JPH0472375B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a fine pattern forming method using a charged particle beam, that is, a charged particle beam lithography method.

(Prior Art) A conventional pattern forming method has been based on a mask original pattern transfer technique using an optical exposure mask. However, as pattern dimensions became finer to the extent that they approached the wavelength of the light used for exposure, forming patterns using this method reached its theoretical limits.

In recent years, charged particle beam irradiation technology has come to be used as a technology for forming finer patterns than conventional optical exposure technology. Charged particle beam irradiation technology has also been improved, from a method in which a charged particle beam with a round cross section narrowed to about 0.5 microns is used to fill in a pattern in one stroke, to a method in which a cross section of several microns to several tens of microns formed into a rectangular shape is used. Methods using charged particle beams with specific dimensions have come into widespread use, and the time required to fill in patterns has been significantly shortened.

A specific example of the conventional drawing method will be described below. The figure is divided into irradiation regions of several tens of nanometers to several tens of micrometers in size as shown in Figure 1a, and charged particle beams with the same cross-sectional dimensions are used to create a charged particle beam layer in each irradiation region. Each irradiation area is sequentially irradiated with a charged particle beam until the irradiated area and the non-irradiated area have a sufficiently large dissolution rate ratio with respect to the developer (hereinafter referred to as appropriate irradiation time). That is, the first irradiation area 1 is first irradiated for an appropriate irradiation time, and then the second irradiation area 2 is irradiated, as shown by arrow 3, to draw a figure.

(Problem to be solved by the invention) However, when trying to form a pattern with a charged particle beam with a large current amount because it has a large cross-sectional dimension of several tens of microns, The energy of the irradiated particle beam, which did not pose a problem when particle beams were used, began to produce unfavorable results in pattern formation. The substrate is locally heated by the charged particle beam incident on the substrate, and the temperature of the substrate surface within the irradiation area and the charged particle beam layer in contact with the substrate rises as shown in FIG. 1b. because,
The rise in temperature of the substrate and the charged particle beam layer formed on the substrate is due to the fact that the energy of the incident charged particles is converted into heat in the charged particle beam layer and the substrate, and heat is diffused in the sensitive particle beam layer and the substrate. Since it is determined by the balance between irradiation energy and thermal diffusion, in the case of a charged particle beam incident with a large cross-sectional shape, the temperature rise at the center of the charged particle beam layer and the substrate irradiated by the charged particle beam is This is because the size increases in proportion to the cross-sectional size of the charged particle beam. Furthermore, this geometrical distribution of temperature rise has the effect of significantly smoothing out the irradiated charged particle pattern, which has a sharp rectangular cross-section. Usually, the charged particle beam layer is made of a polymeric material and thermally decomposes at several hundred degrees Celsius. A thermochemical reaction occurs even if it does not result in thermal decomposition. Normally, when patterning a charged particle beam layer with a charged particle beam, an incident charged particle beam of several keV to several tens of keV is inelastically scattered in the charged particle beam layer and the substrate. The region where energy of several eV is distributed is considered as the pattern formation region. The shape of the region where this energy of several eV is distributed almost faithfully reproduces the sharp rectangular cross-sectional shape of the irradiated charged particle beam, which is why charged particle beam lithography technology is used for forming fine patterns. . However, as mentioned above, the blurred pattern distribution of several hundred degrees Celsius is superimposed on the sharp distribution of several eV, resulting in only a blurred pattern being obtained, resulting in a significant deterioration of the resolution of the charged particle beam layer.

To make the explanation more concrete, A shown in Figure 2
The temperature rise at a point is expressed by the following equation (1).

T (P, Z ₁ , Z ₂ , R, t ₁ , C, ρ, D) = 2P/Cρ∫ ^Z2 _Z1 ∫ ^R ₀ ∫ ^t1 ₀ 2пr・(4пDt) ^-3/2・exp[−(r ² +z ² )/4Dt] dt・dr・dz
...(1) However, T (degrees) is the temperature rise.

P (W/cm ³ ) is the amount of heat generated per unit time and unit volume, and the heating area is from the depth Z ₁ (cm) from the substrate surface.
Area of radius R (cm) up to Z ₂ (cm).

t ₁ (sec) is the irradiation time.

D (cm ² /sec) is the thermal conductivity of the substrate, which is the specific heat C
Thermal diffusivity divided by the product of (Joule/(g・deg)) and density ρ (g/cm ³ ).

Z ₁ (cm) is stated in the explanation of P.

Z ₂ (cm) is stated in the explanation of P.

R (cm) is stated in the explanation of P.

C (Joule/(g・deg)) is described in the explanation of D.

ρ (g/cm ³ ) is described in the explanation of D.

Changes in the irradiated charged particle dose due to the temperature rise in the charged particle beam layer that result in a residual film rate of 50% in the residual film rate curve after development processing of the charged particle beam layer due to a temperature rise during the appropriate irradiation time. When the value exceeds 15% (hereinafter referred to as thermal alteration of the charged particle beam), it was not possible to obtain the desired shape with high accuracy.

On the other hand, if this problem is solved by reducing the amount of charged particle beam irradiation per unit time or unit area, or by reducing the cross-sectional dimensions of the charged particle beam, the pattern drawing time will be significantly reduced. The advantage will be lost.

At this point, the advantage of increasing the cross-sectional dimensions of the charged particle beam in order to significantly shorten the pattern writing time no longer satisfies the requirement to obtain fine patterns with high precision, and on the contrary, it has become a hindrance. It's on.

In order to make the content of the explanation more clear, a case will be explained in which an electron beam, which is widely used today, is used as a charged particle beam. This is for illustrative purposes only and is not intended to limit the invention.

In the case of a monodisperse polystyrene electron beam sensitive resist, an example will be shown below to show how the resist pattern shape and resist sensitivity differ depending on whether or not the resist temperature is increased by electron beam irradiation. FIG. 5a is a cross-sectional photograph of a pattern of monodisperse polystyrene on a Si substrate after development.
The interval between the marker lines at the bottom of the photo is 0.5 μm. Figure 5b shows the Si substrate in Figure 5a using SiO ₂
I just changed it to a board. Other conditions, i.e.
The electron beam irradiation conditions, development processing conditions, etc. are all the same. Figures 5a and 5b are 12.5μm×
This is a pattern obtained when irradiated with a 12.5 μm rectangular electron beam. The protruding film at the lower right of FIG. 5b was produced when the substrate and resist were cut to obtain a cross-sectional photograph, and is not relevant to this explanation. The portion relevant to this explanation is the depressed portion of the resist caused by electron beam irradiation and development processing, which is located in the center of FIGS. 5a and 5b.
For the SiO ₂ substrate, the thermal diffusivity of the substrate is two orders of magnitude lower than that of the Si substrate, so the accelerated energy of the irradiated electron beam cannot be rapidly thermally diffused, and the monodisperse polystyrene layer is heated, reducing the sensitivity. Changes to high sensitivity side,
The cross-sectional shape of the pattern deteriorated. Si substrate and _SiO2
FIG. 6 shows how much the sensitivity of the monodisperse polystyrene layer to the electron beam differs depending on the substrate. Monodisperse polystyrene is a negative resist. That is, the higher the electron irradiation dose, the higher the normalized residual film rate after development. The normalized residual film ratio is the film thickness after development divided by the film thickness before electron beam irradiation. In FIG. 6, curve 20 is a sensitivity curve of monodisperse polystyrene provided on a SiO ₂ substrate, and curve 21 is a sensitivity curve of monodisperse polystyrene provided on a Si substrate. The sensitivity on _SiO2 substrate is
Compared to that on the substrate, the electron beam dose has changed to 15% lower. The electron beam irradiation conditions and development conditions in FIG. 6 are the same as those for obtaining FIG. 5. As shown in Figures 5 and 6, the pattern shape and sensitivity differ depending on the substrate material.
This has become apparent as electron beam patterns have been drawn under conditions of large current due to the large electron beam cross-sectional dimensions of 10 μm or more, and in the case of conventional electron beam cross-sectional dimensions of 1 μm or less There were no problems as long as a normal electron beam pattern drawing device was used.

Now, in FIG. 5b, more striped resist remains between the resist patterns than in FIG. 5a, and it is impossible to etch the substrate using the resist pattern as a mask. As mentioned in the explanation of Figure 6, this means that if the irradiation electron beam dose with a residual film rate of 50% changes by 15%, it will become impossible to form a resist pattern as shown in Figure 5b. ing. The resist pattern shown in FIG. 5, that is, the design dimension of the distance between the resist layers is 1.5 μm. Even considering the case of pattern formation of 2 μm to 3 μm, if the irradiated electron beam dose changes by about 20% with a 50% film remaining rate, the fifth
It is thought that pattern formation becomes impossible, similar to b in the figure.

Although the cases of Si substrate and SiO ₂ substrate are compared in FIG. 5 and FIG. 6, this is not a comparison of the difference in the material of the substrate, but an investigation of the degree of resist pattern formation due to temperature. In other words, when a good pattern can be formed on a Si substrate with good thermal diffusion, we conducted an experiment to see how much temperature rise would cause defects in resist pattern formation on a SiO ₂ substrate with poor thermal diffusion. Furthermore, a simulation for specifically examining the value of the temperature rise will be described next.

Next, the Si substrate described in Figures 5 and 6
Let's consider how much temperature change occurred in the resist layer compared to the SiO ₂ substrate. The distribution of the electron beam incident on the resist layer and the underlying substrate has been elucidated by TEE Everhart and others. (TE Everhart and
PHHoff, J. Appl. Phys. 42, 5837 (1971)) By solving the heat diffusion equation, the heat distribution in the resist layer and substrate can be determined.
The temperature increase distribution can be determined analytically or by computer simulation.

To make the explanation more concrete, A shown in Figure 2
The temperature rise at a point is expressed by equation (1).

Let's simulate how much the temperature of the electron beam sensitive layer increases due to the irradiation electron beam according to this calculation formula. Usually, an electron beam-sensitive layer with a thickness of about 1 micron is provided on a substrate such as silicon or glass, and an acceleration energy of about 20 keV and 0.4 A /
A typical case is irradiation with an electron beam with a current density of about cm ² . (For example, JBX-6 variable rectangular electron beam lithography system manufactured by JEOL Ltd.) For convenience of calculation, a circularly shaped electron beam with a width of 14 μm in diameter was used.
Figure 7 shows an example of irradiation under the conditions of 20 keV and 0.4 A/cm ² . The acceleration energy of the irradiated electron beam is
In the case of 20keV, most of the irradiated electron beam passes through the electron beam-sensitive layer made of polymer and becomes heat in the substrate, so that the electron beam rises above the substrate surface, which is the contact surface between the electron beam-sensitive layer and the substrate. It showed the temperature. It is also the temperature rise at the center of the shaping electron beam where the temperature rise is highest. For comparison, when the substrate is silicon with a thermal diffusivity of 8.7×10 ^-1 cm ² /sec23 and 8.1×10 ^-1
22 was shown when the substrate was glass with cm ² /sec.
The surface of the glass substrate shows a temperature increase of about 300 degrees by continuous electron beam irradiation for 500 μsec. When the optical transfer mask is a substrate, a metal chromium layer of about 800 Å is provided on the glass, but the film is so thin that the metal chromium layer only has an effect on reducing the temperature rise by a few percent. Therefore, when patterning a normal optical transfer mask using an electron beam, the case 22 of the glass substrate shown in FIG. 7 corresponds.

Next, FIG. 8 shows the limiting conditions for combinations of irradiation current density, rectangular width, and irradiation time when the allowable temperature increase is 200 degrees. The horizontal axis is the radius of the irradiation area and the vertical axis is the irradiation time using the irradiation current density of 24 in the case of 0.4 A/cm ² and 25 in the case of 4 A/cm ² as parameters. The meaning of the curves 25 and 26 shown in FIG. 8 is that the upper right side of the curves 25 and 26 is a temperature increase region of 200 degrees or more. i.e.
In the case of an irradiation current density of 0.4 A/cm ² and an electron beam rectangular width of 10 μm, it is shown that unless the electron beam irradiation time is 150 μsec or less, the substrate surface temperature will rise by 200 degrees or more. Figure 8 shows the case of a glass substrate, and in the case of a silicon substrate, as shown in Figure 7, the temperature rise is one order of magnitude lower than that of a glass substrate.
The 200 degree limit curve generally shifts to the upper right. 8th
The figure shows an example clearly showing that the method of the present invention is required when using an electron beam.

It is an object of the present invention to eliminate the above-mentioned conventional drawbacks and to provide a method for writing patterns without increasing pattern drawing time.
Another object of the present invention is to suppress the temperature rise of a charged particle beam layer and provide a charged particle beam lithography method that has never existed before.

(Means for Solving the Problems) In the present invention, first, the continuous irradiation time t is the limit at which the desired shape cannot be obtained with high accuracy due to the temperature rise of the charged particle beam layer accompanying the irradiation of the charged particle beam. ₀ is determined in advance using the substrate material, charged particle beam material, irradiation acceleration voltage, charged particle beam current density, and area as parameters. Next, as in the conventional Figure 1a, the figure is divided into irradiation areas of several tens of nanometers to several tens of micrometers in size, as shown in Figure 3a, and a charged particle beam of the above-mentioned size is irradiated. However, if two or more irradiation areas with a distance of 4√ ₀ (cm) between each irradiation area are defined as a set, first, the time t during which no other alteration occurs in the first irradiation area 5 of the first set 4. Irradiate the charged particle beam for ₀ seconds, then move to the next irradiation area 6 of the same group 4, irradiate it with the charged particle beam, and then move to the next irradiation area 7 of the same group 4 after t ₀ seconds. The steps are continued sequentially as shown by arrow 8 in FIG. 3, and all the irradiation areas in the first set 4 are irradiated. While the other irradiation areas are being irradiated in this manner, the first irradiation area 5 is cooled by heat conduction within the substrate. If the time during which the other irradiation areas are irradiated is longer than the time during which the first irradiation area 5 is first irradiated, the temperature of the substrate surface in the first irradiation area 5 has returned to the temperature before irradiation. Therefore, irradiation of the first irradiation area 5 is started again as indicated by the arrow 9, and the charged particle beam is irradiated for _t0 seconds. After this, irradiation of other irradiation areas in the same group is performed as shown by arrow 10. At this time,
In other irradiation areas as well, the temperature of the substrate surface returns to the temperature before irradiation. This procedure is continued until the total irradiation time in each irradiation area reaches the appropriate irradiation time. t If the proper exposure time is achieved in a time shorter than ₀ seconds,
No further irradiation is performed.

In this way, after the first set of irradiation is completed,
Move to the second set 11 and continue the same procedure as the first set.

FIG. 4 shows the procedure described above.

It should be noted that the other deterioration of the charged particle beam layer mentioned here refers to the residual film rate curve after development of the charged particle beam layer mentioned in the section of the problem to be solved by the invention. The fluctuation value due to the temperature rise in the charged particle beam layer of the irradiated charged particle dose is 15% when the rate is 50%.
If it exceeds , it means that the desired graphic shape could not be obtained with high accuracy.

(Function) The temperature changes of the substrate surface and the charged particle beam layer set therein in each irradiation area at this time are shown in Figure 3b.
become that way. At temperature rise 12, the charged particle beam irradiation time is always t ₀ seconds or less than t ₀ seconds,
The substrate surface temperature does not exceed the thermal alteration temperature of the charged particle beam layer, and no thermal alteration of the charged particle beam layer occurs. Also,
When the temperature drops, the substrate surface returns to its original temperature at 13.
Next, even when the temperature rises, the charged particle beam irradiation time is
t is ₀ seconds or less than t ₀ seconds, and the substrate surface temperature does not exceed the thermal alteration temperature of the charged particle beam layer.

To make the explanation more concrete, A shown in Figure 2
The temperature rise at a point is expressed by equation (1). Also,
As a result of the temperature drop after the charged particle beam irradiation is cut off, the temperature difference from the initial temperature before irradiation becomes a value expressed by equation (2).

T (P, Z ₁ , Z ₂ , R, t ₁ , t ₂ , C, ρ, D) = 2P/Cρ∫ ^Z2 _Z1 ∫ ^R ₀ ∫ ^t1+t2 _t2 2пr・(4пDt) ^-3
/2・exp[−(r ² +z ² )/4Dt]dt・dr・dz
...(2) However, T (degrees) is the temperature rise.

P (W/cm ³ ) is the heat generation amount per unit time and unit volume during irradiation, and the heat generation area is an area with radius R (cm) from depth Z ₁ (cm) to Z ₂ (cm) from the substrate surface. .

t ₁ (sec) is the irradiation time.

t ₂ (sec) is the elapsed time after irradiation cutoff.

D (cm ² /sec) is the thermal conductivity of the substrate, which is the specific heat C
Thermal diffusivity divided by the product of (Joule/(g・deg)) and density ρ (g/cm ³ ).

Z ₁ (cm) is stated in the explanation of P.

Z ₂ (cm) is stated in the explanation of P.

R (cm) is stated in the explanation of P.

C (Joule/(g・deg)) is described in the explanation of D.

ρ (g/cm ³ ) is described in the explanation of D.

(Example) Hereinafter, an example of the present invention will be described in parallel with a conventional method.

80 nm of Cr is deposited on the fused silica substrate, and 300 nm of electron beam sensitive resin PMMA is applied on top of this. Since Cr and PMMA are thin, they do not contribute to heat conduction, and their thermal conductivity is determined by the thermal conductivity characteristics of the fused silica substrate (thermal diffusivity D =
8.1×10 ^-3 cm ² /sec). acceleration voltage
Two 10 μm square shapes separated by 100 μm were drawn using an electron beam of 20 keV, current density of 400 mA/cm ² , and size of 10 μm square.

As shown in Figure 9b, figure A was irradiated for 90 μsec, then figure B was irradiated for 90 μsec, which was repeated 5 times, and finally figure A was irradiated for 50 μsec and figure B was irradiated for 50 μsec. As a result, there was no abnormality due to thermal alteration. You can draw shapes in
It was 10 μsec and 2%. This 10 μsec increase is due to the stabilization time required to set the electron beam at the irradiation position and before commencing irradiation, which will become shorter as electronic circuits advance.

On the other hand, when each figure is sequentially irradiated with the appropriate irradiation time of 500 μsec for PMMA as shown in Figure 9a using the conventional method, thermal alteration occurs in both figures 100 μsec after the start of irradiation, and the desired figure shape cannot be obtained. I couldn't. In order to avoid thermal deterioration, the desired shape could be obtained when the current density was reduced to 1/2, but the time required for drawing was doubled to 2 msec.

(Effects of the Invention) Therefore, according to the present invention, a figure free from abnormalities due to thermal alteration can be drawn without increasing the drawing time due to a reduction in current density to avoid thermal alteration of the charged particle beam layer. You can get the desired effect.

In the above description, especially in the description of the examples, specific charged particle beams, charged particle beam layers and their film thicknesses, specific substrates, and specific exposure conditions have been described, but other charged particle beams, e.g. _{The charged} particle By appropriately setting the continuous irradiation time _t0 , which is the limit at which the desired shape cannot be obtained with high accuracy due to the temperature rise of the line layer, it is possible to prevent other charged particle beam layers such as AZ, P
The present invention can also be applied to (MMA-MA) etc. by changing the number of repetitions to obtain an appropriate irradiation time. Further, in the description, each irradiation time is set to t ₀ , but the present invention can also be applied by setting each irradiation time to an arbitrary time equal to or less than t ₀ as long as the number of repetitions is not increased.

[Brief explanation of drawings]

FIG. 1a is a diagram showing an example of a conventional graphic drawing method. FIG. 1b is a diagram showing the temperature rise on the substrate surface in each irradiation area in the conventional graphic drawing method. Figure 2 is a schematic diagram for explaining Equations (1) and (2). To make the explanation more concrete, it is approximated by a cylindrical coordinate system, and from the surface with a radius R indicated by diagonal lines in the figure. depth Z ₁ and
The heat distribution is limited within the range of Z ₂ . Point A at the center of the heat generation distribution in the figure (that is, the center of the irradiation area) is taken as a representative point. FIG. 3a is a diagram showing an embodiment of the graphic drawing method of the present invention. FIG. 3b is a diagram showing the temperature rise on the substrate surface in each irradiation area in the graphic drawing method of the present invention. FIG. 4 is a flowchart showing the implementation procedure of the present invention. FIGS. 5a and 5b show the surface of the charged particle beam layer and This is a scanning electron micrograph of a cross section. FIG. 5a shows a case where the device is not affected by heat much, and FIG. 5b shows a case where it is affected by a large amount of heat. FIG. 6 is a diagram showing the sensitivity curve of monodisperse polystyrene to electron beams. FIG. 7 is a diagram showing the temperature rise on the substrate surface due to electron beam irradiation. FIG. 8 is a diagram showing combination limiting conditions of irradiation current density, radius of irradiation area, and irradiation time in electron beam irradiation. FIG. 9a is a diagram showing an example of a conventional graphic drawing method. FIG. 9b is a diagram showing an embodiment of the graphic drawing method of the present invention. In the figure, each symbol indicates the following.
1...First irradiation area, 2...Second irradiation area, 3...Irradiation order, 4...First set, 5...First irradiation area, 6...
...Second irradiation area, 7...Third irradiation area, 8...Irradiation order, 9...Irradiation order, 10...Irradiation order, 11...
...Second set, 12...Temperature rise during charged particle beam irradiation, 13...Temperature fall when charged particle beam is not irradiated, 2
0...Sensitivity curve of monodisperse polystyrene on a glass substrate, 21...Sensitivity curve of monodisperse polystyrene on a silicon substrate, 22...Temperature rise when electron beam is irradiated on a glass substrate, 23...Silicon substrate Temperature rise when electron beam is irradiated on the top, 24...
...Combination of irradiation time and radius of irradiation area such that the substrate surface temperature rise is 200 degrees when the irradiation current density is 0.4A/ ^cm2 , 25...Irradiation current density
A combination of the irradiation time and the radius of the irradiation area such that the substrate surface temperature rise is 200 degrees in the case of 4A/cm ² , A...The intersection of the extension line of the center of the heat generation distribution and the substrate surface, Z ₁ ... ...distance of heat distribution from the substrate surface,
Z ₂ ... Distance of heat distribution from the substrate surface, R ... Radius of heat generation distribution.

Claims (1)

[Claims] 1 In a charged particle beam lithography method in which a charged particle beam is irradiated onto a charged particle beam layer on a substrate to draw a figure, the desired figure is drawn by the temperature rise of the charged particle beam layer accompanying the irradiation of the charged particle beam. The continuous irradiation time _t0 , which is the limit at which the shape cannot be obtained with high accuracy, is determined in advance using the substrate material, charged particle beam material, irradiation acceleration voltage, current density of the charged particle beam, and area of the charged particle beam as parameters, and then drawn. Dozens of power figures
The irradiation area is divided into irradiation areas ranging in size from nm to several tens of μm, and the distance between each irradiation area is 4√ ₀ cm (D (cm ² /sec) is the specific heat C (Joule/(g・deg)), which is the thermal conductivity of the substrate. )) and the density ρ (g/cm ³ ), t ₀
(sec) is the continuous irradiation time in each irradiation area) At least two irradiation areas separated from each other are set as a set, and charged particles are first applied to the first irradiation area of one set of irradiation areas for a time of t ₀ seconds or less. The procedure is to irradiate the charged particle beam, then move the charged particle beam to the next irradiation area in the same group and irradiate it, and then move it to other irradiation areas in the same group and irradiate it so that the total irradiation dose for each irradiation area is appropriate. A charged particle beam lithography method characterized by continuing until the exposure dose is reached, then transferring the charged particle beam to the next group and performing the same procedure, and irradiating the irradiation areas of all other groups in the same way.