JPS6332178B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates generally to lithography, and more particularly to lithography using metal masks and heavy energetic particles.

Photolithography technology is widely used in the field of semiconductor technology. A typical conventional method uses a mask with transparent and opaque areas. This mask is placed on the photoresist layer. Ultraviolet light is used to expose portions of the photosensitive layer located beneath the transparent portions of the mask to change the molecular structure of those portions.
This method has the disadvantage that the mask itself is relatively fragile and the resolution is limited primarily by the diffraction of the ultraviolet radiation. Other methods have been developed that use focused electron beams and X-rays to solve this diffraction problem. Although these methods substantially improve resolution, they require complex equipment, are relatively expensive, and have limited yields.

The purpose of the present invention is to overcome the problems of conventional methods. The gist of the invention is a processing method for forming a pattern in a resist layer, the method comprising the step of arranging on the resist layer a mask having regions opaque to radiation and regions transparent to radiation. The treatment method consists in bombarding the mask with particles having an atomic weight of 1 or more.

A preferred embodiment of the invention uses high energy particles such as alpha particles or protons to pattern a resist layer.
In practicing the invention, a mask consisting of a metal foil and a patterned metal layer attached thereto is placed over the resist layer. A flood beam of high-energy particles is directed toward the mask. While metal foils are transparent to the energetic particles, the combination of metal foil and patterned metal layer is substantially impermeable to those particles. Some of these particles pass through the metal foil and strike the resist layer, changing the molecular structure of those portions of the resist layer. Utilizing changes in the molecular structure of a portion of the resist layer,
Portions of the resist layer are selectively removed to form the desired pattern.

FIG. 1 is a diagram illustrating a preferred embodiment of the method of the present invention. This method uses a stable and durable metal mask. High resolution is achieved because high-energy protons or alpha particles with a small scattering angle are used to expose the photosensitive material (resist). This method is fast for a given resist. This is because a large luminous flux (projection beam) can be obtained over a wide area, and the energy loss rate of particles in the resist is high. The mask is preferably a metal, especially beryllium. Beryllium has good mechanical properties and dissipates heat more easily than some other materials such as Mylar. However, other materials can also be used.

The term "resist" is used herein to mean any material whose molecular structure can change when exposed to radiation.

In FIG. 1, an insulating layer 12 of silicon oxide is formed on the upper surface of a silicon semiconductor substrate 10. As shown in FIG. A layer of photosensitive material (resist) 14 is deposited on the top surface of silicon oxide layer 12.
comprising a permeable region (to alpha particles and protons) and an opaque region consisting of a beryllium foil 16 and a patterned gold layer 18 deposited on the lower surface of the beryllium foil 16; A metal mask 11 is arranged to cover the resist layer 14. A projected beam of high-energy particles such as protons or alpha particles is directed by arrows 20A to 20H.
The upper surface of the beryllium foil 16 is irradiated as shown in FIG. The thickness of the beryllium foil 16 and patterned gold layer 18 is such that particles are substantially blocked by the beryllium foil 16 and patterned gold layer 18 and impinge on a portion of the underlying resist layer 14. It has been selected so that it does not occur. However, the beryllium foil 16 allows particles to pass through the beryllium layer 16 to the underlying resist layer 16.
The resist layer 14 is thin enough to collide with a portion of the resist layer 14 with enough energy to change the molecular structure of the resist layer 14.

When particles impact resist layer 14, the molecular structure of this layer changes. This type of resist layer is called a positive resist layer because the molecular bonds are broken (separated) by the absorbed radiation but there is no further reaction. In addition, the groups formed by the broken bonds may later react with nearby polymer molecules to form cross-links, and these types of resists are classified as having negative properties. Ru. In general, both separation and cross-linking occur simultaneously in resist structures, and their classification (positive or negative characteristics)
The determination is made depending on which mechanism, separation or cross-linking, is dominant. Due to this change in molecular structure, the resist layer 14 can be selectively dissolved to remove predetermined portions of the resist layer 14 and form a desired pattern in the resist layer 14. In addition to selective dissolution, complete depolymerization of materials such as Teflon also allows selective removal by mechanical methods (brushing).

After the resist layer 14 is exposed to light and certain portions are removed by selective dissolution, the remaining portions form a protective mask for the silicon oxide layer 12. The exposed portions of silicon oxide layer 12 are then removed using conventional methods. Additionally, this method can be used to form patterns in other materials;
It can also be used as other photolithography methods commonly used in the semiconductor field.

Here, high-energy particles range from 0.5 to 50 MeV
(mega electron volts). This range is primarily limited by the equipment utilized to generate the particles. This particle range can therefore be expanded by the development of particle generators.

To better understand how the thicknesses of beryllium foil 16 and patterned gold layer 18 are selected, the proton penetration range in gold, carbon, and beryllium is illustrated in FIG.
For example, curve 22 for beryllium is approximately
It shows that protons with an energy of 1 MeV penetrate to a depth of about 14 microns into beryllium. Similarly, curve 26 for gold shows that protons with an energy of 1 MeV penetrate to a depth of about 6 microns of gold. Beryllium and gold were chosen as the curves illustrated in FIG. 2 for understanding the invention because these are the preferred materials for the mask described with respect to FIG. From these curves, one skilled in the art can determine whether the beryllium foil 16 and the gold layer 18 (the first
(Figure) can select the appropriate thickness for. Furthermore, the curve for carbon is shown because the permeation properties of carbon are very similar to those of most commercially available resists used in the semiconductor industry; This is because the characteristic curve for is not available.

Under these circumstances, the carbon curve is similar to that of the Mead Chemical Company.
This includes a general indication of the range of particle penetration in commercially available resists such as Cop, available from Co., Ltd. The thickness of the beryllium foil 16, patterned metal 1 such that the molecular structure of the resist layer 14 changes.
In order to select the thickness of 8 and the energy of the particles, it is necessary to know the characteristics of the resist.

Figure 3 shows α in beryllium, carbon and gold.
3 shows a curve illustrating the penetration range of particles. For example, curve 28 shows that alpha particles with an energy range of about 4 MeV penetrate beryllium to a depth of about 17 microns. curve 32
As exemplified by, alpha particles with an energy of 4 MeV penetrate gold only to a depth of about 7 microns. Using these curves, one skilled in the art can determine whether the beryllium foil 16 and the gold layer 18
An appropriate thickness (FIG. 1) and an appropriate energy level of the alpha particles can be selected. As in the previous example, FIG. 3 shows a curve for carbon as representing the penetration depth of alpha particles in the resist. This was also done for convenience; the curve for carbon is
Although it is easily available, the curve for resist must be obtained experimentally. It is also known that the penetration properties of carbon and most commercially available photoresists are relatively similar. Accordingly, the carbon permeation curve 30 is shown as representative of the permeation curves of commonly commercially available photoresists.

Using the curves illustrated in FIGS. 2 and 3, a suitable thickness for the beryllium layer illustrated in FIG. 1 was found to be about 25.4 microns (1 mil). Similarly, the thickness of the patterned metal is about 1 micron. Furthermore, the energy of the particles (alpha or protons) is chosen to give the desired range of penetration in the resist layer. Using this type of construction, beryllium and gold masks are relatively rigid and easy to handle. This mask is extremely durable and can be used almost permanently. This is a significant improvement over the brittle masks typically used in the ultraviolet photolithography process described above.

Collimated (projection) beams of protons and alpha particles have significantly more mass than electrons, so as they travel through a material, interactions with electrons in atoms do not significantly change their direction. However, after multiple scattering, the degree of deflection with respect to the original direction becomes measurable, setting a lower limit for this type of lithography. In order to determine these limits, explanation will be made with reference to FIG. FIG. 4 is a partial cross-sectional view of a mask consisting of a 25.4 micron (1 mil) thick beryllium foil 16 and 1 micron gold covering a 1 micron thick resist layer. Using this diagram, the angle θ can be estimated based on multiple scattering theory. This angle θ is such that 96% of the particles enter any point on the surface of the beryllium foil 16.
is the angle of the scattered range. As is clear from FIG. 4, this angle θ determines the amount of cut under the metal 18 and the resolution of the method. The width of the cut is indicated by dimension D. Using well-known formulas, the angle θ is approximately 1 degree 32 minutes for protons and approximately 0 degrees 18 minutes for alpha particles. This results in a dimension D of 0.08 microns for protons and a dimension D of 0.016 microns for alpha particles.
These numbers are in contrast to the 1-5 micron resolution obtained using conventional ultraviolet lithography methods.

FIG. 5 shows three curves illustrating the rate of energy loss for alpha particles, protons and electrons as a function of residual range. These curves are important in that the rate of energy loss of the particles determines the rate of change in the molecular structure of the resist. In Figure 5, curves for electrons, protons and alpha particles are included. As is clear from these curves, the highest energy loss rate is for α particles, followed by protons and electrons. It is therefore clear that the rate of energy loss for alpha particles and protons is much higher than that for electrons. Moreover, protons and alpha particles can be easily generated using commercially available equipment. Alpha particles and protons also have high resolution capabilities, as mentioned above.

FIG. 6 shows a curve showing the relationship between resist dissolution rate and proton dosage. As can be seen from this curve, at a dose of approximately 10 ¹³ protons/cm ²
A decomposition rate of 100 A°/sec is obtained. As is clear from this, with this method, not only the resolution can be improved, but also the speed can be very high.

The dissolution rate is also high for the actual dose of alpha particles.

FIG. 7 is a partial diagram showing a test pattern used to illustrate the invention. This portion of the test pattern consists of a group of standard test patterns (6th and 7th) known as "USAF1951". This test pattern confirmed that the method of the present invention could be implemented.

In this demonstration, a proton projection beam and a mask consisting of a Mylar foil and a patterned layer of gold applied thereto were used. This is because masks made entirely of metal as mentioned above were not available.

Although the invention has been described with respect to protons and alpha particles, other particles can also be used.
However, in the field of semiconductor technology, protons and alpha particles are particularly effective in the following respects.
(1) Their mass is sufficient to reduce errors due to scattering to some low value, and low enough to penetrate masks of appropriate thickness at appropriate initial energies. (2) When these particles capture electrons, they become gases (helium and hydrogen) that become part of the semiconductor crystal structure. (3) Crystal damage in semiconductors is limited to dislocations near the surface. These dislocations can be reduced or eliminated using known semiconductor processing methods.

Other possible energetic particles include nitrogen ions and noble gases. However, the use of these particles would require the use of thin, flimsy masks. Oxygen ions may also be used. However, these ions may chemically bond with the semiconductor. Electrons may also be used. However, since the mass per particle is low, scattering increases and resolution deteriorates.

Additionally, a mask can be used with a patterned metal foil (beryllium in the preferred embodiment) adjacent to the resist layer. However, such a method would likely result in reduced resolution since most of the scattering would occur in the metal foil.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing an example of use of the method of the present invention;
Figure 2 is a diagram illustrating the penetration range of protons in beryllium, carbon, and gold; Figure 3 is a diagram illustrating the penetration range of alpha particles in beryllium, carbon, and gold; Figure 4 is a diagram illustrating the resolution of the method of the present invention. figure to do,
Figure 5 is a diagram illustrating energy loss as a function of residual range for electrons, protons and alpha particles;
The figure is a diagram illustrating the dissolution rate of a resist layer, and FIG. 7 is a diagram showing a portion of a mask used when actually carrying out the method of the present invention. DESCRIPTION OF SYMBOLS 10... Semiconductor substrate, 11... Mask, 12... Silicon oxide insulating layer, 14... Resist layer, 16... Beryllium foil, 18... Patterned gold layer, 2
0A to 20H...High-energy particle projection beam,
In addition, in the figures, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Scope of Claims] 1. A processing method for forming a predetermined pattern on a resist layer, the method comprising applying a metal mask comprising a beryllium foil and a patterned gold layer attached to the beryllium foil to the resist layer. and bombarding the mask with particles having a predetermined energy, thereby changing the molecular structure of a portion of the resist layer under the radiation-transparent region of the mask, thereby changing the resist layer portion. 1. A processing method for forming a pattern on a resist layer, the method comprising: a step of selectively removing . 2. The processing method according to claim 1, wherein the particles are high-energy α particles. 3. The processing method according to claim 1, wherein the particles are protons.