JP2004296740A - Solid state imaging device and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent the extrusion of an adhesive when a spacer is pasted to a chip substrate. <P>SOLUTION: A chamfered part 5b is formed at the edge of the pasting end face 5a of a spacer 5 disposed to surround a solid state image sensor 3 formed on the upper surface of a solid state image sensor chip 4. The chamfered part 5b can be formed easily with high precision by performing isotropic dry etching before the spacer 5 is formed by anisotropic dry etching. Even if an adhesive 10 extrudes when the spacer 5 is pasted to a chip substrate 4a, it is received in the chamfered part 5b and does not flow to the solid state image sensor 3. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a solid-state imaging device using a wafer level chip size package structure and a method for manufacturing the solid-state imaging device.
[0002]
[Prior art]
Digital cameras using a solid-state imaging device and a semiconductor memory instead of a silver salt film have become widespread. In addition, portable telephones that can be taken easily by incorporating a solid-state imaging device and a semiconductor memory, and small electronic devices such as electronic notebooks are also widespread. Therefore, downsizing of the solid-state imaging device is desired.
[0003]
One mounting method for reducing the size of a solid-state imaging device is a wafer level chip size package structure (hereinafter abbreviated as wafer level CSP) that completes mounting of the solid-state imaging device at a wafer level without using a package (hereinafter referred to as wafer level CSP). For example, see Patent Document 1). In this solid-state imaging device using the wafer level CSP, a spacer is arranged on the upper surface of the solid-state imaging device chip so as to surround the solid-state imaging device, and a cover glass for sealing the solid-state imaging device is attached on the spacer. Thus, a solid-state imaging device is formed. A CCD, which is a solid-state imaging device, destroys a photodiode when irradiated with α rays. Therefore, α-ray shielding glass or a glass material that does not generate α-rays from itself is used for the cover glass.
[0004]
The post-process of the solid-state imaging device is performed as follows. First, an inorganic material substrate, such as a silicon wafer (hereinafter referred to as a spacer wafer), which serves as a spacer substrate, is bonded to a transparent glass substrate which serves as a cover glass substrate with an adhesive or the like. A resist mask having a spacer shape is formed on the spacer wafer using photolithography, and a portion not covered with the resist mask is etched. Thereby, a large number of spacers are formed on the glass substrate. An adhesive is applied to the end face of each spacer, and a glass substrate is bonded to a chip wafer on which a large number of solid-state imaging elements are formed. Thereafter, by dicing the glass substrate and the wafer, a large number of solid-state imaging devices having a wafer level CSP structure are completed.
[0005]
[Patent Document 1]
Japanese Patent Laid-Open No. 2002-231921
[Problems to be solved by the invention]
The periphery of the solid-state image sensor must be securely sealed with a spacer and a cover glass so that dust, moisture and the like do not enter the solid-state image sensor. For this reason, in bonding the spacer and the chip wafer, it is necessary to uniformly apply an adhesive to the entire area of the bonding end surface of the spacer. An adhesive having a low viscosity is suitable for uniformly applying the adhesive, but this low-viscosity adhesive has a characteristic that it easily protrudes from the bonding end surface of the spacer. For this reason, the protruding adhesive flows into the solid-state imaging device, which deteriorates the yield of the solid-state imaging device.
[0007]
In addition, α-ray shielding or low α-ray glass that is a material for the cover glass is expensive and hinders cost reduction of the solid-state imaging device. Further, although not disclosed in Patent Document 1, the α-ray shielding glass has a different thermal expansion coefficient from that of silicon used as a spacer material. Therefore, when bonding glass substrates and spacer wafers, use a thermosetting adhesive that is inexpensive, highly reliable, and has a short curing time because of concerns about warpage and breakage due to differences in the coefficient of thermal expansion. I could not.
[0008]
An object of the present invention is to solve the above-described problems, and to eliminate the deterioration of the yield of the solid-state imaging device due to the protrusion of the adhesive between the spacer and the chip wafer and the increase in cost due to the α-ray shielding glass. And
[0009]
[Means for Solving the Problems]
In order to solve the above-described problems, the solid-state imaging device of the present invention is provided with a chamfered portion that accommodates the protruding adhesive at the edge of the end surface bonded to the chip substrate of the spacer. Thereby, even if the adhesive protrudes from between the spacer and the chip substrate, the adhesive can be retained in the chamfered portion, so that the adhesive does not flow onto the solid-state imaging device.
[0010]
When forming a spacer on the transparent substrate, a chamfered portion that isotropically etched to a depth in the middle of the spacer base material laminated on the transparent substrate and accommodates an adhesive in the edge portion of the spacer After that, anisotropic etching is performed so as to penetrate the spacer base material to form the spacer. Thereby, the spacer which has a chamfer part can be easily formed only by an etching process.
[0011]
The transparent plate is formed of a material having a coefficient of thermal expansion similar to that of the spacer, and is coated with a material that shields specific radiation on at least one surface facing the solid-state imaging device. Accordingly, the transparent plate can be configured at a lower cost than when a transparent plate having a shielding property against specific radiation is used, and the cost of the solid-state imaging device can be reduced. Further, even if any heat treatment is performed after bonding the spacer and the transparent plate, the spacer and the transparent plate are not warped or damaged due to the difference in thermal expansion coefficient. Furthermore, since the coating of the radiation shielding material can be performed either before or after the formation of the spacer, the degree of freedom in configuring the production line can be increased.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 and FIG. 2 are a perspective view and a cross-sectional view showing a main part of a solid-state imaging device having a wafer level CSP structure manufactured by the manufacturing method of the present invention. The solid-state imaging device 2 includes a rectangular solid-state imaging element chip 4 provided with the solid-state imaging element 3, a frame-shaped spacer 5 attached on the chip 4 so as to surround the solid-state imaging element 3, and the spacer 5 It consists of a transparent cover glass 6 which is attached on top and seals the solid-state imaging device 3.
[0013]
The solid-state image pickup device chip 4 includes a rectangular chip substrate 4a, a solid-state image pickup device 3 formed on the chip substrate 4a, and a plurality of connection terminals 8 used for connection to an electronic device in which the solid-state image pickup device 2 is incorporated. It consists of. The solid-state image sensor chip 4 is formed by forming a large number of solid-state image sensors 3 and connection terminals 8 on a chip wafer and dicing the wafer for each solid-state image sensor 3. The thickness of the chip substrate 4a is, for example, about 300 μm.
[0014]
The solid-state image sensor 3 is composed of a CCD, for example. A color filter and a microlens are stacked on the CCD. The connection terminal 8 is formed by printing on the chip substrate 4a using, for example, a conductive material. The connection terminal 8 and the solid-state imaging device 3 are connected by a wiring layer formed on the chip substrate 4a.
[0015]
The spacer 5 is formed of an inorganic material such as silicon, and has a width dimension of, for example, about 200 μm and a thickness of about 10 to 200 μm. The spacer 5 and the chip substrate 4a are joined by an adhesive. A chamfered portion 5b is formed at the edge of the end surface 5a bonded to the chip substrate 4a of the spacer 5 so that the cross section of the spacer 5 has a substantially wineglass shape. The chamfered portion 5b accommodates the adhesive 10 protruding from under the spacer 5 when the spacer 5 and the chip substrate 4a are bonded together using the adhesive 10, and the adhesive 10 flows onto the solid-state imaging device 3. To prevent.
[0016]
As the cover glass 6, a transparent glass having a thermal expansion coefficient close to that of the material of the spacer 5, for example, “Pyrex (registered trademark) glass” or the like is used. The inner surface of the cover glass 6 is coated with an α-ray shielding material 12 in order to prevent destruction of the photodiode of the CCD. The cover glass 6 also has a function of reinforcing the solid-state imaging device 2 and has a thickness of about 500 μm, for example.
[0017]
FIG. 3 is a flowchart showing a post-process of the solid-state imaging device, “formation of spacer on glass substrate (first process)”, “bonding of glass substrate and chip wafer (second process)”. , “Dicing (third step)”. FIG. 4 is a flowchart showing the first to seventh steps of the first step.
[0018]
As shown in FIG. 5A, in the first step, the α-ray shielding material 12 is coated on one surface of a wafer-like glass substrate 14 that is a base material of the cover glass 6. As the α-ray shielding material 12, an amorphous fluororesin (for example, “Cytop (Asahi Glass)”) having high transparency and excellent thin film formation, liquid polyimide, or the like is used. As a coating method for the α-ray shielding material 12, spin coating, spray coating, or the like can be used. Moreover, if the alpha ray shielding material 12 which has photosensitivity is used, it can be hardened in a short time by irradiating the apply | coated alpha ray shielding material 12 with an ultraviolet-ray. As described above, since the α-ray shielding glass can be constituted by the inexpensive transparent glass 14 and the α-ray shielding material 12, it is possible to contribute to the low cost of the solid-state imaging device 2.
[0019]
As shown in FIG. 5 (B), in the second step, a spacer wafer 16 serving as a base material of the spacer 5 is adhered to the surface of the glass substrate 14 on which the α-ray shielding material 12 is applied by an adhesive 17. It is pasted together. A thicker glass substrate 14 and spacer wafer 16 (for example, in the case of a silicon wafer, a standard wafer of t625 μm in the case of φ6 inch size) is used. And after bonding together, the glass substrate 14 and the wafer 16 for spacers of required thickness can be obtained by grinding and grinding | polishing in a 3rd step. Thereby, material cost can be suppressed and handling property can be improved.
[0020]
Since the adhesive 17 needs to be applied thinly (for example, t10 μm or less) and uniformly on the glass substrate 14 using a spin coating method or the like, a low viscosity of 500 cps or less is preferable. Moreover, as the kind of the adhesive 17, since the raw material which has a thermal expansion coefficient close | similar to a silicon | silicone is used for the glass substrate 14, a cheap and reliable thermosetting type adhesive with a short hardening time should be used. Can do. In addition to the thermosetting adhesive, a room temperature curable adhesive, a UV curable adhesive, or the like can be used. When a UV curable adhesive is used, the adhesive 17 can be cured in a short time by irradiating ultraviolet rays from the glass substrate 14 side.
[0021]
In addition, as a selection criterion for the adhesive 17 used for bonding the glass substrate 14 and the spacer wafer 16, removal suitability can be given. Although details will be described in the seventh step, in this embodiment, unnecessary adhesive 17 remaining on the glass substrate 14 is removed by ashing. Therefore, selecting a low molecular weight adhesive as an adhesive that can be easily removed by ashing is also effective for improving the manufacturing efficiency of the solid-state imaging device 2.
[0022]
As an adhesive having a high removal efficiency by ashing other than a low molecular weight adhesive, an adhesive that does not contain a C═C bond or has a low bonding ratio can be selected. This is because the binding force of C = C is not easy to break even with decomposition energy by oxygen plasma.
[0023]
An alignment sticking device is used for bonding the glass substrate 14 and the spacer wafer 16 together. The alignment sticking apparatus applies the adhesive 17 to the upper surface of the glass substrate 14 by using a spin coating method, and the spacer wafer 16 whose position in the XY direction and the rotational direction is adjusted by the orientation flat on the glass substrate 14. After the superposition, the glass substrate 14 and the spacer wafer 16 are heated to cure the adhesive 17.
[0024]
When the glass substrate 14 and the spacer wafer 16 are bonded together, no bubbles or voids should be formed between them. This is because the layer of the adhesive 17 not only bonds the glass substrate 14 and the spacer wafer 16 but also has a function of securely sealing the solid-state imaging device 3. Therefore, the alignment sticking apparatus performs the bonding of the glass substrate 14 and the spacer wafer 16 in a chamber capable of forming a working environment in a reduced pressure to vacuum state (for example, 10 Torr or less). In addition, the glass substrate 14 and the spacer wafer 16 may be bonded by anodic bonding, fusion bonding, direct bonding, room temperature bonding, or the like without using any adhesive or inclusions.
[0025]
As shown in FIG. 5C, in the third step, grinding and polishing are performed to reduce the thickness of the bonded glass substrate 14 and spacer wafer 16. In addition, the glass substrate 14 and the wafer 16 for spacers of the thickness after completion can also be used, and in this case, the 3rd step for the purpose of grinding and polishing can be omitted.
[0026]
As shown in FIG. 5D, in the fourth step, a resist mask 19 is formed on the upper surface of the spacer wafer 16. The resist mask 19 is produced using a known photolithography technique. First, an unexposed resist is applied on the spacer wafer 16. Next, the resist is exposed through an exposure mask in which the pattern of the spacer 5 is formed, and development processing is performed. As a result, a large number of resist masks 19 in the shape of the spacers 5 are formed on the spacer wafer 16.
[0027]
It should be noted that the resist mask 19 needs to be thick enough that the resist mask 19 itself is not consumed by the etching gas when the spacer wafer 16 is etched by the fifth and sixth steps of dry etching. . Further, when the resist mask 19 is formed, not only the pattern of the spacer 5 but also a resist pattern of an alignment mark and an outer peripheral ring provided on the outer periphery of the wafer is formed.
[0028]
In the fifth and sixth steps, a large number of spacers 5 are formed on the glass substrate 14 by etching. For the processing of the spacer 5, isotropic dry etching and anisotropic dry etching are used. Therefore, it is preferable to use a parallel plate type RIE (reactive ion etching) apparatus or the like corresponding to both isotropic and anisotropic etching. When the height of the spacer 5 exceeds 100 μm, an ICP (inductively coupled plasma) type dry etcher is preferably used in order to maintain dimensional accuracy and ensure high productivity.
[0029]
First, isotropic dry etching is performed on the spacer wafer 16 by a dry etcher. As a result, as shown in FIG. 6A, the portion of the spacer wafer 16 that is not covered with the resist mask 19 and the portion under the resist mask 19 are etched at the same rate. The purpose of this isotropic dry etching is to form the chamfered portion 5b that accommodates the protruding adhesive 10 at the edge of the bonding end surface 5a of the spacer 5, so that the spacer wafer 16 has a predetermined width and depth. Therefore, it is necessary to control so as to be etched.
[0030]
Next, anisotropic dry etching is performed with the same dry etcher. As a result, as shown in FIG. 6B, a large number of spacers 5 having vertical side surfaces and chamfered portions 5b processed at high precision on the edges are formed on the glass substrate. When it is necessary to suppress the squareness of the side surface of the spacer 5 to ± 5 ° or less, it is preferable to use a Bosch process that can proceed with vertical processing while preventing side etching of the side surface.
[0031]
The Bosch process refers to a process in which etching and an operation called deposition for coating the entire workpiece with a polymer that is not eroded by the etching are alternately repeated. When the Bosch process is used for manufacturing the spacer 5, the polymer coated in the deposition process remains on the surface of the glass substrate 14 (hereinafter referred to as polymer residue), and the transparency of the glass substrate 14 is impaired. Therefore, in order to eliminate the polymer residue, it is preferable to reduce the amount of polymer coating in the deposition process just before the spacer 5 is processed. Further, the etching rate may be lowered in accordance with the decrease in the coating amount.
[0032]
Moreover, in order to reduce the polymer residue by the Bosch process, the following method can also be taken. That is, the Bosch process is terminated just before the etching through the spacer wafer 16 is completed, and the formation of the spacer 5 is completed by normal etching without performing deposition.
[0033]
As shown in FIG. 6C, in the seventh step, the adhesive 17 and the resist mask 19 remaining on the glass substrate 14 are removed. For the residual organic matter removal treatment, an ashing treatment for removing (ashing) the organic matter using oxygen plasma is used. In this ashing process, since the glass substrate 14 is not contaminated, there is no need for a step of cleaning the glass substrate 14 after the residual organic substances are removed. The ashing process may be performed on the dry etcher immediately after the etching of the spacer 5 or may be performed by a dedicated asher. Further, in order to realize the composition (ashing characteristics) of the adhesive 17 and the target processing speed, a fluorine-based, hydrogen-based, or argon-based gas may be used or added to oxygen.
[0034]
As described above, the etching process of the spacer 5 and the removal of the adhesive 17 and the resist mask 19 are processed in the dry consistent process, so that the processing can be finished in a clean state of the glass substrate 14, and the cleaning process is performed. Without being bonded to the chip wafer on which the solid-state imaging device 3 is formed.
[0035]
Note that a wet process can be used to remove the adhesive 17 and the resist mask 19. In this wet treatment, the residual organic matter is decomposed (dissolved) by immersing the glass substrate 14 in a chemical solution such as a solvent, strong acid, or strong alkaline solution. As an advantage of the wet processing, the entire glass substrate 14 can be cleaned by a chemical solution cleaning process in addition to the reduction of the apparatus cost and the improvement of the throughput.
[0036]
As described above in the first to seventh steps of the first process, the glass substrate 14 and the spacer wafer 16 are first integrated, and then the spacer 5 is processed and residual organic substances are removed. Therefore, the process can be simplified as compared with the case where the thin and brittle spacer wafer 16 is handled alone.
[0037]
In the second step, as shown in FIGS. 7 and 8B, a glass substrate 14 on which a large number of spacers 5 are formed and a chip wafer 21 on which a large number of solid-state imaging elements 3 are formed are bonded together. . First, as shown in FIG. 8A, the adhesive 10 is thinly and uniformly applied on the spacer 5, the alignment mark, and the outer ring of the glass substrate 14. For applying the adhesive 10 to the spacer 5, apply the adhesive thinly and uniformly on a plate such as another sheet or wafer, and transfer it to the spacer 5, alignment mark, and outer ring of the glass substrate 14. The method is used. According to this method, a thin layer of the adhesive 10 having a uniform thickness can be formed on the spacer 5 without being influenced by the handling property of the adhesive 10 represented by viscosity.
[0038]
Further, as another application method of the adhesive 10, a method of using a dispenser according to the handling property of the adhesive 10, or a method of spray coating in a lump by covering only the region to be opposed to the solid-state imaging device 3. A method using screen printing technology can be selected.
[0039]
As described above, in the method in which a large number of spacers 5 are bonded together to the chip wafer 21, the position accuracy and the bonding process are more important than the case where the spacers 5 separated one by one are arranged on the chip wafer 21. Is also realistic. Further, the glass substrate 14 and the chip wafer 21 are bonded to each other and the solid-state imaging device 3 is sealed before the dicing step, so that the concern about dust can be completely canceled.
[0040]
As shown in FIGS. 8C and 8D, in the third step, after the adhesive 10 that bonds the glass substrate 14 and the chip wafer 21 is sufficiently cured, the glass substrate 14 and the chip wafer 21 are cured. And dicing. A dicing tape 23 is attached to the back surface of the chip wafer 21 so that the solid-state imaging devices 2 do not fall apart after dicing. Further, a protective tape 24 for preventing contamination may be attached to the surface of the glass substrate 14 in order to prevent contamination with grinding scraps or grinding liquid. Instead of the protective tape 24, a surface protective coating that can be easily applied and removed can be applied.
[0041]
For dicing the glass substrate 14, a grindstone formed of diamond or CBN abrasive grains of about # 400 to # 1500 is used. Since the glass substrate 14 is a hard and brittle material, chipping of the cutting edge and wear of the grindstone become problems during cutting, and the cutting speed cannot be increased. Therefore, instead of processing with a single grindstone, it is advisable to adopt a multi-blade configuration in which several grindstones are assembled on a shaft and multiple pieces are cut at once. As a result, the processing efficiency of about 0.1 to 10 mm / s can be doubled by the number of blades assembled.
[0042]
After the dicing of the glass substrate 14 is completed, the chip wafer 21 is diced in the same manner. Thereafter, the dicing tape 23 and the protective tape 24 are peeled off to complete the solid-state imaging device 2.
[0043]
In the above embodiment, the α-ray shielding material 12 is applied to the glass substrate 14 before the spacer 5 is formed. However, as shown in FIG. 9A, the spacer 30 is formed and the adhesive and the resist mask are formed. The α-ray shielding material 32 may be applied to the glass substrate 31 after the removal of and as shown in FIG. In this case, for example, an α-ray shielding material 32 having ultraviolet curing properties is used, and ultraviolet rays are irradiated from the glass substrate 31 side after application. Thereby, since the alpha ray shielding material 32 on the spacer 30 is not hardened by ultraviolet rays, it can be easily removed as shown in FIG.
[0044]
【The invention's effect】
As described above, according to the solid-state imaging device and the manufacturing method of the solid-state imaging device of the present invention, even if the adhesive protrudes from between the spacer and the chip substrate, it protrudes from the chamfered portion formed at the edge of the spacer. Since the adhesive is accommodated, the adhesive does not flow into the solid-state image pickup element and the like does not cause a defect. Further, since the chamfered portion can be formed by performing isotropic etching before anisotropic etching used when forming the spacer, it can be processed with low cost and high accuracy.
[0045]
In addition, the transparent plate is provided with a shielding performance against specific radiation by applying a coating material, and thus can be configured at low cost. In addition, since the transparent plate can be made of a material having a thermal expansion coefficient similar to that of the spacer, even if heat treatment is performed when the transparent plate and the spacer are provided integrally, the difference in thermal expansion coefficient No warping or damage due to Furthermore, since the coating material may be applied before or after the spacer is formed on the transparent substrate, the degree of freedom in configuring the production line can be improved.
[Brief description of the drawings]
FIG. 1 is an external perspective view showing a configuration of a solid-state imaging device manufactured using the present invention.
FIG. 2 is a cross-sectional view of a main part showing a configuration of a solid-state imaging device.
FIG. 3 is a flowchart illustrating a procedure of a post-process of the solid-state imaging device.
FIG. 4 is a flowchart showing a procedure of a first step.
FIG. 5 is a fragmentary cross-sectional view showing the state of the glass substrate and spacer wafer in the first step to the fourth step of the first step.
FIG. 6 is a cross-sectional view of the principal part showing the state of the glass substrate and the chip wafer in the fifth to seventh steps of the first process.
FIG. 7 is a perspective view showing a bonded state of a glass substrate and a chip wafer.
FIG. 8 is a cross-sectional view of the main part showing the state of the glass substrate and spacer wafer in the second and third steps.
FIG. 9 is a cross-sectional view of a main part showing a coating state of an α-ray shielding material according to another embodiment of the present invention.
[Explanation of symbols]
2 Solid-state imaging device 3 Solid-state imaging device 4 Solid-state imaging device chip 5 Spacer 6 Cover glass 10 Adhesive 12 α-ray shielding material 14 Glass substrate 16 Spacer wafer 19 Resist mask 21 Chip wafer

Claims (4)

In a solid-state imaging device in which a frame-shaped spacer surrounding the solid-state imaging element is attached to the chip substrate on which the solid-state imaging element is formed with an adhesive, and the spacer is sealed with a transparent plate.
A solid-state imaging device, wherein a chamfering portion that accommodates the protruding adhesive is provided at an edge of an end surface bonded to the chip substrate of the spacer. In a solid-state imaging device in which a frame-shaped spacer surrounding the solid-state imaging device is disposed on the chip substrate on which the solid-state imaging device is formed, and the spacer is sealed with a transparent plate.
The transparent plate is formed of a material having a coefficient of thermal expansion similar to that of a spacer, and a material for shielding specific radiation is coated on at least one surface facing the solid-state imaging device. Solid-state imaging device. The process of forming a number of spacers surrounding the solid-state image sensor on a transparent substrate, the chip wafer on which a large number of solid-state image sensors are formed, and the spacer on the transparent substrate are bonded together with an adhesive, and each solid-state image sensor is bonded with a spacer. A solid-state imaging comprising: enclosing and sealing each solid-state imaging device with a transparent substrate; and dividing the chip wafer and the transparent substrate into each solid-state imaging device to form a large number of solid-state imaging devices. In the device manufacturing method,
The step of forming the spacer includes
Laminating a spacer base material to be a spacer material on a transparent substrate;
Forming a spacer-shaped mask on the surface of the spacer substrate;
Performing isotropic etching to a depth in the middle of the spacer base material, and forming a chamfered portion for accommodating an adhesive in a portion to be an edge of the spacer;
And a step of forming the spacer by performing anisotropic etching so as to penetrate the spacer base material. The process of forming a number of spacers surrounding the solid-state image sensor on a transparent substrate, the chip wafer on which a large number of solid-state image sensors are formed, and the spacer on the transparent substrate are bonded together with an adhesive, and each solid-state image sensor is bonded with a spacer. A solid-state imaging comprising: enclosing and sealing each solid-state imaging device with a transparent substrate; and dividing the chip wafer and the transparent substrate into each solid-state imaging device to form a large number of solid-state imaging devices. In the device manufacturing method,
A method for manufacturing a solid-state imaging device, comprising: coating a transparent substrate before the spacer is formed or a transparent substrate after the spacer is formed with a material that shields specific radiation.

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