Porous-type anodization

Electrochemical micromachining and surface microstructuring based on porous-type anodization of patterned films... 

This chapter is not intended to be a comprehensive review of literature. For example, micromachining based on porous-type anodization of the entire A1 substrate, subsequent lithography, and selective and anisotropic etching of unprotected regions of porous AI2O3 will not be covered in this chapter. This technique has been described in other publications.6-11 This chapter represents our attempt to draw attention to the unique capability of localized porous-type anodization of A1 for the fabrication of A1 or AI2O3 microstructures and to demonstrate some examples of fabricated or feasible devices. In addition, this chapter is intended to summarize advances made in this area from the 1970s to the present time. 

Figure 1. Process flow (1) photolithography (may include an optional step of negative mask transfer by barrier-type anodization), (2) porous-type anodization, (3) chemical etching of either (3a) porous AI2O3 or (3b) Al.

Figure 2. Cross-sectional schematics of microstructures (a) features of A1 and porous AI2O3 obtained after localized porous-type anodization, (b) trapezoidal features of A1 obtained after chemical etching of porous AI2O3. Reproduced from Ref. 32 with permission from IOP Publishing Ltd.

Figure 4. Regions of Al-0.5%Cu alternating with regions of porous AI2O3 obtained upon completion of localized porous-type anodization of 3-pm-thick Al-0.5%Cu films. The inset is a magnified image of the interface between Al-0.5%Cu and porous AI2O3. Reproduced from Ref.32 with permission from IOP Publishing Ltd.

In addition to voltage, the rate of porous-type anodization depends on the electrolyte composition, acidity, and temperature. These variables affect the rate of AI2O3 dissolution at the pore bottoms in the isotropic way and are not expected to increase EF. Another variable to consider is the depth of porous-type anodization. 

Cosse et al. showed that EF decreases as the thickness of anodized Al-0.5%Cu films increases from 3 to 10 J.m.32 Kikuchi et al. reported that EMM of Al specimens resulted in the hemispherical 20-pm-deep microgrooves.40 In this case, porous-type anodization of patterned Al substrates was isotropic. These results suggest that localized anodization becomes isotropic as the anodization process proceeds. These conclusions are consistent with those obtained with other EMM methods, which indicate that EF decreases as the depth of metal removal increases.13,17... 

In order to evaluate EF, one has to consider the dimensions of features undergoing porous-type anodization, which are typically 1-100 pm wide. Data reported in literature suggest that EF decreases when the width of unmasked areas decreases. For example, Renshaw showed the hemispherical cavities of porous AI2O3 formed by porous-type anodization through defects in the nonporous surface oxide films.41 Brevnov et al. reported the radial propagation of pores initiated at the anodization mask defects, which results in... 

In conclusion, the requirement to obtain a high aspect ratio features dictates a necessity to increase EF. As discussed in this section, the anisotropy of localized anodization depends on the process conditions, such as anodization voltage, electrolyte composition and temperature, the depth of porous-type anodization, the presence of alloying elements in Al, and the dimensions of features, which undergo porous-type anodization. As shown, voltage is the primary variable, which allows one to increase EF. At the same time, localized anodization of patterned Al substrates became isotropic as the depth of porous AI2O3 formation increases and the width of anodized features decreases. These trends have to be taken into account while utilizing porous-type anodization for EMM of Al substrates. 

Figure 8. Regions of 9.5-pm-thick Al-0.5%Cu phase alternating with regions of porous AI2O3 obtained upon completion of porous-type anodization. Pitch is 15 pm. Porous-type anodization time is 80 min. Reproduced from Ref.47 with permission from ECS - The Electrochemical Society.

This section will review selected applications and devices produced by using localized porous-type anodization and, optionally, selective etching of either porous AI2O3 or Al. The fabrication of these devices shares the common technological issues and challenges discussed in Sect. Ill the choice of a reliable mask material, the fidelity of the mask transfer, the trapezoidal profile of metallic features, the... 

Prior to the introduction of Cu electroplating, the primary method used to form a multilevel structure of interconnections in integrated circuit applications was A1 and Al-alloy metallization.49 Localized porous-type anodization was developed in the 1970s to obtain planar interconnection metallization for multilevel large-scale integration (LSI) 26,46,50 For example, Schwartz and Platter showed that the subtractive etching for A1 interconnects could be substituted... 

Experimental conditions for porous-type anodization were identified, which favor the formation of almost rectangular features with the pattern transfer accuracy of 15%.53... 

In addition to the multilevel metallization and formation of interconnects, anodic processing of A1 was employed for the fabrication of integrated passive components thin film capacitors and inductors.56,57 For example, localized porous-type anodization of A1 films was used to convert 20- am-thick A1 to the dielectric layer of porous AI2O3 and to define metal-dielectric-metal structures.56 The... 

In addition to the anodic processing of A1 films, localized porous-type anodization, combined with selective etching of either A1 or porous AI2O3, can be used for the surface microstructuring of thick A1 substrates.7,30,58 As an example, Fig. 13 demonstrates a schematic presentation of processing steps. Photolithography (Step 1) was used to define 25-pm-diameter circular features separated by 37.5 pm pitch. Time and conditions of localized porous-type anodization (Step 2) were chosen such that U exceeded half of the pitch. Consequently, under chosen process conditions and feature dimensions, any three adjacent anodized areas overlapped. Finally,... 

Figure 15. Process flow for surface microstructuring with undercut being less than half of the pitch. (1) photolithography, (la) top view and (lb) cross-section view (2) porous-type anodization (3) application of an epoxy on the top and selective etching of A1 from the back and (4) selective etching of porous AI2O3. SEM images of microstructured A1 surface after Step 4, tilted samples with different shapes of anodized/etched features (left and middle) and separated features of porous AI2O3 on the epoxy film, after Step 3 (right). Images reproduced from Ref.30 with permission from ASME.

As shown in the previous section, localized porous-type anodization and selective etching of A1 can be used for the fabrication of complex microstructures composed from freestanding porous AI2O3. As an example, Fig. 16a and b demonstrates SEM and optical images of microstructured porous AI2O3 substrates, which are utilized for sensor applications (Fig. 16c, d). The lateral dimensions of freestanding ceramic substrates were controlled by the mask design and process conditions. The thickness was controlled by the current density and duration of anodization (2 min anodization... 

Figure 16. (a) SEM of gas microsensor substrate prepared by localized porous-type anodization of Al. (b, c) Optical images of gas microsensor substrate without and with electrodes, (d) Packaged humidity sensor with microheater. Images reproduced from Ref. 7 with permission from ECS - The Electrochemical Society and from Ref. 69 with permission from ASME. 

EMM can be applied to either Al films, typically deposited on SiCVSi substrates, or Al foils. The dimensions of metallic features are determined by the same rules the etch factor, the depth of porous-type anodization, the mask design, and process conditions. In addition to the fabrication of metallic microstructures, EMM can be used to produce microstructured ceramic substrates composed of porous AI2O3. For the fabrication of both types of 3D microstructures by localized porous-type anodization, the following technological problems have to be addressed the reliability of a mask material, the fidelity of the mask transfer, volumetric expansion of porous AI2O3 during anodization, and the effect of the mask design on the rate of porous-type anodization and on the completion of anodization of the entire thickness of Al without traces of Al islands. 

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