JPH05259064A - Substrate heating mehtod and device - Google Patents

Abstract

PURPOSE:To shorten a baking time after light exposure, to restrain acid produced by the irradiation with an energy beam from diffusing, and to enhance a pattern in dimensional controllability. CONSTITUTION:A substrate 3 is mounted on a support pad 4 after pattern irradiation and cooled down by a cooler 5. Resist 6 on the substrate 3 is irradiated with an energy beam 2, exposed to light, and baked. By this setup, a substrate can be shortened in baking time, acid produced by the irradiation with an energy beam can be restrained from diffusing, and a pattern can be enhanced in dimensional controllability. Moreover, a substrate can be enhanced in productivity owing to a decrease in baking time.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a substrate heating apparatus for heating a substrate on which a thin film such as a resist is deposited, and more particularly to a substrate heating apparatus for shortening the heating time.

[0002]

2. Description of the Related Art The method currently used in microfabrication of semiconductors follows the following procedure. First, an organic or inorganic thin film called a resist is deposited on a semiconductor substrate to be processed by a method such as spin coating. After depositing the resist, a heat treatment (hereinafter referred to as baking) is generally performed in order to scatter the solvent in the resist. This baking is performed by allowing the substrate to stand for a certain period of time on a hot plate set to a constant temperature, and also plays a role of enhancing the adhesiveness of the resist to the substrate. Then, after irradiating an energy ray such as an ultraviolet ray or an electron beam according to a desired pattern, the semiconductor substrate is dipped in a developing solution. In this development process, a difference in dissolution rate in the developing solution occurs between the pattern-irradiated portion and the pattern-unirradiated portion, so that a desired pattern can be formed.

With the progress of resist materials, the above-mentioned base
In addition to black, materials that require baking after energy beam irradiation have appeared. For example, Journal of Vacuum Science and Technology, Vol. 6, 3
79-383 (1988) (J. Vac. Sci. Tec
hnolo. B6 , 379-383 (1988).), there is a high-sensitivity resist called "chemically amplified resist". In the case of a negative type resist, it is composed of a novolac resin, an acid generating substance, and a cross-linking substance which constitute the resist skeleton.
When an energy beam such as an electron beam is irradiated, an acid (hydrogen ion) is generated from the acid generating substance contained in the irradiation part, and this acid acts as a catalyst during the baking treatment described later to form a crosslinking substance. By acting, the crosslinking reaction of the novolac resin proceeds. As a result, the novolac resin in the energy-irradiated portion is polymerized, the solubility in the developing solution is significantly lowered, and the pattern remains after development. As a result, a negative pattern is obtained.

In the case of a positive type resist, a dissolution inhibiting substance is contained in place of the cross-linking substance, and the dissolution of the novolak resin in the developing solution is suppressed. The acid generated in the energy ray irradiation part acts on the dissolution inhibiting substance during the baking treatment described later, and acts as a catalyst for reducing the dissolution inhibiting effect by the decomposition reaction of the dissolution inhibiting substance. As a result, the peripheral novolac resin is dissolved in the developing solution, and a positive pattern is obtained.

In the conventional resist, the energy is directly applied to the cross-linking substance or the dissolution inhibiting substance and the reaction proceeds, but in the chemically amplified resist, the energy is imparted to the acid generating substance. A pattern can be obtained if an acid is generated and the catalytic reaction proceeds. It is generally known that an acid is generated from an acid generating substance by applying low energy. Therefore, it can be generally said that the chemically amplified resist has high possibility of high sensitivity. Here, a feature of many chemically amplified resists is that the baking process is required as described above for the purpose of providing energy for promoting a catalytic reaction by an acid. This bake (hereinafter,
The baking after the exposure to the energy beam is referred to as the baking after the exposure) is performed by, for example, leaving a substrate coated with a resist on a hot plate heated at 100 ° C. for 5 minutes. It was a thing. Here, the baking temperature and the time are different depending on the resist.

This post-exposure bake is not required in the conventional resist, and if a chemically amplified resist is used, a new process is required in the resist process. Therefore, from the viewpoint of improving productivity, it is desirable to shorten this processing time as much as possible. However, when the post-exposure bake is performed by a hot plate as described above, the temperature of the resist rises slowly because the substrate in contact with the hot plate is first heated and then the resist on the substrate is heated. ..
Further, since the heat energy applied to the resist from the hot plate is dissipated from the upper surface of the resist, a long heating time is required on the hot plate in order to allow the catalytic reaction in the resist to proceed sufficiently. For this, hot play
However, there is a limit in shortening the baking time after coating.

The inventors have found the following points regarding the chemically amplified resist. As shown in FIG. 2A, the substrate 10
A latent image 13 of the pattern is formed on the resist 11 applied on the surface by irradiating the energy beam 12. Acid 14 is generated in the area of the latent image 13 of this pattern. The catalytic reaction proceeds during the above-mentioned post-exposure bake, but as shown in FIG. 2B, the generated acid 14 is an effective catalytic reaction due to thermal diffusion in the lateral direction of the pattern. The area expands. As a result, the pattern formed after development spreads in comparison with a desired size, and in the case of a negative type, the shape spreads and deteriorates from the latent image as shown in FIG. 2C. It was found that this spread increased as the baking time after exposure increased. The length of diffusion differs depending on the resist, but for example, SAL6 known as a negative electron beam electron beam resist.
In the case of 01-ER7 (registered trademark, Shipley Co., Ltd.), it may reach several tens of nanometers if the baking time after exposure is long. Therefore, in forming a pattern of about 100 nm, the spread of the pattern size due to the diffusion of acid cannot be ignored. Therefore, in forming a fine pattern, it is essential to suppress the diffusion of acid as much as possible.

Therefore, as one method of solving the above-mentioned problem, there is a method of applying an electric field during the post-exposure bake as described in, for example, JP-A-3-159114. This is to suppress the diffusion of the generated acid in the horizontal direction of the pattern by applying an electric field in the vertical direction of the resist. However, even in this method, since the post-exposure baking is performed using the hot plate, the processing time cannot be shortened, and the diffusion of the acid in the lateral direction of the pattern cannot be significantly suppressed.

For the purpose of shortening the baking time, it is considered as one of the solutions that baking is performed by a known anneal device using an infrared lamp. This heats the substrate by irradiating the substrate with strong infrared rays, and has the ability to raise the substrate temperature to around 1000 ° C. within 1 second. The heat dissipation is also suppressed, and the processing time can be significantly shortened as compared with the conventional anneal using an electric heating furnace. If this is used, a large temperature rise during heating can be obtained, and the treatment time can be shortened. However, in general, the same equipment is used stably at a temperature of 250 ° C. or higher at which the resist deteriorates.
Since it does not have a cooling mechanism, it is necessary for baking after exposure.
There is a problem that it is difficult to control the temperature around 0 ° C.
For this reason, the apparatus alone could not be applied to a post-exposure bake.

[0010]

SUMMARY OF THE INVENTION An object of the present invention is to provide a post-exposure base which shortens the processing time, which was a problem in the prior art.
It is an object of the present invention to provide a substrate heating apparatus capable of performing high-speed heat treatment, in which the temperature can be controlled in a necessary temperature region by a post-exposure baking or the like in order to perform heating.

[0011]

The above-mentioned object is a substrate heating method in which a substrate on which a thin film or the like is adhered is subjected to a heat treatment at a temperature equal to or lower than the melting point of the substrate, in which the substrate is cooled before the heat treatment is started. A substrate heating method including: and a support table that supports at least the substrate to perform the heat treatment on the substrate, and an energy ray source for irradiating an energy ray for the heat treatment at a temperature equal to or lower than the melting point of the substrate. In the substrate heating device, a substrate heating device having a mechanism for cooling the substrate before starting the heat treatment, a substrate heating device in which the energy rays are infrared rays, and a mechanism for cooling the substrate cool the support base. A substrate heating device that is a mechanism, a substrate heating device that is a mechanism that cools the substrate sprays a cooling medium onto the substrate, and a mechanism that rotates the support table, and It is achieved by implementing a substrate heating apparatus installed in the source - the substrate heater becomes uniform, and unnecessary energy in the heat treatment - the energy - filter that removes a line - of energy.

[0012]

Operation: Energy for heating the substrate in FIG.
There is a radiation source 1, from which energy rays 2 such as infrared rays are radiated and reach the substrate 3. A resist 6 is applied to the substrate 3. Here, the substrate 3 is mounted on a support base 4, and the support base 4 is connected to a cooler 5. Board 3
After the installation, the cooler 5 is operated to cool the support base 4, and the substrate 3 and the resist 6 are set to have a low temperature.
The energy ray 2 radiated from the energy ray source 1 stably has a power to raise the substrate 3 to about 250 ° C. or higher when the substrate 3 is not cooled. However, it does not realize a temperature higher than the melting point of the substrate and is not used for the purpose of melting and recrystallizing the substrate. Although the heat capacity of the support base 4 is larger than that of the substrate, the heat capacity of the support base 4 is set so that the temperature can be increased by heating with the energy rays 2.
Therefore, by heating the substrate 3 and then irradiating it with the energy beam 2 from the energy beam source 1, it is possible to realize a substrate heating capable of performing a heat treatment at a temperature necessary for post-exposure baking, for example, 100 ° C. in a short time. You can Here, in addition to cooling the support table, cooling of the substrate 3 can be realized by spraying a medium such as cooled gas. Here, the support base 4 is provided with a rotation mechanism, and the substrate 3 by heating the energy rays 2 is added.
May have a uniform temperature distribution. Further, since the substrate 3 is cooled before being irradiated with the energy beam 2, it is desirable that the substrate 3 portion is installed in the vacuum container 7 for frost prevention. Here, a vacuum pump 8 is connected to the vacuum container 7.

[0013]

EXAMPLES The present invention will be described in detail below with reference to examples.

(Embodiment 1) In this embodiment, a negative resist will be described with reference to FIG.

A resist 6 is deposited on the substrate 3 which has been subjected to a usual hydrophobic treatment by a spin coating method to a thickness of, for example, 0.1 μm. As the resist 6, for example, a chemically amplified resist SAL601-ER7 is used. Here, as the substrate 3, in addition to a semiconductor substrate such as silicon, GaAs, or InP,
It is also possible to use a metal substrate such as Al or an insulating substrate such as silicon dioxide. Hereinafter, description will be made assuming silicon.

The baking after coating is carried out, for example, at 80 ° C. for 2 minutes on a hot plate to scatter the solvent in the resist.
A latent image of a pattern is formed by irradiating the substrate 3 with an electron beam at an acceleration voltage of 50 kV. However, it goes without saying that the acceleration voltage is not limited to 50 kV. This pattern
In the latent image portion of the battery, an acid is generated from the acid generating substance by the energy of the electron beam.

The substrate 3 is set on the support 4. At this time, it is desirable that the substrate 3 be in good thermal contact with the support base 4 by a vacuum chuck or the like. After that, the cooler 5 connected to the support base 4 operates, and together with the support base 4, the substrate 3
And the resist 6 is cooled. The cooling temperature can be set arbitrarily, but by using liquid nitrogen, it is possible to easily cool to about -200 degrees. Here, the temperature is set to −150 ° C., for example. This temperature can be set by heating the heater installed on the support 4. Then, the energy ray 2 is irradiated from the energy ray source 1, and the substrate 3 and the resist 6 are irradiated.
And heating the support 4. Here, a case where a normal halogen lamp is used as the energy ray source 1 will be described. The halogen lamp has the spectrum shown in FIG. Infrared rays having an internal wavelength range of about 0.75 μm to 3 μm are absorbed by the substrate 3, the support 4 and the resist 6 to raise the temperatures of the substrate 3, the support 4 and the resist 6. Usually, a halogen lamp is used for performing high-speed annealing after ion implantation, and a temperature of 250 ° C. or higher can be stably obtained at high speed without cooling the substrate. However, it does not melt the substrate 3. The heat capacity of the resist 6 is very small compared with the heat capacity of the substrate 3 and the support 4. Therefore, it can be considered that the temperature of the substrate 3 and the temperature of the resist 6 during heating are substantially equal. Therefore, by measuring the temperature of the substrate 3,
The temperature of the resist 6 can be known.

FIG. 4 shows the time change of the temperature of the substrate 3 measured by a known pyrometer or thermocouple. The temperature of the substrate 3, which was initially at room temperature, is lowered by cooling the support 4. The temperature rapidly changes with the irradiation of the halogen lamp, and reaches a predetermined temperature of 100 ° C. in about 0.5 seconds. Then, after the infrared irradiation for, for example, 10 seconds necessary for the crosslinking reaction to proceed, the lamp is turned off and the heating is stopped. Here, the shutter may block the infrared rays. Here, needless to say, the reached temperature and the irradiation time are not limited to the above temperatures, and may take different values depending on the resist to be used, as long as the temperature is not high enough to change the quality of the resist. Since the support base 4 is continuously cooled, the temperature of the support base 4 decreases again. This rapid cooling also has the effect of suppressing the diffusion of the acid generated in the resist 6, or the effect of rapidly suppressing the excessive progress of the crosslinking reaction.
Then, by stopping the cooling, the temperature of the support base 4 gradually rises to reach room temperature. At this time, the heat installed in the support base 4
It is desirable to shorten the time required to forcibly heat the substrate 3 to return it to room temperature by using the heater. The ultimate temperature at the time of infrared irradiation can be set by changing the value of the current passed through the halogen lamp and controlling the output power, and the ultimate temperature can be controlled with an accuracy of 1 degree. Then, the time required for the post-exposure bake can be set within 1 minute in total, and the post-exposure bake can be introduced without significantly reducing the productivity.

FIG. 5 shows the temperature change of the substrate 3 when the lamp is turned off and the cooling is stopped at the same time. As can be seen, the temperature gradually drops after the ultimate temperature is reached and heating is completed. This shows the case of applying to a resist that is altered by lowering the temperature after the crosslinking reaction.

Here, since the temperature of the substrate 3 becomes low during cooling, water vapor in the air may be solidified. Therefore, in order to prevent frost, it is desirable to put the substrate 3 in a vacuum to cool it. Therefore, the entire apparatus described above was placed in the vacuum container 7. A vacuum pump 8 such as a turbo molecular pump is connected to the vacuum container 7 so that the degree of vacuum inside the vacuum container 7 is 1 Pa or less. The vacuum vessel 7 may include the energy ray source 1, but the energy ray 2 may be emitted from outside the vacuum vessel 7 without including the energy ray source 1.

After completion of the above processing, development is carried out using a usual alkali developing solution such as tetramethylammonium hydroxide (TMAH). As a result, as shown in FIG. 6A, the acid 2 generated in the latent image 23 portion formed by irradiating the resist 21 coated on the substrate 20 with the electron beam 22 is removed.
4 shows a pattern as shown in FIG. 6 (b) at the time of baking after exposure.
After the development, there is almost no lateral diffusion, and FIG.
It was possible to obtain a pattern faithful to the latent image 23 as shown in FIG.

As described above, it is possible to form a pattern having a high dimensional controllability in which the generated acid is suppressed from diffusing during baking after exposure. Here, the case of the negative resist has been described, but the same applies to the positive resist. Although the spin coating method has been described as a method for forming the above resist, the method is not limited to this, and the chemical vapor deposition (CVD) method, the vapor deposition method, the liquid phase growth method, the Langmuir-Blodgett (LB) method is used. It goes without saying that the same applies to films formed by other methods such as. In the above, a halogen lamp is used as the energy ray source 1, but
It is not limited to this, and may be any one that radiates an energy ray having a heating effect. Therefore, energy
Needless to say, the line 2 is not limited to infrared rays.
Electromagnetic waves such as X-rays, ultraviolet rays and gamma rays, and accelerated electron beams, molecular beams, ion beams and the like may be used. In this embodiment, the energy beam 2 is emitted from the upper surface of the substrate 3 coated with the resist 6, but it may be emitted from the rear surface of the substrate 3 depending on the case. Further, the energy beam irradiated for forming the latent image is not limited to the electron beam, but ultraviolet rays,
Any energy ray having sufficient energy for generating an acid such as an ion ray, an X ray, a gamma ray may be used.

(Embodiment 2) In the above embodiment, the cooler for cooling the support was used for cooling the substrate. Here, the method of directly cooling the resist and the substrate is adopted. As shown in FIG. 7, a resist 36 is formed on the substrate 33, and is placed on a support 34. At this time, it is desirable that the substrate 33 be kept in good thermal contact with the support 34 by a vacuum chuck or the like. And the nozzle 35 for cooling the substrate
Is provided, from which a low-reactive cooling medium 39 such as nitrogen, carbon dioxide, helium, or argon can be sprayed. As a result, the substrate 33 and the resist 36 are also cooled to a low temperature, and the subsequent energy is reduced.
By irradiating the energy beam 32 from the radiation source 31, the effect of baking after the exposure for a short time can be obtained.

Here, as in Example 1, the substrate 3 was cooled during cooling.
Since the temperature of 3 is low, the substrate 3
It is desirable to put 3 in a vacuum to cool. For this reason, the entire apparatus was placed in the vacuum container 37. A vacuum pump 38 such as a turbo molecular pump is connected to the vacuum container 37 so that the degree of vacuum in the vacuum container 7 is 1 Pa or less. The vacuum vessel 37 may include the energy ray source 31, but the energy ray 32 may be emitted from outside the vacuum vessel 37 without including the energy ray source 31.

After the substrate 33 is placed on the support 34, the vacuum pump 38 is operated to bring the inside of the vacuum container to a vacuum degree of 1 Pa or less, and the cooling medium 39 is sprayed from the nozzle 35 onto the substrate. At this time, since the temperature of the gas is low inside the nozzle 35 and the temperature of the gas further decreases due to adiabatic expansion when the gas is ejected from the nozzle 35, the temperatures of the substrate 33 and the resist 36 rapidly decrease, and the first embodiment is described. The same cooling effect as can be obtained. Here, a heater is installed inside the support base 34, and the ultimate temperature can be set by heating the heater. Further, the rise in temperature after the post-exposure bake could be performed in a short time by heating the heater. Although the cooling gas is used as the medium for cooling the substrate here, the substrate may be cooled by using a liquid having a low melting point and a small vapor pressure.

(Embodiment 3) In the above-mentioned embodiment, the supporting base on which the substrate is placed is fixed, but a mechanism for rotating the supporting base is added. As a result, even if the temperature distribution in the substrate is not uniform due to the unevenness of the illuminance of the energy beam from the energy source due to deterioration of the energy source, the temperature distribution in the substrate can be made uniform. It was

(Embodiment 4) In the above Embodiments 1 and 3, the energy ray from the energy ray source was directly irradiated. Therefore, when infrared rays are used as the energy rays, the energy rays may contain a slight wavelength component absorbed by the crosslinking substance or the dissolution inhibiting substance in the resist. As a result, the effect of baking after exposure may be reduced in some cases. Therefore, as shown in FIG. 8, a known filter 49 was inserted between the energy ray source 41, the substrate 43, the resist 46, and the support base 44 on which they were mounted. As a result, light having a wavelength component of unnecessary energy at the time of the post-exposure bake can be cut from the energy beam 42, and the post-exposure bake effect can be enhanced. A cooler 45 is connected to the support base 44 to cool the support base 44. Even when electromagnetic waves such as X-rays, ultraviolet rays, and gamma rays or particle beams such as accelerated electron beams, molecular beams, and ion beams are used as energy rays, the same filter is used to obtain a post-exposure bake. I was able to increase the effect.

Since the temperature of the substrate 43 is low, water vapor in the air may solidify. Therefore, in order to prevent frost, it is desirable to put the substrate 43 in a vacuum to cool it. For this reason, the entire apparatus was placed in a vacuum container 47. A vacuum pump 48 such as a turbo molecular pump is connected to the vacuum container 47, and the degree of vacuum in the vacuum container 47 is 1P.
a or less. The vacuum container 47 is the energy source 41.
May be included, but the energy ray 42 may be irradiated from outside the vacuum container 47 without including the energy ray source 41.

(Embodiment 5) In the above-mentioned Embodiments 2 and 3, the energy rays from the energy ray source were directly irradiated. Therefore, when infrared rays are used as the energy rays, the energy rays may contain a slight wavelength component absorbed by the crosslinking substance or the dissolution inhibiting substance in the resist. As a result, the effect of baking after exposure may be reduced in some cases. Therefore, as shown in FIG. 9, a known filter 60 was inserted between the energy ray source 51, the substrate 53, the resist 56, and the support base 54 on which they were mounted. As a result, it was possible to enhance the post-exposure bake effect by cutting the light of the wavelength component of unnecessary energy from the energy ray 52 during the post-exposure bake. A nozzle 55 for cooling the substrate is provided near the support base 54.
It has a structure in which a cooling medium 59 such as helium or argon can be sprayed. Thereby, the substrate 53 and the resist 5
6 is also cooled to a low temperature and then the energy source 5
By irradiating the energy beam 52 from No. 1, the effect of baking after the exposure for a short time could be obtained. X as energy line
Electromagnetic waves such as rays, ultraviolet rays and gamma rays, and accelerated electron rays,
Even when using particle beams such as molecular beams and ion beams,
By using the same filter, the effect of the post-exposure bake could be enhanced.

Here, since the temperature of the substrate 53 becomes low, water vapor in the air may solidify. Therefore, in order to prevent frost, it is desirable to put the substrate 53 in a vacuum to cool it. For this reason, the entire apparatus was placed in the vacuum container 57. A vacuum pump 58 such as a turbo molecular pump is connected to the vacuum container 57, and the degree of vacuum in the vacuum container 57 is 1P.
a or less. The vacuum container 57 is the energy source 51.
May be included, but the energy ray 52 may be emitted from outside the vacuum container 57 without including the energy ray source 51. Although the cooling gas is used as the medium for cooling the substrate here, the substrate may be cooled using a liquid having a low melting point and a small vapor pressure.

Although the post-exposure bake of the resist has been described in the above embodiments, the application is not limited to this, and a substrate on which a thin film other than the resist such as spin-on-glass (SOG) is deposited at high speed. It could be widely used for heating.

[0032]

As described above, according to the present invention, the baking time can be shortened. Therefore, it is possible to suppress the diffusion of the generated acid and form a pattern having good dimensional controllability. became. In addition, the shortening of the baking time was also effective in improving productivity.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment of the present invention.

FIG. 2 is a diagram showing a problem of the conventional method.

FIG. 3 is a spectrum diagram of a halogen lamp.

FIG. 4 is a diagram showing a change in substrate temperature over time (No. 1).

FIG. 5 is a diagram showing a change in substrate temperature with time (No. 2).

FIG. 6 is a diagram showing an effect of using the present invention.

FIG. 7 is a diagram showing an example in which a cooling medium is used for cooling.

FIG. 8 is a view showing an embodiment in which a filter for cutting unnecessary energy for heating is installed by a method of cooling a support for cooling.

FIG. 9 is a diagram showing an embodiment in which a filter for cutting unnecessary energy for heating is installed by a system using a cooling medium for cooling.

[Explanation of symbols]

1, 31, 41, 51 ... Energy-source, 2, 12, 3
2, 42, 52 ... Energy lines, 3, 10, 20, 3
3, 43, 53 ... Substrate, 4, 34, 44, 54 ... Supporting stand, 5, 45 ... Cooler, 6, 11, 21, 36, 46,
56 ... Resist, 7, 37, 47, 57 ... Vacuum container,
8, 38, 48, 58 ... Vacuum pump, 13, 23 ... Latent image, 14, 24 ... Acid, 22 ... Electron beam, 35, 55 ... Nozzle, 39, 59 ... Cooling medium, 49, 60 ... Filter.

Claims (7)

[Claims] 1. A method for heating a substrate, in which a substrate having a thin film or the like deposited thereon is subjected to a heating treatment at a temperature equal to or lower than the melting point of the substrate, which includes a treatment for cooling the substrate before starting the heating treatment. Substrate heating method. 2. A substrate heating device comprising at least a support for supporting the substrate for heat treatment of the substrate and an energy beam source for irradiating an energy beam for heat treatment at a temperature below the melting point of the substrate. A substrate heating apparatus, comprising a mechanism for cooling the substrate before starting the heat treatment. 3. The substrate heating apparatus according to claim 2, wherein the energy rays are infrared rays. 4. The substrate heating apparatus according to claim 2, wherein the mechanism for cooling the substrate is a mechanism for cooling the support base. 5. The substrate heating apparatus according to claim 2, wherein the mechanism for cooling the substrate is a mechanism for spraying a cooling medium on the substrate. 6. The substrate heating apparatus according to claim 2, wherein the temperature distribution on the substrate is made uniform by providing a mechanism for rotating the support base. 7. A substrate heating apparatus according to claim 2, wherein a filter for removing energy rays of unnecessary energy in said heat treatment is installed in said energy ray source.