氧化对超薄氧化物的表面粗糙度的影响

Surface roughness of ultra-thin oxide formed by various oxidation, ie. dry, wet and rapid thermal oxidation. isinvestigated by atomic force microscopy measurement Surface roughness is identified as "DRY" > "RTO > “WEr”Wet oxidation makes smoother surface than other oxidation methods. lt is considered that oxidation affects on surfaceroughness and on reliability of ultra-thin oxide.

1. Introduction

The reliability of gate oxide becomesof greatimportance as the gate oxide thickness of MOS devices isapproaching to the physical limit of directtunnelingRecently, many studies of the direct-tunneling gate oxidehave been carried out[l].The reliability of ultra-thin oxides is related to severalissues such as soft-breakdown and the gate leakage currentIt is reported that a “strained layer’ near SiO,/Si interfacestrongly affects on the ultra-thin gate oxide reliability. It isalso known that the TDDB is decreased, as the built-incompressive strain is increased/2].In this study, among many concerns for the direct-tunneling oxide such as effect of leakage current, chargedensity, defect density, roughness, determination of theaccurate oxide thickness, method of formation etc, weinvestigated relationship between roughness and oxidationcondition.Surface roughness of tunneling oxide formed by variousoxidation processes is investigated by atomic forcemicroscopy measurement.

2. Experimental

Starting material is 4 inch, boron doped p-type (100)oriented single crystal silicon wafer. After cleaning(HFlast), ultra-thin oxides with 30 A thickness were thermallygrown as shown in Table 1. Oxides were formed in a rapidthermal process at 900C , in an O, ambient at 800 'C andin pyrogenic ambient at 800 C (The formed oxides aredenoted in this paper RTO, DRY, and WET, respectively).Oxide thicknesses were monitored by ellipsometer.Then 30A oxides were removed by wet etching in 10:lHF solution. On each samples atomic force microscopymeasurements were carried out. Table l. shows processsequence of each sample.

3. Results

AFM measurement was done after removingoxidesformed by different oxidation methods. Table 2. showssilicon surface roughness data of each sample.