SPM cantilever and fabricating method thereof

First, a problem in the cantilever disclosed in Japanese Patent No. 2624873is that it becomes impossible to accurately ascertain the surface irregularities of a sample,-even though the terminal end of the probe can be sharpened.

Also, since the silicon oxide is etched away in a short time period by fluoric acid, the fabrication of a sharpened probe portion of the order of nanometers with controlling variance thereof is difficult due to the variance in etching if a plurality of cantilevers each having probe portion are to be fabricated within a wafer.

Of the cantilever disclosed in Japanese Patent No. 2984094...

[0039] The probe portion 2 is inclined by an angle of 54.7 degrees corresponding to the lattice plane (111) of silicon with respect to the lever portion 1 at its boundary 4 to the lever portion 1. Further, two sides 2a, 2b of the probe portion 2 that contain the terminal end portion 3 are bent toward the inner side. In other words, it is formed into such a shape that, of the portions having the same width as the lever portion 1, those portions unnecessary as the probe portion 2 are cut off. By such construction, the weight of the probe portion 2 is reduced so that a drop in resonance frequency can be prevented. At the same time, an aspect ratio of the configuration of the terminal end portion 3 of the probe portion 2 is improved so that a sample of narrower measurement intervals can be measured. Further, a damping in cantilever oscillation due to the air or water surrounding the cantilever can be reduced. The terminal end portion 3 of the probe portion 2 has a radius of curvature of 20 nm or less and is very slender as having its length exceeding the plate thickness (film thickness), i. e., the probe structure has a probe terminal end portion of high aspect ratio.

[0040] The SPM cantilever having such construction makes it possible to achieve an SPM cantilever having a probe portion of the structure by which a sufficient measurement down to a bottom surface of concave is possible even if the sample has surface irregularities of relatively large aspect ratio. Also, since the terminal end of the probe portion can be brought into contact substantially vertically with the sample to be measured, measurements at high resolution become possible.

[0045] By as described using a silicon nitride film having a higher silicon content, in particular the silicon nitride film containing more silicon than an elemental ratio of 3:4 between silicon and nitrogen, warping at the lever portion or crack at the terminal end of the probe portion can be prevented. Further, the reaction of silicon atoms within the silicon nitride film and oxygen atoms is accelerated during the oxidation, resulting in an advantage of promoting the sharpening effect. Furthermore, use of CVD method in forming the silicon nitride film makes it possible to adjust the elemental ratio between silicon and nitrogen within the silicon nitride film so that a fine film having less internal stress be formed. Also, since the silicon nitride film can be formed on a substrate of various shapes or materials, the scope of its application can be widened.

[0051] The manner of the terminal end portion of the probe portion 17 at the time of such low-temperature thermal oxidation treatment is shown in an enlarged manner in FIG. 5A. In particular, when a low-temperature thermal oxidation treatment is performed to oxidize the silicon nitride film 16 for forming the probe portion 17, oxidation is advanced not only from the front side of the silicon nitride film 16 which will become the terminal end portion of the probe portion 17 but also from the back side and two lateral faces of the silicon nitride film 16 which will become the terminal end portion of the probe portion 17. Accordingly, the plate-like silicon nitride film 16 is oxidized from the front and back sides and also from the two lateral faces thereof so that the silicon oxide film 20 be formed. A high aspect ratio is thereby achieved of the terminal end portion of the probe portion 17 where the sharpening is more intense toward its tip.

[0056] A further explanation will be made below with respect to the reason why silicon nitride having greater silicon content is used as the silicon nitride film for constructing the probe portion and lever portion of SPM cantilever according to the first embodiment. In particular, the reason is that the reaction between silicon atoms in the silicon nitride film and oxygen atoms in the oxygen atmosphere is accelerated during the sharpening step so that an advantage of promoting the surface oxidation results. Specifically, while an elemental ratio between silicon (Si) and nitrogen (N) of the silicon nitride film in the range of 0.75

[0057] In the range of Si/N>1.3, on the other hand, since Si is polycrystallized and causes a precipitation, the surface coarseness is worsened and it becomes disadvantageous for example for a cantilever in which a smooth lever portion surface or probe portion surface is desired. Also in a subsequent silicon etching process, an etchant may permeate from the polycrystallized silicon portion to result an abnormal etching. To prevent such abnormal etching, the above described elemental ratio range is used.

[0058] By such fabricating method, the cantilever made of silicon nitride having a radius of curvature at the tip of the probe portion sharpened to 20 nm or less and at the same time having a high aspect ratio thereof can be readily fabricated without using a high precision apparatus which is expensive. Further, since the probe portion having the sharpened terminal end portion can be fabricated in a well regulated manner and at high yield, costs can also be reduced. Furthermore, batch fabrication and processing of a plurality of cantilevers at wafer level become possible.

[0059] In particular, unlike the conventional method in which a resist mask is prepared and an underetching is effected while regulating the etchant and time, the length of the sharpened portion of the probe terminal end portion in the fabricating method according to the present embodiment is not restricted by the limitation due to the permeation of etchant under the mask so that the sharpened portion (terminal end portion) having greater length can be made. Further, in the fabricating method according to the present embodiment, it is adequate that only the oxidized portion formed on the silicon nitride film surface be entirely etched away by fluoric acid. It is thereby also possible to secure far greater processing margin for example in control of the temperature and concentration of etchant and control of etching time as compared to an isotropic wet etching of which control is generally considered to be difficult.

[0060] In the case where a further improvement in the aspect ratio of the probe portion or further sharpening of the tip of the terminal end portion is desired, the above described process, i.e., the forming of an oxide film by low-temperature thermal oxidation treatment and the removing of the oxide film by fluoric acid can be repeated for a plurality of times to additionally promote the sharpening. In particular, while a long time is required to form an oxide film of desired film thickness on the silicon nitride film by one time of low-temperature thermal oxidation treatment, a similar result as the forming of the oxide film of the desired film thickness can be obtained in a shorter time period by repeating a plurality of times the forming processing of an oxide film having a film thickness thinner than the desired film thickness. Also, the sharpening of the probe portion can be additionally promoted by previously forming an optimal acute angle pattern of silicon nitride film.

[0063] Further, of the cantilever according to the present embodiment, the probe portion is located on the free end of the lever portion. The positioning at the time of measuring (scanning) of the probe portion to a location to be observed on the sample can thus be easily effected in a short time by attaching it to SPM apparatus into which an optical microscope i...