RU2651639C1 - Method of local etching of silicon dioxide - Google Patents

Abstract

FIELD: microelectronics.

SUBSTANCE: invention relates to microelectronics, methods for monitoring and analyzing the structure of integrated circuits, and to processes of liquid etching. Summary of the invention: aligning the local thick unevenness of the silicon dioxide layer on the surface of an IC chip formed during the sequential removal of topological layers, is produced by means of local liquid etching, which is performed by "painting" the area with a thicker layer of silicon dioxide by means of a pointed porous rod saturated with an etchant.

EFFECT: aligning the local uneven thickness of the silicon dioxide layer.

1 cl, 1 dwg

Description

The invention relates to microelectronics, methods for monitoring and analyzing the structure of integrated circuits (ICs), mainly during micro- and macroscopic studies, to liquid chemical etching processes.

During the analysis of the structure of microcircuits, sequential removal of topological layers is applied. For this, plasma-chemical and liquid etching processes are used, as well as planarization processes of the dielectric surface. Planarization is carried out by the method of abrasive grinding and, in general, gives good results, but there are crystals that cannot be ground without the formation of a wedge, local areas of under-grinding or regrinding. This is due to the fact that the grinding speed of even a homogeneous material depends on the geometry of the surface topography. In areas where the relief is formed only by rare narrow protruding elements, the grinding speed is much higher than in areas where there are many wide elements or the density of the elements is very high. In these cases, the thickness of the residual layer is formed. Without its elimination, further analysis of the IP crystal is impossible, since etching of a dielectric of a nonuniform thickness leads to damage to the underlying layers.

This defect is especially pronounced when processing ICs with copper metallization, which have smaller layer thicknesses than ICs with aluminum metallization. Eliminate or reduce the distortion of the thickness of the dielectric can be local etching of those areas where the layer thickness is greater.

There is a method of local etching of silicon dioxide using a photomask, which consists in applying a layer of photosensitive varnish (photoresist) to the surface of the sample, exposing it to ultraviolet radiation in the right places through a special template that transmits light in some places and does not allow in others, dissolving the exposed or vice versa unlit, depending on the type of photoresist, sites and etching of a layer of silicon dioxide in open areas by liquid or plasma-chemical methods [1]. Its disadvantage is the complexity of implementation and the need to manufacture a special template for each specific case.

There is a method of local etching of non-magnetic materials, in which a magnetic fluid is used as a mask to protect individual areas of the surface from etching, which is held over predetermined areas and moved along the surface by a magnetic field [2]. The disadvantage of this method is the inability to isolate in the magnetic fluid covering the entire surface of the IC chip an open small area where etching should be carried out. This method is only suitable for protection against etching of an island on the surface when the rest is etched. In addition, magnetic fluid does not allow the protection of micron-sized regions, since it is not possible to obtain droplets of such sizes.

Known methods of maskless local etching by exposing the surface to a focused ion beam, which are used, for example, for marking diamonds, preparing samples for transmission electron microscopy, creating dimples on the surface of a sample, or depositing a modified coating [3, 4, 5]. The area of influence and the amount of material removed can be easily set using a computer that controls the beam. The disadvantage of these methods is the need to use sophisticated expensive equipment and low efficiency in operation, since all actions with the sample are carried out in a deep vacuum.

A known method of photolytic gas etching of silicon dioxide, which consists in the fact that the sample is placed in a special gas medium (a mixture of sulfur hexafluoride with argon), which under normal conditions does not cause etching of silicon dioxide, but when the surface of the sample is illuminated with short-wave ultraviolet radiation, etching occurs [6 ]. To ensure the locality of etching, it is sufficient to focus the radiation on a sufficiently small area of the sample in the desired region. The disadvantages of this method include the need to have special equipment: a source of short-wave radiation with a focusing system and an etching chamber, as well as the difficulty of controlling the focus of invisible ultraviolet rays and low etching rates.

A known method of local maskless etching of silicon dioxide in a gas discharge, involving the formation of a discharge directly above those parts of the surface that must be etched [7]. To do this, on the surface of the electrode located above the processed material, a negative image of the desired etching pattern is engraved. In this case, the discharge is localized at the protruding elements of the electrode, and the discharge volume is extremely small. The method allows to etch the pits ranging in size from 0.01 to 100 mm, but to achieve the intended purpose it is of little use. Its disadvantage is the difficulty of forming an electrode with a negative image of the desired etching pattern, which will be different for each particular case. The difficulty arises when trying to accurately align the protruding region of the electrode with the region where you want to etch, since the gap between the protruding part of the electrode and the surface to be treated must be very small.

Closest to the proposed invention, the technical solution is a method for local etching of materials by applying to specified for etching the surface of the polymer granules previously held in the etching composition before swelling [8]. The disadvantage of this method to achieve the stated goal is the difficulty of handling micron-sized granules that need to be removed from the etching composition and placed on a given area of the treated surface. Wet granules stick to the tweezers or spatula with which they are removed from the solution, so it is difficult to place them on a given surface area. In addition, the stock of the etchant in micron granules of micron sizes is small, the process of applying them to the surface will have to be repeated many times to etch the layer of the required thickness.

The objective of the invention is to eliminate the thickness variation of the silicon dioxide layer on the IC chip by local etching of areas of greater thickness.

The problem is solved in a method for local etching of silicon dioxide mainly during micro- and macroscopic studies, including localization of the etching surface and its interaction with the etching composition by placing polymer granules pre-conditioned in the etching composition on predetermined surface areas, characterized in that instead of using polymer granules a pointed rod of porous material, and the etching of a layer of silicon dioxide is carried out only when the layer touches the tip the rod, which is held on the surface subjected to etching. Such a rod can be a thin wooden stick (for example, a match) or a rod of pressed synthetic fibers (for example, the writing unit of a felt-tip pen).

Distinctive features of the invention are the use of a porous rod to feed the etchant into a given region of the sample and etching only when the tip of the rod touches it, which is carried out along the surface to be etched. The core material is the know-how of the method, which provides the best result in microscopic and macroscopic studies.

This set of features ensures the achievement of a technical result, namely, that the use of a pointed porous rod allows to reduce the area in which etching can be performed without affecting other areas where this is not required. This is critical when etching silicon dioxide on the surface of an IC chip, where areas of a few microns in size need to be etched. Etching only in the area where the IP surface touches the porous rod saturated with the etchant, and by conducting it along the IP surface, the etching stops almost immediately after the rod is displaced to another point. At a constant speed of movement of the rod, this provides a constant etching rate, and the thickness of the etched layer is determined by the time the rod was in the contact area and the number of passes of the rod in one place. The constant shift of the rod to a new point during etching allows you to quickly evaluate the residual thickness of the etched layer by changing its color, which greatly simplifies the etching process.

In FIG. 1 shows the result of applying the proposed method, which shows the surface of the IC crystal: a) after carrying out the planarization process by abrasive grinding, b) after local etching of areas with a thicker layer of silicon dioxide. In the memory matrix, the elements are located very densely, so it is polished much more slowly than the elements located next to it. Dark stripes along the contour of the matrix (in fact they are colored), marked with the numbers 1, 2, 3 in FIG. 1a, characterize the degree of heterogeneity of the thickness of the dielectric above it. In FIG. 1b, after local etching, the thickness of SiO ₂ above the matrix is almost the same as above the elements around it.

Concrete example

For local etching, a thin wooden rod, which was a pointed match, was immersed with a sharp end in the etchant for a few minutes - a solution of hydrofluoric acid. The IS crystal was mounted on a microscope stage with a large working length of the lens (MBS-10), which does not invert the image. Since hydrofluoric acid vapors are very volatile and corrode glass, the microscope objective was covered with a thin protective glass, and etching was carried out under a hood. In addition, due to the high volatility of hydrofluoric acid vapors, as a result of which the etchant droplets could condense on neighboring areas of the crystal and cause unwanted etching there, a fan was used to blow around the treatment area and prevent droplets from forming. Excessive etchant was removed with a cloth with a cloth removed from the rod so that no drop would form on the surface of the IC crystal at the point of contact, the size of which is difficult to control. Under the microscope, the tip of the rod was slowly drawn along the surface of the IP crystal, "painting" the desired area. In the microscope, this area is clearly visible, since dielectrics of different thicknesses have different color shades. Since the thickness and area of the removed layer were quite large, several such rods were prepared in advance. The etchant is quickly consumed and evaporates, and its stock in the rod is not large.

The minimum size of the area in which this method of local etching can be applied depends on the degree of sharpening of the porous rod and the size of the pores. For a match it is about 10 microns. For the porous core of a felt-tip pen - 100 microns.

In the examples shown in the prototype, granules with a diameter of 1 mm were used, and the etching area was 2 mm. For etching in areas of several tens of microns in size, granules with micron sizes will have to be used. The complexity of working with such small granules was mentioned above.

Information sources

1. VLSI technology / Ed. S. Zee. T. 1. - M .: Mir, 1986. - S. 357-358.

2. USSR Author's Certificate No. 1516508 A1, publ. 05/23/1989.

3. RF patent No. 2199447, publ. 02/27/2003.

4. European patent No. 1879011, publ. 01/16/2008.

5. RF patent №2328548, publ. 07/10/2008.

6. RF patent No. 2257641, publ. 12/10/2004.

7. Abramov A.V., Abramova E.A., Surovtsev I.S. Etching of materials with a localized gas discharge / A.V. Abramov, E.A. Abramova, I.S. Surovtsev // Letters in ZhTF. 2001. - T. 27. No. 3. - S. 45-48.

8. USSR author's certificate No. 1481267 A1 - prototype, publ. 05/23/1989.

Claims (2)

1. A method of local etching of silicon dioxide, including localization of the etching surface and its interaction with the etching composition, characterized in that the etching of the silicon dioxide layer is carried out only when the layer is touched by the tip of the rod of a porous material saturated with the etchant, which is conducted along the surface to be etched. 2. The method according to p. 1, in which a thin wooden stick, such as a match, or a synthetic fiber compacted material, such as a writing pen of a felt-tip pen, is used as a rod of porous material.

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