metallic foils and substrates. Other examples of maskless EMM include microfinis

26、hing of components and removal of unwanted layers of thin films by electromilling 8. #EMM in conjunction with a photoresist mask is of considerable interest in microelectronic fabrication. Through-mask EMM involves selective metal dissolution from unprotected areas of a one- or two-sided photoresist

27、patterned workpiece. Through-mask metal removal by wet etching is accompanied by undercutting of the photoresist and is generally isotropic in nature. In isotropic etching, the material is removed both vertically and laterally at the same rate. This is particularly the case in chemical etching, wher

28、e the etch boundary usually recedes at a 45 deg angle relative to the surface 2. In EMM, however, the metal-removal rate in the lateral direction may be significantly reduced through proper consideration of mass transport and current distribution 5. Etch factor is defined as the ratio of the amount

29、of straight-through etch to the amount of undercut 6. For applications requiring high aspect ratio, minimized undercutting of the photoresist and a high value of etch factor are desirable. #EMM performance criteria #The metal-removal rate, microfeature profile, surface finish, and uniformity of meta

30、l removal are some of the performance criteria that determine the technical feasibility of a metal-removal process. In EMM, these criteria are dependent on the ability of the system to provide desired mass-transport rates, current distribution, and surface film properties at the active surface （Figu

31、re 2）. An understanding of the metal-electrolyte interaction under high-rate anodic dissolution conditions is a prerequisite for optimizing process parameters such as electrolyte composition and voltage/current. The development of precision tools requires an understanding of the influence of hydrody

32、namics, current distribution, and process parameters on the EMM performance. A precision tool should provide conditions of desired current distribution and a high rate of uniform mass transport at the dissolving surface 5. #In through-mask processes, additional issues related to lithography processi

33、ng are critical to achieving desired performance. Production of the master artwork, surface preparation, choice of proper photoresist, and imaging are extremely important in the successful implementation of an etching process. Since parts produced by this process are a direct reflection of the master artwork, it is essential that all aspects of preparing th