JPS633452B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electron beam exposure apparatus that projects a slit image.

Electron beam exposure using a method of projecting a slit image is a technique in which an electron beam is irradiated onto a slit having, for example, a rectangular opening, and an image at the position of the slit is projected onto a wafer. In this method, the current density distribution within the rectangular electron beam is essentially uniform, and the edges around the beam are sharp. In this respect, it can be said that it is a completely different method from the previous method used for circular beams, which project a crossover image and have a Gaussian beam distribution.

However, in reality, the projected image formed by the method of projecting the slit image also exhibits a sloped current density distribution at its edge portions.

Figure 1a shows the planned rectangular projection image, Figure 1b
1 shows the actual current density distribution on the X-axis of the rectangular projection image shown in FIG. 1a. This beam expansion is explained as being due to the Coulomb force between the electrons forming the electron beam (space charge effect). Therefore, the larger the total beam current I,
The rising width d of the current density at the edge portion of the slit projection image increases. This rising width d is more remarkable when the total current I is increased while keeping the slit opening constant, rather than when the slit opening is widened while keeping the current density constant.

If the exposure amount at the edge of the projected image is tilted in this manner, the edges will not be sharply cut when etching the resist. Therefore, the beam current I naturally has an upper limit. The allowable upper limit dmax of the rise width d is
If it is 0.1 μm, Imax is approximately 1 μA. Imax
If is determined, the area of the projected image has a maximum value Smax, which is given by Smax=Imax/ _I0 . I ₀ is the current density of the beam. Normally, Smax will be smaller than 10 μm ² .

In this way, if a practically necessary Smax is determined, I ₀ will become small, and in order to increase I ₀ , it is necessary to suppress Smax. In other words, the limitation of this slit image projection method is that it is necessary to suppress the intensity of the beam when attempting to draw a large projected image.
Incidentally, in the above-mentioned method of projecting a crossover image, the beam has a Gaussian distribution even if a slit is provided in the middle, so the spreading effect due to the Coulomb force can be ignored compared to the extent of the spreading of the Gaussian distribution. It is something.

There is a variable-shaped spot method, which is an application example of the slit image projection method and has recently been attracting attention due to its high-speed drawing.
FIG. 2 illustrates a typical example of a variable beam type optical system.

In FIG. 2, an electron beam emitted from an electron gun 20 is irradiated by an irradiation lens 22 onto a first beam shaping slit 22 having, for example, a square aperture. Using this correct aperture as a virtual source of the beam, a condenser lens 23 projects this square image onto a second beam shaping slit 24 which also has a square aperture. The degree of weight is given by a deflector 25 provided midway, and as a result, the second
The amount and shape of the beam passing through the slit 24 changes. This means that electron beams of various shapes, in this case rectangular beams, are formed here. Furthermore, the beam at the position of the second slit 24 is made a new virtual light source, and its image (slit image) is transferred to the condenser lens 24.
6. Wafer 28 which is a sample is detected by objective lens 27.
projected on top. Selection of the projection position on the wafer 28 is performed by a deflector 29. The shape of the projected image projected onto the wafer 28 is rectangular as shown in FIG. 1a.

In such a variable beam type optical system, the beam spreading effect due to the above-mentioned Coulomb force is even more significant. As described above, in this variable beam method, electron beams of various shapes and beam amounts are formed by controlling the passage of the electron beam between a plurality of slits. Since each shot has a different size of projected image, the rising width of the edge portion of the projected image is different for each shot. Therefore, the difference in size between the planned pattern and the actually etched pattern varies depending on the size of the projected image, resulting in a decrease in pattern accuracy.

``The Coulomb repulsion between beam electrons puts a practical limit on the maximum spot diameter'' is ``Variable''.
Spot Shaping for Electron Beam
Lithography,”14th Sycnp on Electron, Ion
and Photon Beam Technology (Balo Alto),
Scanning Electron Beam Lithography I,
2:10, May 1977.

The present invention has been made in view of the above circumstances, and includes a method in which an electron beam is irradiated onto a slit, an image of the slit position of the electron beam that has passed through the slit is projected onto a sample, and a gas is introduced into the apparatus. When neutralizing the space charge of the electron beam with positive ions generated due to the impact of the electron beam, The electron beam is irradiated by satisfying the relationship expressed by (p: pressure of the gas, α ₀ : ionization coefficient of the gas, V: acceleration voltage of the electron beam, t: one shot exposure time of the sample). An object of the present invention is to provide an electron beam exposure apparatus that can quickly neutralize ions and obtain an exposure pattern with sharp edges.

An embodiment in which the present invention is applied to a variable beam exposure apparatus will be described in detail below with reference to the drawings.

FIG. 3 shows a preferred embodiment of an electron beam exposure apparatus according to the present invention, which accommodates the optical system shown in FIG. It has an electron gun 31 along an electron beam flow path, a liner tube 32 in which optical systems 21 to 27 and 29 shown in FIG. 2 are arranged, and a sample chamber 33 in which a wafer 28 is housed.

In Fig. 3, 34 is an anode that extracts electrons;
5 and 36 are grounded vacuum pumps, which are electrically isolated from the main body by insulators 37 and 38, respectively. Further, the anode 34 may be grounded via the insulator 39.

As mentioned above, the liner tube 32 and the sample chamber 33 are given a negative bias by the variable power supply 40 in order to prevent positive ions from flowing out. 41 is an aperture for preventing scattering. Further, gas is introduced between the gas introduction device 42 and the main body via a flow rate control valve 43.

Now, let us consider the case where gas is introduced into the electron gun 31 or the sample chamber 33. The electron beam emitted from the electron gun 31 collides with this gas to generate positive ions. The position of the electron beam has a lower potential than the surroundings due to the presence of a negative space charge, ie, the electrons that make up the electron beam. Therefore, the positive ions flow into a space where the potential is low and where the electron beam exists.

The light then flows along the beam path to the cathode, that is, the electron gun 31, which has a lower potential. An equilibrium distribution occurs when the generation of positive ions and their outflow to the cathode space are balanced. At this time, the density distribution of positive ions is determined by the slope of the potential in the direction of the beam flow path (electron gun -
(between samples), it is high near the wafer 28 and low near the cathode.

Now, if ion neutralization progresses, the potential in the direction across the electron beam is flattened by the presence of positive ions in the electron beam, and the Coulomb repulsion between the electrons that make up the electron beam is suppressed, and the beam The spreading effect disappears.

As mentioned above, the density distribution of positive ions has a maximum near the sample surface, so when gas is introduced into the apparatus, ions are first neutralized near the sample surface as the amount of gas introduced increases.
The pressure of the gas inside the device at this time is defined as critical pressure Pc. Therefore, the gas pressure required to neutralize the electron beam in the liner tube is
It will be more expensive than PC. This critical pressure Pc is F−S−
According to the H model, 100pipcd=0.86φ ^1/2 (M _H /M) ^1/2
It is given by (I ₀ /V ₀ ^1/2 ) ^1/2 . Here, M _H is the mass of the hydrogen atom, M is the mass of the gas molecule, and V ₀ is the acceleration voltage I ₀
is the beam current, Pi is the number of positive ions generated while one electron travels 1 cm at 0°C and 1 Torr, and d is the beam path length.

Further, it is given by φ=1/2+ln(a/b), where a is the radius of the liner tube and b is the radius of the electron beam. Regarding this F-S-H model, M.
E. Hlines G.W. Hobbmar and J.A.
Salon：Positiul−ion drainage in magnetically
focused electvon beans, J. Appl phys 26
(1955) 1157. This is a model when the beam path is stationary and steady.

However, in the case of electron beam light, the beam is spatially moved (scanned) or at least exposed intermittently. Therefore, when neutralizing ions, it is necessary to neutralize the ions quickly within a time equal to or less than at least the exposure time (time during which a beam is continuously irradiated onto one point on the sample surface) t. In this case, the gas pressure P is (P is the pressure of the gas, α ₀ is the ionization coefficient of the gas, and V is the acceleration voltage of the electron beam) It is necessary to satisfy the following relationship.

According to the inventors, acceleration voltage 20KV beam current
If the beam was 10μA, the length of the liner tube was 0.1m, and air was used, then the beam would be Pc10 ^-5 Torr when the beam was stationary, and ion neutralization would proceed if the pressure inside the liner tube was set to about 10 ^-4 Torr. On the other hand, the time from when the beam is turned on until ions are created and ion neutralization progresses is based on the air pressure inside the liner tube, P = 10 ^-2 Torr.
It is about 20nsec and is inversely proportional to the air pressure. The on-off rise and fall times of normal electron beam exposure equipment are about 20 nsec, and the shot time for one pattern is about 1000 nsec to 1 μsec, so at approximately P = 10 ^-2 Torr, ion neutralization progresses sufficiently. be.

Note that although positive ions are generated by introducing the gas, the generated ions flow toward the cathode along the electron beam path, shortening the life of the cathode. To deal with this, the degree of vacuum in the electron gun chamber is increased to, for example, 10 ^-6 Torr or higher, and the potential in the liner tube space is lower than that of the anode.
For example, it may be lowered by several +V. Positive ions are trapped in the liner tube, making it difficult for them to flow toward the cathode, and because positive ions are less likely to be lost, less gas is required to be introduced into the device.

On the other hand, considering the mean free path of electrons, the degree of vacuum in the liner tube is preferably better than 10 ^-4 Torr.

By the way, in the normal slit image projection method, the beam width is successively reduced and projected. For example, in the optical system shown in FIG. 2, the space where the beam is narrowed most narrowly extends from the objective lens 27 to the wafer 28. The beam spreading effect between the two is the largest. Therefore, near the sample surface
If the vacuum level is between 10 ^-2 and 10 ^-3 Torr, and the vacuum inside the liner tube is better than 10 ^-4 Torr, the beam spreading effect can be effectively suppressed without impairing the function of the exposure equipment. .

The mass of the introduced gas should be small so that the positive ions quickly neutralize the space charge of the electron beam. It goes without saying that the gas may be not only air but also general gases such as hydrogen, rare gases, nitrogen, metals, and oxygen. Among these, hydrogen is preferred, and if a rare gas is used, no reaction will occur within the device.

In a method of projecting a slit image, such as a variable beam method, a problem arises in that the slits 22 and 24 are charged. This is because organic substances such as hydrocarbons, which are decomposed from resists floating inside the device, are ionized by the impact of the electron beam, and the positive ions move into the electron beam, burning the hydrocarbons into the slits. , and the slit becomes electrically charged. Therefore, the electron beam passing through the slit causes positional fluctuations and astigmatism. In contrast,
If positive ions other than hydrocarbons, such as hydrogen ions, are predominant in the apparatus, most of the positive ions present in the beam will be hydrogen ions, so the above-mentioned charging phenomenon will be reduced. If metal ions are dominant, the conductivity of the slit will not be lost and charging can be prevented.

[Brief explanation of the drawing]

Figures 1a and b are diagrams for explaining the rectangular pattern created by the conventional slit image projection method and its current density distribution, Figure 2 is a diagram for explaining the optical system of the general variable beam system, FIG. 3 is a diagram for explaining one embodiment of the electron beam exposure apparatus of the present invention. 20... Electron gun, 22... Wafer, 34... Anode, 32... Liner tube, 33... Sample chamber, 35, 36... Vacuum pump, 37, 38, 3
9... Insulator, 40... Variable power supply, 41... Anti-scattering aperture, 42... Gas introduction device, 43...
…valve.

Claims (1)

[Claims] 1. An electron beam is irradiated onto a slit, an image of the slit position of the electron beam that has passed through the slit is projected onto a sample, and a gas is introduced into the device to absorb the impact of the electron beam. In ion neutralizing the space charge of the electron beam with the positive ions invented accordingly, (p: pressure of the gas, α ₀ : ionization coefficient of the gas, V: acceleration voltage of the electron beam, t: one shot exposure time of the sample). 2. The electron beam exposure apparatus according to claim 1, wherein the liner tube and the sample chamber are maintained at a lower potential than a vacuum pump and an anode for adjusting the degree of vacuum within the apparatus. 3 The electron gun chamber should be kept at 10 ^-6 Torr or more, the liner tube should be kept at 10 ^-4 Torr or more, and the sample area should be kept at about 10 ^{-3 or} more.
The electron beam exposure apparatus according to claim 1, which has a degree of vacuum of 10 ^-2 Torr.