Metal-enhanced HF etching of Si is an electroless method used to produce porous Si. Such etching generally uses not only metal-modified Si but also an oxidizing agent, such as hydrogen peroxide or metal ions. Pd exhibits high activity in enhancing the HF etching of Si without an oxidizing agent even under dissolved-oxygen-free and dark conditions. Electrolessly deposited Pd particles on n-type Si enhance the HF etching of Si but produce no porous layer. Patterned Pd films localize the etching under the boundary of the Pd deposited areas, and thus Pd can produce a microetch pattern on Si with a simple immersion in the HF solution. This etching reaction is explained by electron injection into the conduction band of Si due to the Pd-enhanced anodic oxidization of Si with water and the cathodic hydrogen evolution on Pd with the injected electrons.Porous silicon ͑Si͒ is usually prepared by electrochemical etching under an anodic bias in a fluoride-containing solution. 1-3 Metalenhanced HF etching of Si has attracted considerable attention as a new electroless method that can produce porous Si by immersing metal-modified Si in an HF solution without bias. 4-15 Such etching generally uses not only metal-modified Si but also an oxidizing agent, such as hydrogen peroxide, 5,10,12-14 peroxodisulfate, 8 permanganate, 8 or metal ions. 7 We reported the preparation process 6,9,11,15 of porous Si using metal-particle-enhanced HF etching of Si without a particular oxidizing agent as well as its application for solar cells 6,9,16 and metal nanorod formation. 17 This etching generally proceeds by a local galvanic cell mechanism requiring photoillumination or dissolved oxygen. Palladium ͑Pd͒ exhibits high activity in enhancing etching under dissolved-oxygen-free and dark conditions. 15,18 In this paper, we describe the unique behavior of Pd on HF etching of Si and micropattern formation. 19
ExperimentalSingle-crystalline n-type Si wafers ͓Yamanaka Semiconductor, CZ ͑100͒, ca. 1 ⍀ cm͔ were cut into pieces ͑1 ϫ 1 cm͒, washed with acetone, and etched with CP-4A ͑a mixture of HF, nitric acid, acetic acid, and water͒ and a 7.3 M ͑M = mol dm −3 ͒ HF solution. Metal particles were deposited on Si wafers by electroless displacement deposition from a 1 mM metal-salt ͑PdCl 2 or H 2 PtCl 6 ͒ solution containing 0.15 M HF. 20 Pd thin films were deposited onto n-Si wafers by the electron-beam evaporation method. The thicknesses of the deposited films were measured using a quartz crystal microbalance ͑ULVAC, CRTM-5000͒ adjusted by gravimetry. The Pd films were patterned by attaching a mask ͑85 m square holes and 40 m space͒ to the n-Si wafers during deposition. The metaldeposited n-Si wafers were immersed in a 7.3 M HF aqueous solution at 298 K under dark conditions. The etching time was 24 h unless otherwise specified. Argon gas ͑Ͼ99.9999% purity, Taiyo Nippon Sanso, grade-1͒ bubbling was applied to the HF solution before and during etching to remove the dissolved oxygen from the solution. The average etch rate of Si was measured using a gravi...
