The process of photoanodic etching of deep pores in lightly doped n Ϫ -Si wafers was investigated. On a 6 in. wafer, more than 1 billion pores, regularly distributed over the wafer in a hexagonal array with a pitch of 3.5 m, were obtained using an electrolytic back contact. The diameter of the pores was about 2 m and depths up to 400 m could be achieved. Kinetic experiments revealed that the etching process was diffusion controlled in a solution containing HF, H 2 O, and ethanol. Electrochemical experiments showed that H 2 evolution took place at the back side of the Si wafer and that H 2 and O 2 evolved at the Pt cathode and anode, respectively.

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High-density MOS capacitors have been fabricated with ∼ 30 nF/mm2 specific capacitance on highly-doped Si-wafers with arrays of macropores with ∼ 2 μm diameter. Using the Bosch process [1] these pores were dry-etched to depths of ∼ 30 μm or more. The enlarged Si-surface thus obtained serves as a substrate for capacitors fabricated by fully MOS-compatible processing.Wafers were fabricated with a top electrode of poly-Si and Al and ‘ONO’ (i.e. oxide / nitride / oxide) dielectric stacks showing 7–10 MV/cm electrical breakdown field and leakage < 1 nA/mm2 @ 20 V. These wafers were thinned to 380 μm and sawn into dies, representing 40 nF capacitors. Typically low loss factors such as ESR < 50 mΩ and ESL < 20 pH and resonance frequencies of ∼ 0.1 GHz were found for 40 nF capacitor dies. Next, 40 nF dies were mounted by wire bonding on Al2O3 or laminate substrate as supply-line decoupling capacitors in complete GSM power amplifier test modules. RF decoupling and transmission were measured and compared to identical test modules with conventional discrete ceramic capacitors. The MOS capacitors showed very efficient decoupling, resulting in superior signal stability as measured in the 0 – 1 GHz range (less noisy, free from oscillations).The new capacitor is very suitable for integrated decoupling purposes, e.g. supply-line decoupling in RF wireless communication and analog and mixed-signal systems.