JP2001042304A - Liquid crystal display device and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a liquid crystal display device hardly increasing cross talk even when a reflective pixel electrode to be formed on the uppermost layer is superposed on scanning lines and signal lines, capable of being driven by low power and giving pictures high in numerical aperture and contrast. SOLUTION: An interlayer insulation film 11 is formed on thin film transistors(TFTs) formed on an insulation substrate 1 and has contact holes 12 formed so that their positions are aligned with those of TFTs. The film 11 is constituted of a first interlayer insulation film 11a and a second interlayer insulation film 11b which are made from photosensitive resins having the same UV sensitivity. An irregular pattern controlled in height is formed on the film 11a by exposing the film 11b with light. The reflective pixel electrode 13 is formed on the film 11b by connecting it with TFTs through the holes 12.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device such as a reflection type liquid crystal display device which performs display by reflecting light incident from the outside, and a method of manufacturing the same.

[0002]

2. Description of the Related Art Liquid crystal display devices are being actively researched and developed as one of flat panel displays instead of CRTs. In particular, the liquid crystal display device has a small power consumption and a thin shape. It has been put to practical use for TVs, notebook computers, car navigation systems, portable terminal devices, and the like. As a driving method of a liquid crystal display device, an active matrix type TFT array using a thin film transistor (hereinafter abbreviated as TFT) as a switching element is mainly used in terms of high quality display. Liquid crystal displays have a transmission type and a reflection type. Since the reflective type does not require a backlight light source like the transmissive type, low power consumption can be realized, and it can be said that the reflective type is suitable for use in portable terminals and the like. In order to improve the display characteristics of the reflection type liquid crystal display device, it is effective to increase the effective display area of the pixel portion of the liquid crystal display panel to increase the light use efficiency, that is, to increase the aperture ratio of the pixel.

In a reflection type liquid crystal display device, a first insulating substrate provided with a grid-like electrode, a TFT portion, a reflective pixel electrode and the like, and a second insulating substrate provided with a color filter, a black matrix, a counter electrode and the like are provided. The transparent insulating substrate (counter substrate) is opposed to and bonded to the substrate, and liquid crystal is injected between the two substrates. In order to increase the efficiency of light utilization, do not apply a scattering film (forward scattering plate method) on the incident light side, and use the first insulating substrate to obtain scattered light with good directivity. Liquid crystal display devices have been proposed. This reflective liquid crystal display device obtains good scattered light by providing irregularities on the surface of the reflective film and pixel electrode.

[0004]

Japanese Patent Application Laid-Open No. Hei 8-184846 discloses a reflection type liquid crystal display device using this structure and having unevenness formed by photolithography. However, in the case of the technology disclosed in Japanese Patent Application Laid-Open No. 8-184846, a concave / convex pattern is formed by coating and baking a polyimide film, coating, exposing and developing a resist, and etching using this as a mask. After the removal, a resist is applied, exposed and developed again, and etching is performed using the resist as a mask to form a contact hole pattern. Therefore, an increase in the number of processing steps, an increase in cost, and a decrease in yield have been caused.

A method of forming a reflector in which unevenness is formed on an interlayer insulating film by changing the depth in the film thickness direction by exposing the mask while changing the transmission amount of the mask in multiple steps is disclosed in Japanese Patent Laid-Open No. 7-1989.
No. 19 discloses this. JP-A-7-198919
In the case of the technique disclosed in Japanese Patent Application Laid-Open No. H11-209, a mask that changes the transmission amount in multiple stages is used. However, in order to obtain good scattering characteristics, it is necessary to eliminate a flat place. It is necessary to form irregularities of about 200 to 300 in the inside, and it is necessary to form 1.44 million pixels in total number of pixels and to make the inter-pixel shape of the irregularities uniform in order to eliminate reflection unevenness. The manufacture of a mask capable of such exposure is very expensive and difficult. Further, it is difficult to stably obtain a developing speed distribution caused by the exposure amount distribution, a developing speed distribution due to the degree of application of the developing solution, and the like, which hardly have a margin to these and have good reflection characteristics without unevenness.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problem, and does not cause an increase in crosstalk even when a reflective pixel electrode formed on the uppermost layer is overlapped with a scanning line and a signal line. It is a first object of the present invention to provide a liquid crystal display device which can drive a liquid crystal with low power and can obtain a high contrast screen with a high aperture ratio. A second object is to obtain a method for manufacturing such a liquid crystal display device.

[0007]

In the liquid crystal display device according to the present invention, the contact is formed on the switching element on the substrate, and the contact is formed so that the surface opposite to the substrate has irregularities and is aligned with the switching element. An interlayer insulating film having a hole formed therein; and a pixel electrode formed on the uneven surface of the interlayer insulating film and connected to the switching element via a contact hole formed in the interlayer insulating film. It is composed of a first photosensitive resin layer formed on the element and a second photosensitive resin layer formed on the first photosensitive resin layer. An interlayer insulating film formed on the transistor on the substrate, the surface opposite to the substrate having irregularities formed thereon and a contact hole formed so as to be aligned with the drain electrode; A pixel electrode connected to a drain electrode via a contact hole formed in the interlayer insulating film, wherein the interlayer insulating film includes a first photosensitive resin layer formed on the transistor and the first photosensitive resin layer. It is constituted by a second photosensitive resin layer formed on the conductive resin layer.

The second photosensitive resin layer is formed by exposing a predetermined pattern whose height is regulated by the first photosensitive resin layer. Further, the first photosensitive resin layer and the second photosensitive resin layer have different photosensitivity. At least one of the first photosensitive resin layer and the second photosensitive resin layer is colored.

In the method of manufacturing a liquid crystal display device according to the present invention, a first step of forming a switching element on a substrate and a second step of forming a first photosensitive resin layer on the switching element And a third step of exposing the first photosensitive resin layer to form a contact hole so as to align with the switching element, and forming a second photosensitive resin layer on the first photosensitive resin layer. A fourth step of forming and a fifth step of exposing the second photosensitive resin layer to form a predetermined pattern including a contact hole aligned with the contact hole formed in the first photosensitive resin layer And forming a pixel electrode on the second photosensitive resin layer so as to be connected to the switching element via a contact hole formed in the first photosensitive resin layer and the second photosensitive resin layer. Six It is intended to include a degree.

Further, a first step of forming a switching element on the substrate, a second step of forming a first photosensitive resin layer on the switching element, and a second step of forming a first photosensitive resin layer on the switching element. A third step of forming a second photosensitive resin layer having a different photosensitivity from the one photosensitive resin layer, and switching by exposing the first photosensitive resin layer and the second photosensitive resin layer. A fourth step of forming a contact hole so as to match the element, a fifth step of forming a predetermined pattern by exposing the second photosensitive resin layer, and connecting to the switching element through the contact hole As a result, the method includes a sixth step of forming a pixel electrode on the second photosensitive resin layer.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 Hereinafter, embodiments of the present invention will be described with reference to the drawings. High aperture ratio pixel TF
To obtain a T-array, the array substrate is made flat with an interlayer insulating film made of a photosensitive resin, and is overlapped with a control electrode or the like under the interlayer insulating film, and a pixel electrode having a large area is formed on the interlayer insulating film. It is effective to form and connect the pixel electrode to the drain electrode through a contact hole formed in the interlayer insulating film. The invention provides a low induction (<4)
Was formed into a laminated structure and used as an interlayer insulating film.

FIG. 1 is a block diagram of a first embodiment of the present invention.
It is sectional drawing which shows an FT array substrate. In the figure, 1 is an insulating substrate, 2a is a gate electrode formed on the insulating substrate 1, 3 is a common electrode wiring formed on the insulating substrate 1,
4 is a gate insulating film formed on the gate electrode 2 and the common electrode wiring 3, 5 is an amorphous silicon film (hereinafter referred to as an a-Si film) which is a semiconductor film formed on the gate insulating film 4, and 6 is an a-Si film. An impurity-doped low-resistance amorphous silicon film (hereinafter referred to as n + -a-S) formed on the Si film 5.
and an ohmic contact film. 7a
Denotes a source electrode formed on the n ⁺ -a-Si film 6, and 8 denotes a drain electrode formed on the n ⁺ -a-Si film 6. Reference numeral 9 denotes a channel portion formed between the source electrode 7a and the drain electrode 8. Reference numeral 10 denotes a passivation film formed so as to cover the source electrode 7a, the drain electrode 8, and the channel portion, and reference numeral 11 denotes an interlayer insulating film formed on the passivation film 10, which is a first interlayer which is a first photosensitive resin layer. It is composed of an insulating film 11a and a second interlayer insulating film 11b which is a second photosensitive resin layer. Reference numeral 12 denotes a contact hole formed in the interlayer insulating film 11 so as to match the drain electrode 8, and reference numeral 13 denotes a reflective pixel electrode formed on the interlayer insulating film 11 and connected to the drain electrode 8 through the contact hole 12.

FIG. 2 shows T according to the first embodiment of the present invention.
FIG. 3 is a plan view showing an FT array substrate. FIG.
The section taken along line -A 'is shown. In FIG. 2, 3, 7a,
8, 9, 12, and 13 are the same as those in FIG. Reference numeral 2 denotes a gate electrode wiring serving as a scanning line connected to the gate electrode 2a, and reference numeral 7 denotes a source electrode wiring serving as a signal line connected to the source electrode 7a. 3A to 3D are cross-sectional views illustrating a manufacturing process of the TFT array substrate according to Embodiment 1 of the present invention, and FIGS. 3A to 3D illustrate the respective manufacturing processes. 3, 1, 2a, 3-6, 7a, 8-13
Are the same as those in FIG.

Next, a method of manufacturing a TFT array substrate of a liquid crystal display according to Embodiment 1 of the present invention will be described with reference to FIG. As shown in FIG. 3A, a Cr film is formed on an insulating substrate 1 made of glass or the like by using a sputtering method or the like.
A gate electrode wiring 2 and a common electrode wiring 3 which are scanning lines are formed by photolithography. Next, plasma CV
A gate insulating film 4 made of silicon nitride by using the D method or the like;
a-Si film 5, n ⁺ -a-Si film 6 doped with impurities
Are sequentially formed, and a-Si is formed using a photolithography method.
A semiconductor layer is formed by patterning the film 5 and the n ⁺ -a-Si film 6. Next, a channel portion 9 of the semiconductor layer and a source electrode wiring 7 and a drain electrode 8 which are signal lines are formed by a sputtering method and a photolithography method.
Form T (first step).

One end of the drain electrode 8 has a structure in which the gate insulating film 4 made of an inorganic insulating film is interposed and the common electrode wiring 3 formed of a low-layer low-resistance metal is opposed to form a capacitor in the pixel electrode 13 area. Next, a passivation film 10 for protecting the transistor is formed by a CVD method or the like (FIG. 3A).
Acrylic resin with photosensitivity and low dielectric constant (<4)
The coating is applied so that the step due to the FT and the control wiring is eliminated and the surface is flattened (second step), and the contact hole 12 is formed on the drain electrode 8 facing the common electrode wiring 3 and forming a capacitor by photolithography. Is provided (third step) and baked to form a first interlayer insulating film 11a (FIG. 3B). Next, the same photosensitive resin as that of the first interlayer insulating film 11a is applied on the first interlayer insulating film 11a (fourth step), and the gate electrode wiring 2, the source electrode wiring 7 and the capacitor are formed by photolithography. A convex pattern is formed in the pixel excluding a part of the formation position, and a punched pattern matching the contact hole 12 provided in the first interlayer insulating film 11a is formed (fifth step), and the second interlayer insulating film 11b is formed. To form

The interlayer insulating film 11 is made of a positive-type photosensitive acrylic resin (PC-335 manufactured by JSR: i-line or h-line photosensitive product). The first interlayer insulating film and the second interlayer insulating film are both thick. It was applied to a thickness of about 2 μm and exposed and developed at 400 mj / cm ² using an h-line stepper exposure machine. The height of the convex pattern provided on the second interlayer insulating film 11b is regulated on the upper surface of the first interlayer insulating film 11a. As a developing solution, a weak alkaline developing solution (TMAH 0.4 wt%) was used. After the development, each layer is baked at 200 to 230 ° C. for about 1 hour. Next, the passivation film 10 in the contact hole 12 is etched to form the drain electrode 8.
Is exposed in the contact hole 12 (FIG. 3).
(C)). At this time, the passivation film 10 at the terminal (including the transfer electrode) contact portion is also removed (not shown). A highly reflective film of Al or the like is formed on the interlayer insulating film 11 in the contact hole 12, and a reflective pixel electrode 13 corresponding to each pixel portion is formed using a resist mask (sixth step).

The reflective pixel electrode 13 is connected to the drain electrode 8 via a contact hole 12 formed in the interlayer insulating film 11 and the passivation film 10 (FIG. 3).
(D)). The reflection pixel electrode 13 is formed on the second interlayer insulating film 11b having a height-regulated convex pattern, and a reflection surface without spots can be obtained. As the reflection film, Al
Silver or the like having high reflection may be used other than the film. The interlayer insulating film 11 is formed sufficiently thick, and the reflective pixel electrode 1
3 can be formed so as to overlap the scanning line and the signal line.
It has a pixel top layer structure, can drive liquid crystal sufficiently with low power, has a high aperture ratio, and has a high contrast screen. A TFT array substrate thus formed;
A reflective liquid crystal display device is formed by forming an alignment film on the surface of a counter substrate on which a counter electrode is formed on another transparent insulating substrate and then facing the alignment film, and injecting a liquid crystal material during this.

According to the above steps, a TFT array substrate having low power, high performance, and high aperture ratio can be stably obtained, and a TFT array substrate capable of reducing costs by reducing display defects can be manufactured. In addition, either one of the interlayer insulating films 11
By forming the layer or both layers with a colored (black resin) resin, reflection from unnecessary portions can be suppressed. Further, the size of the convex pattern of the interlayer insulating film 11 may be randomly large or small. In the first embodiment, the passivation film 10 such as a nitride film is provided, but the same effect can be obtained without it.

Embodiment 2 4A to 4D are cross-sectional views illustrating a manufacturing process of the TFT array substrate according to Embodiment 2 of the present invention, and FIGS. 4A to 4D illustrate the manufacturing process. In FIG. 4, 1, 2a, 3 to 6, 7a, and 8 to 13
Are the same as those in FIG.

In the second embodiment, a method basically similar to that of the first embodiment is used. The TFT shown in FIG. 4A is formed in the same manner as in FIG. 3A of the first embodiment (first step). Next, as shown in FIG.
The photosensitive organic resin having a low dielectric constant (<4) is formed in two layers on the insulating substrate 1 on which the electrodes and the like are formed, that is, the formation of the first interlayer insulating film 11a (the second step) and the formation of the second The interlayer insulating film 11b is formed (third step), exposed and developed using a photolithography method, and is appropriately formed on the gate electrode wiring 2, on the source electrode wiring 7, and inside the pixel except for a part of the capacitance forming position. And a contact hole is formed on the drain electrode 8 facing the common electrode wiring 3 and forming a capacitance.

The photosensitive organic resin which is the interlayer insulating film 11 is 2
It has a layer structure, and the UV of the first interlayer insulating film 11a of the first layer is
The sensitivity is configured to be lower than the UV sensitivity of the second interlayer insulating film 11b of the second layer, and the contact hole 12 is formed by UV light 1 which is an exposure amount capable of exposing the photosensitive organic resin of the first layer. (Fourth step) is used to form the unevenness in the pixel.
A division exposure method in which exposure was performed using V light 2 (fifth step). After exposure, a weak alkaline developer (TMAH 0.4wt
%), And baked at 200 to 230 ° C. for about 1 hour to form an interlayer insulating film 11 having unevenness in a pixel and a contact hole 12.

By extremely changing the UV sensitivity of the photosensitive organic resin of the first layer and the second layer, the photosensitive organic resin of the first layer is almost completely exposed to the UV light 2 exposing the pattern for forming the unevenness in the pixel. At the time of development without exposure, the photosensitive organic resin of the first layer functions as a stop layer, and the depth of the unevenness of the photosensitive organic resin of the second layer is defined. Next, the drain electrode 8 is exposed in the contact hole 12 as shown in FIG. 4C, and the source electrode wiring 7 and the gate electrode wiring 2 are formed on the interlayer insulating film 11 as shown in FIG. A reflective pixel electrode 13 that overlaps and is connected to the drain electrode 8 via the contact hole 12 is formed (sixth step).

In the method of manufacturing the reflection type liquid crystal display device according to the second embodiment, the same effect as in the first embodiment can be obtained. In the second embodiment, the conventional method of forming a photosensitive organic resin by a spin method is not suitable, and the photosensitive resin is formed by a slit-and-spin method or a method of laminating a film having a photosensitive agent layer formed on two layers. It is suitable to form.

[0024]

Since the present invention is configured as described above, it has the following effects. An interlayer insulating film formed on the switching element on the substrate and having a contact hole formed so as to be aligned with the switching element and having an uneven surface on the surface opposite to the substrate; and an uneven surface of the interlayer insulating film. A pixel electrode connected to the switching element via a contact hole formed in the interlayer insulating film, wherein the interlayer insulating film is formed on the switching element by a first photosensitive resin layer and the first photosensitive resin layer. Since it is constituted by the second photosensitive resin layer formed on the conductive resin layer, it is possible to form irregularities on the interlayer insulating film by exposing.

Further, an interlayer insulating film formed on the transistor on the substrate and having a contact hole formed so as to have irregularities on the surface opposite to the substrate and to be aligned with the drain electrode; Formed on uneven surface,
A pixel electrode connected to the drain electrode via a contact hole formed in the interlayer insulating film, wherein the interlayer insulating film is formed on the first photosensitive resin layer formed on the transistor and on the first photosensitive resin layer; Because it is constituted by the second photosensitive resin layer formed in, by exposing,
Irregularities can be formed on the interlayer insulating film.

The second photosensitive resin layer can form a predetermined uneven pattern whose height is regulated by the first photosensitive resin layer by exposure. Further, since the first photosensitive resin layer and the second photosensitive resin layer have different photosensitivities, the contact holes and the concavo-convex pattern can be easily formed by changing the exposure amount.

Further, since at least one of the first photosensitive resin layer and the second photosensitive resin layer is colored, reflection from unnecessary portions can be suppressed.

In the method for manufacturing a liquid crystal display device according to the present invention, a first step of forming a switching element on a substrate and a second step of forming a first photosensitive resin layer on the switching element And a third step of exposing the first photosensitive resin layer to form a contact hole so as to align with the switching element, and forming a second photosensitive resin layer on the first photosensitive resin layer. A fourth step of forming and a fifth step of exposing the second photosensitive resin layer to form a predetermined pattern including a contact hole aligned with the contact hole formed in the first photosensitive resin layer And forming a pixel electrode on the second photosensitive resin layer so as to be connected to the switching element via a contact hole formed in the first photosensitive resin layer and the second photosensitive resin layer. Six Since including the extent, by exposing, it is possible to form an uneven pattern on the second photosensitive resin layer.

Furthermore, a first step of forming a switching element on the substrate, a second step of forming a first photosensitive resin layer on the switching element, and a second step of forming a first photosensitive resin layer on the switching element. A third step of forming a second photosensitive resin layer having a different photosensitivity from the one photosensitive resin layer, and switching by exposing the first photosensitive resin layer and the second photosensitive resin layer. A fourth step of forming a contact hole so as to match the element, a fifth step of forming a predetermined pattern by exposing the second photosensitive resin layer, and connecting to the switching element through the contact hole As described above, since the sixth step of forming a pixel electrode on the second photosensitive resin layer is included, a pattern of concavities and convexities can be formed on the second photosensitive resin layer by exposure.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a TFT array substrate according to a first embodiment of the present invention.

FIG. 2 is a plan view showing a TFT array substrate according to the first embodiment of the present invention.

FIG. 3 is a sectional view illustrating a manufacturing process of the TFT array substrate according to the first embodiment of the present invention;

FIG. 4 is a cross-sectional view showing a step of manufacturing a TFT array substrate according to Embodiment 2 of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Insulating substrate, 2 Gate electrode wiring, 2a Gate electrode, 3 Common electrode wiring, 4 Gate insulating film, 5
a-Si film, 6 n ⁺ -a-Si film, 7 source electrode wiring, 7a source electrode, 8 drain electrode, 9
Channel section, 10 passivation film, 11 interlayer insulating film, 11a first interlayer insulating film, 11b second interlayer insulating film, 12 contact holes, 13 reflective pixel electrode.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H01L 21/336 (72) Inventor Keisuke Nakaguchi 997 Miyoshi, Nishi-Koshi-cho, Kikuchi-gun, Kumamoto Prefecture Advanced Display Co., Ltd. (72) Inventor Takamitsu Ishikawa 997 Miyoshi, Nishi-Goshi-cho, Kikuchi-gun, Kumamoto Prefecture F-Term in Advanced Display Co., Ltd. EA06 EA07 FA04 FB01 FB20 GB01 5F110 AA09 AA18 CC07 DD02 EE04 EE44 FF03 FF30 GG02 GG15 GG45 HK09 HK16 HK33 HK35 HM04 NN03 NN24 NN27 NN35 NN36 NN40 QQ09 QQ19

Claims (7)

[Claims] 1. A liquid crystal display device comprising: a substrate having a plurality of scanning lines and a switching element formed at an intersection of a plurality of signal lines formed to intersect the scanning lines; Formed on the switching element,
An interlayer insulating film in which concavities and convexities are formed on a surface opposite to the substrate and a contact hole is formed so as to match the switching element, formed on the concavo-convex surface of the interlayer insulating film, and formed on the interlayer insulating film; A pixel electrode connected to the switching element through a contact hole, wherein the interlayer insulating film is formed on the first photosensitive resin layer formed on the switching element and on the first photosensitive resin layer. A liquid crystal display device comprising a second photosensitive resin layer formed as described above. 2. A plurality of scanning lines, a plurality of signal lines formed so as to intersect the scanning lines, a gate electrode and a signal line arranged at the intersection of the scanning lines and the signal lines and connected to the scanning lines. A liquid crystal display device having a substrate on which a transistor having a source electrode and a drain electrode connected to the substrate is formed. The liquid crystal display device is formed on the transistor on the substrate. An interlayer insulating film in which a contact hole is formed so as to match with a pixel electrode formed on an uneven surface of the interlayer insulating film and connected to the drain electrode through a contact hole formed in the interlayer insulating film; The interlayer insulating film comprises a first photosensitive resin layer formed on the transistor and a second photosensitive resin layer formed on the first photosensitive resin layer. A liquid crystal display device comprising: 3. The second photosensitive resin layer, wherein a predetermined pattern whose height is regulated by the first photosensitive resin layer is formed by exposure. The liquid crystal display device as described in the above. 4. The method according to claim 1, wherein the first photosensitive resin layer and the second photosensitive resin layer have different light sensitivities.
Or the liquid crystal display device according to claim 2. 5. The liquid crystal display according to claim 1, wherein at least one of the first photosensitive resin layer and the second photosensitive resin layer is colored. apparatus. 6. A first step of forming a switching element on a substrate, a second step of forming a first photosensitive resin layer on the switching element, and exposing the first photosensitive resin layer. A third step of forming a contact hole so as to be aligned with the switching element, a fourth step of forming a second photosensitive resin layer on the first photosensitive resin layer, A fifth step of exposing the conductive resin layer to form a predetermined pattern including a contact hole that matches the contact hole formed in the first photosensitive resin layer, the first photosensitive resin layer and the second step A liquid crystal display comprising a sixth step of forming a pixel electrode on the second photosensitive resin layer so as to be connected to a switching element via a contact hole formed in the photosensitive resin layer. Equipment made Construction method. 7. A first step of forming a switching element on a substrate, a second step of forming a first photosensitive resin layer on the switching element, and a first step of forming a first photosensitive resin layer on the switching element. The third step of forming a second photosensitive resin layer having a different photosensitivity from the photosensitive resin layer of the above, by exposing the first photosensitive resin layer and the second photosensitive resin layer, the switching element A fourth step of forming a contact hole so as to match with, a fifth step of forming a predetermined pattern by exposing the second photosensitive resin layer, connected to the switching element via the contact hole A method for manufacturing a liquid crystal display device, comprising: forming a pixel electrode on the second photosensitive resin layer.