FR2733342A1 - Active matrix LCD screen panel prodn. - Google Patents

Abstract

A method of manufacturing a plate of an active matrix liquid crystal display screen, and a plate obtained by this method. The method of the invention comprises forming on a screen plate first tiles (124) forming pixels, address rows and columns (122) and control transistors; electrical insulation of columns, lines and transistors; then forming second tiles (158) forming pixels, the second tiles covering the first tiles (124) and being larger than the first tiles.

Description

METHOD FOR MANUFACTURING A PLATE OF A SCREEN
LIQUID CRYSTAL AND ACTIVE MATRIX DISPLAY, AND
PLATE OBTAINED BY THIS PROCESS
DESCRIPTION
Technical area
The present invention relates to a method of manufacturing a plate of a liquid crystal display screen and an active matrix, as well as a plate of a display screen obtained according to this method.

 The invention applies especially to the production of display cells in which the plate of the invention is associated with a counter-plate to form the main planar faces of a screen. Such screens can be used as a display screen of a computer terminal or as a projection screen, for example.

State of the art
A liquid crystal display screen generally comprises two plates kept at a distance from each other and forming walls of a cell filled with liquid crystals.

 A layer of liquid crystals sandwiched between the plates of the screen has optical properties which can be modified locally in order to form an image. The local modification of the optical properties results from the application of a voltage between conductive armatures, disposed respectively on each plate and mutually forming capacitors. The liquid crystal filling the space between the plates constitutes a dielectric for each capacitor. An active matrix display screen comprises a plurality of image points or pixels each comprising at least one capacitor of the type described above and at least one control transistor preferably produced using thin film techniques also known as TFT ('Thin Film Transistor "). In this type of screen, an electronic memory formed essentially by the capacitors of the image points distributed over the entire surface of the flat screen, stores a video signal throughout the duration of an image and the liquid crystals placed between the plates of the capacitors of each pixel are excited according to the polarization of the pixel.

In a known display screen structure, as described for example in document (1)
FR-A-2 679 057, one of the plates of the screen comprises on a transparent substrate, such as a glass plate, a plurality of conductive armatures arranged in rows and columns and forming with one or more against -electrodes of a counterplate of the capacitors of the active matrix.

 Transistors of the TFT type, associated respectively with the pixels comprise a first channel electrode, for example the drain, connected respectively to an armature of the pixel and a second channel electrode, for example the source, connected to an addressing column.

 Figures 1 to 7 provide a better understanding of the structure and an embodiment of active matrices of display screens of a known type.

 In a first operation illustrated in section in FIG. 1, an optical mask 12 called "black grid" is first formed on a glass substrate 10. This optical mask, for example in chrome, makes it possible to protect the control transistors of the active matrix from ambient light which is capable of causing parasitic photocurrents in these transistors and of disturbing the operation of the display screen. As shown in Figure 2, which is a top view of the structure of Figure 1, the mask 12 has openings 14. These openings correspond substantially to the locations of the frames of the pixels of the active matrix.

 After the formation of a first passivation layer 18, visible in FIG. 3, a layer 20 of an electrically conductive and transparent material, for example of indium tin oxide or ITO, is deposited over the whole of the plate.

 Using a mask. etching, not shown, is formed in the metal layer 20 of the conductive patterns which correspond to the addressing columns 22 and the frames 24 of the pixels. As shown in Figure 4, the frames 24 are substantially aligned with the openings 14 of the optical mask 12 while the addressing columns 22 coincide with the opaque parts of the mask 12. The distance separating the edge of the is designated by R opening 14 of the mask 12 and the nearest edge of an addressing column 22.

 Furthermore, a slight overlap can be noted between the armatures 24 and the optical mask 12. This overlap creates an additional capacitor for storing the video signal applied to the pixel. This capacitor has a reduced reinforcement area but benefits from an excellent dielectric, formed by the passivation layer 18.

 The layer 18 is, in fact, generally made of a better electrical insulating material than the liquid crystal. An image signal applied to a pixel is therefore stored on the one hand in the capacitor formed by the armature 24 of the pixel and a counter-electrode on the counter-plate (not shown) and on the other hand in the capacitor formed by a peripheral part of the armature 24 and the optical mask 12, which is, for this purpose, connected to a constant potential relative to that of the counter-electrode (not shown) of the counter-plate.

 The overlap between the frames 24 of the pixels and of the mask 12 also appears in FIG. 5, which is a top view of the structure of FIG. 4.

 I1 also appears in FIG. 5 that the frames 24 are equipped with an appendage 26, and that a track forming an elbow 28, connected to the columns 22, surrounds each appendage 26.

 Transistors referenced 30 in FIG. 6 of the TFT type are formed between the columns 22 and the armatures 24. The columns 22 and the armatures 24 respectively form the channel electrodes, that is to say the drain and the source, of the transistors 30 The transistors 30 are formed by a successive stack of a semiconductor layer 32 for example of hydrogenated amorphous silicon, an insulating layer 34 for example of silicon nitride and a conductive layer 36, for example of aluminum. These layers are etched to also form address lines 38 visible in FIG. 7, which is a top view of the structure of FIG. 6. The transistors 30 are formed at the intersection of the lines 38 with the columns 22 and appendages 26 of the frames 24. For a more detailed description of such screens, reference may be made, for example, to document (1) referenced above.

 To improve the brightness or the luminosity of such a display screen, one solution consists in increasing the area of the pixels relative to the total area of the screen. When the display screen includes an optical mask, this consists in increasing the size of the openings of this mask. To this end, the opening ratio denotes the ratio between the surface area of the openings in the mask 14 relative to the total surface area.

 One of the main limitations of the opening rate of a display screen is related to the formation of the frames and the addressing column tracks.

 Indeed, during the etching of the columns 22 and of the frames 24 of the pixels, problems of physical separation of these parts appear. To obtain a good production efficiency of screen plates with good physical and electrical separation of columns and frames, it is necessary to leave a large spacing between them. By way of example, a spacing of 8 μm is typically left between the addressing columns and the pixel frames on screens of the 250 mm diagonal type.

This spacing is still 6 μm on screens of the 94 mm diagonal type.

 The necessary spacing between the pixel frames and the addressing columns therefore prevents the relative size of the frames from being increased, and consequently the screen opening rate.

 Another limitation to the rate of opening of the screens, when they are equipped with an optical mask, is linked to the precision of alignment of the pixel frames and of the addressing columns with respect to the optical mask. this alignment precision is typically of the order of 2.5 μm.

 Yet another limitation to the opening rate can be imposed by the possible need for additional storage capacity, defined by the covering of the frame and the optical mask which is of the order of 4 to 5 μm.

 It follows that, in a plate structure as shown in FIGS. 4 and 5, the distance R between the columns and the corresponding edge of an opening of the optical mask is of the order of 8 μm to 12 μm.

 An object of the present invention is therefore to propose a method of manufacturing a liquid crystal display screen making it possible to eliminate the problems of physical separation between the columns and of the pixel arrays during engraving, and to increase thus opening this screen.

 Another object of the present invention is to provide a method of manufacturing a display screen which allows excellent alignment between the conductive patterns of the screen with an optical mask for protecting the transistors.

 Yet another object is to propose a method for manufacturing a display screen requiring a minimum number of masking steps.

Statement of the invention
To achieve these goals, the present invention relates more precisely to a method of manufacturing a plate for a liquid crystal and active matrix display screen, the plate forming one face of the display screen and being equipped with '' a plurality of conductive armatures made of a transparent material, defining image points and arranged according to lines and columns of armatures, of a plurality of control transistors connected respectively by a first channel electrode to each armature, of tracks electrically conductive, said addressing columns, connected respectively to a second channel electrode of each control transistor respectively of an armature column, the address columns being respectively disposed between the columns of conductive armature, and electrically tracks conductive, called address lines connected respectively to an electrode, this control of the control transistors respect ive of a reinforcement line, the method comprising the following stages - formation of first blocks and columns
addressing in a layer of conductive material
transparent electric covering a plate of
substrate, the first blocks being arranged in
rows and columns of cobblestones, and spacing
being respectively preserved between each column
and the blocks of each column of blocks, - formation of the address lines and the transistors
control.

According to the invention, the method further comprises - the formation of electrical insulation of the columns
addressing lines and transistors
the electrical insulation extending in
part of said spacing around and along
addressing lines and columns, - the formation of second blocks larger than the
first paving stones, covering the first paving stones and
extending to the electrical insulation of
addressing columns, the second blocks forming,
with the first paving stones, the conductive armatures
image points.

 Thanks to the invention, the distance between the addressing columns and the first blocks, which are formed during the same etching step can be chosen to be very large since it does not condition the final size of the frames of the pixels. this amounts to producing the first small blocks, the simultaneous etching of which with the column tracks poses no problem of physical separation.

 The risk of short circuit between these parts is therefore reduced.

 Furthermore, the second blocks can be large also without risk of short circuit with the columns due to the electrical insulation of the latter. This improves the screen opening rate.

 According to a particularly advantageous aspect of the invention which applies more precisely to screens of the type comprising an optical mask for protecting the transistors, it is possible to form the second blocks and / or the isolation of the addressing columns in self-alignment with this optical mask. Thanks to this feature, additional mask formation operations can be avoided. To make a screen according to the invention, only three masking steps are necessary and relate successively to the formation of the optical mask, the etching of the first blocks and of the columns and the etching of the control transistors preferably of the TFT type.

 The engraving of the insulation of the columns or the formation of the second blocks makes use of the optical mask.

 Thus for screens comprising such an optical mask, the method of the invention can be implemented without increasing the number of masks to be produced, compared with known methods.

According to a particular aspect of the invention, the method more precisely comprises the following successive steps - formation of the optical mask on a substrate
transparent, - deposit on the optical mask of a first layer of
passivation and then a first conductive layer
transparent, - etching of the transparent conductive layer for
form the addressing columns and the first
blocks, - formation of the control transistors, - deposit on the whole of the plate of a second
passivation layer, and a resin layer
photosensitive positive, - exposure of the resin through the optical mask
and removal of exposed parts of the resin to
form a first etching mask on the second
passivation layer, etching mask, auto
aligned with the optical mask, defining
the location of said electrical insulation of
addressing columns, - etching of the second passivation layer according to
said first etching mask to form said
electrical insulation, - formation of second paving stones by depositing and placing
form of a second transparent conductive layer.

 Advantageously, the second conductive layer can be formed by depositing a layer of ITO on the entire plate. This layer which covers the patterns of the first etching mask can be shaped by peeling by eliminating this mask.

 As a variant, it is also possible to eliminate the first etching mask, to deposit the transparent ITO layer, and to form thereon a second etching mask to define the size and the location of the second blocks.

 This second mask can advantageously be produced by depositing a negative photosensitive resin which is exposed through the optical mask and through the various transparent layers already formed. The elimination of the non-exposed parts makes it possible to shape the second mask.

 The invention also relates to a structure of a liquid crystal display screen plate, forming a face of the screen and comprising a plurality of conductive armatures defining image points and arranged in rows and columns of armatures, a plurality of control transistors connected respectively by a first channel electrode to each armature and electrically conductive tracks, called address columns, connected respectively to a second channel electrode of each control transistor respectively of a column of armatures , the addressing columns being respectively disposed between the columns of conductive armatures, characterized in that each armature comprises first and second blocks of transparent material, superimposed and mutually in contact, the first block of each armature and the columns d addressing being in the same plane of the structure, and the second block of each frame p parallel to said plane, an area greater than the area, respectively of the first block of each frame.

 The plane of the structure of the plate is understood to mean the plane of one of the layers of material formed on the plate. In this case, the first block and the addressing columns are formed in the same layer of transparent material initially formed on the plate as described above.

 Other characteristics and advantages of the invention will emerge more clearly from the description which follows, given purely by way of illustration and without limitation, with reference to the appended drawings.

Brief description of the figures.

FIG. 1, already described, is a schematic section of a detail of a display screen plate of a known type during manufacture illustrating a step of forming an optical mask,
FIG. 2, already described, is a top view of the detail in FIG. 1,
- Figures 3 and 4, already described, are schematic sections of a detail of a display screen plate of a known type during manufacture, illustrating the formation of pixel frames and addressing columns ,
FIG. 5, already described, is a top view of the detail of FIG. 4,
FIG. 6, already described, is a diagrammatic section of a detail of a display screen plate of a known type during manufacture, illustrating the manufacture of control transistors,
FIG. 7, already described, is a top view of the detail in FIG. 6,
FIGS. 8 and 9 correspond to FIGS. 1 and 2 and illustrate the manufacture of an optical mask from a plate of a display screen according to the invention,
FIGS. 10 and 11 are schematic sections of a detail of a display screen plate according to the invention during manufacture, illustrating the formation of first conductive blocks and of addressing columns,
FIG. 12 is a top view of the detail of the display screen during manufacture of FIG. 11,
FIG. 13 is a schematic sectional view of a detail of a display screen plate according to the invention, during manufacture, illustrating the manufacture of control transistors,
FIG. 14 is a top view of the detail of FIG. 13,
FIGS. 15, 16 and 17 are schematic sections of a detail of a plate of a display screen illustrating the steps for producing electrical insulation of the addressing columns,
FIGS. 18, 19 and 20 are schematic sections of a detail of a display screen plate illustrating the production of second conductive blocks, and
FIG. 21 is a top view of the detail in FIG. 20.

Detailed description of modes of implementing the invention
In FIGS. 8 to 21 concerning the invention, identical references to which the value 100 has been added denote parts which are identical or similar to parts of FIGS. 1 to 7 which relate to the state of the art.

 The steps for manufacturing an optical mask 112 on a substrate plate 110, made of glass for example, illustrated by FIGS. 8 and 9 correspond to those of FIGS. 1 and 2. A layer of chromium is deposited on the substrate which is then engraved to form the mask 112 with openings 114. These openings may however be larger than those of the usual masks as shown in the following description.

 The manufacturing of the screen continues as shown in FIG. 10 by the formation of a passivation layer 118, for example made of silicon oxide, which covers the substrate 110 and the mask 112, and of a layer 120 in one transparent and electrically conductive material such as ITO.

 The layer 120 is then etched through a mask, not shown, to form addressing columns 122 and first blocks 124. As appears in FIG. 11, but also in FIG. 12 which is a top view, the blocks 124 are smaller than the apertures 114 of the optical mask. Unlike the structure shown in FIGS. 4 and 5, there is no overlap or overlap between the blocks 124 and the opaque parts of the mask 112. A large spacing 125 is respectively left between each block 124 and each adjacent column 122.

 These spacings 125 make it possible to limit the risks of short circuit between the columns and the first blocks. R ′ denotes the distance separating an edge of a part of the mask 112 from the edge of the column 122 situated opposite this part of the mask.

 As shown in FIG. 12, the first blocks can each be fitted with an appendage 126, the end of which is surrounded by a portion of conductive track 128 forming an elbow and connected to the addressing column 122 corresponding to the block.

 FIG. 13 shows in section the control transistors 130. A transistor 130 is respectively associated with each pixel.

 Each transistor 130 is formed by the overlap of a control line 138 with a column 122, an appendage 126 and an elbow 128. The column 122 and the elbow 128 form for example the source of the transistor and the appendage 126 the drain. The control lines are formed by a successive deposition of a layer 132 of a semiconductor material such as hydrogenated amorphous silicon of a layer 134 of an electrical insulating material such as silicon nitride and then of a layer 136 of a conductive material such as aluminum, then by etching the stack of these layers. The semiconductor material of the line respectively forms the channel of each transistor of a line of pixels, the insulating material forms the gate insulation and the conductive material forms the gate of each transistor.

 As also shown in FIG. 14, the control lines 138 coincide with an opaque part of the mask 112 as well as the control transistors 130.

The optical mask 112 has the function of limiting parasitic photocurrents in these transistors.

 The manufacture of the screen continues, as shown in FIG. 15 by the deposition on the entire structure of a second passivation layer 140 made of a transparent material such as silicon nitride.

The layer 140 covers the first blocks, the rows and the addressing columns, the transistors 130 and the spacings 125. A layer of positive photosensitive resin 142 is formed on the layer 140. A positive resin is understood to be a product sensitive to UV radiation Any part of this resin that has been exposed to UV will be removed by the developer during development.

 The assembly is then subjected, on the side of the substrate 110, to an exposure symbolized by arrows 144. The photosensitive resin is thus subjected to the action of ultra violet light through the optical mask 112.

 After development of the resin and elimination of the exposed parts, as shown in FIG. 16, a first etching mask 146 self-aligned with the optical mask 112 is obtained.

 For reasons of simplification, the transistors are not shown in FIG. 18 and following. The passivation layer 140 is etched for example by reactive ion etching (GIR) according to the mask 146, then this mask is eliminated. The structure of FIG. 17 is obtained. As shown in the figure, electrical insulations 148 extend along and around the addressing columns 122. The insulations also extend in part of the spaces 125 along the columns, but do not reach the blocks 124 which are smaller than the openings 114 of the mask 112. This is due to the fact that the mask 146 and therefore the insulations 148 are self-aligned on the mask 112.

 Although FIG. 17 does not show it, the electrical insulation 148 also covers the address lines, the control transistors and more generally all the parts coinciding with the opaque regions of the mask.

 The manufacture of the screen continues as shown in FIG. 18 by the deposition of a second layer 150 of transparent and conductive material, for example, of ITO, then by the coating of this layer with a second layer 152 of negative photosensitive resin.

 Negative resin is understood to mean a photosensitive product to UV radiation. All or part of this resin which will have been exposed to UV rays will be insoluble in the developer during development. We then form the negative of the mask. The layer 150 covers both the insulations 148 and the blocks 124.

 A second insolation, represented by arrows 154 is applied on the side of the substrate 110. The resin of the layer 152 is thus subjected to the action of ultraviolet light through the optical mask 112. After insolation, development and elimination of the parts not exposed, a second etching mask 156 is obtained which is self-aligned with the optical mask 112, and more precisely disposed above the openings 114 of the optical mask (FIG. 19).

 This mask is represented in FIG. 19. An etching of the layer 150 of ITO for example according to a technique of etching with reactive ions called RIE ("Reactive Ion Etching") in the parts not protected by the mask 156, or by immersion in a solution containing hydrochloric acid, makes it possible to form second conducting blocks 158 visible in FIG. 20.

 As shown in FIGS. 20 and 21, the blocks 158 which form, with the blocks 124, the frames of the pixels extend as far as the regions protected by the insulation 148.

 The blocks 158 cover not only the blocks 124 but also the spaces 125 not yet occupied by the insulation 148; they are also aligned with the openings 114 of the optical mask 112.

 FIG. 21 makes it clear that the risks of short circuit between the armatures and the control lines and columns are avoided. These risks are avoided on the one hand because the first blocks are sufficiently far from the addressing columns and, on the other hand, because the insulation 148 prevents any contact between the second, larger blocks with the lines of addressing, addressing columns and transistors.

 In addition, as the second tiles are larger, the opening of the screen is improved.

 Furthermore, the correct alignment of the second blocks with respect to the optical mask makes it possible to increase the area of the openings 114.

 For example, while the distance R between the edge of the mask openings and the corresponding columns is of the order of 8 to 12 μm on conventional screens, the distance R ′ can be reduced to approximately 2 or 3 μm in a realization as described. An increase in the screen opening rate of 15 to 30% can be obtained.

 In a particular embodiment of a screen according to the invention, it is possible to overexpose the resin layers 142 and 152, which has the effect of obtaining electrical insulations 148 extending less in the space 125 and / or second pavers larger than the mask openings. An overlap is thus obtained between the mask and the second blocks. This overlap constitutes an additional capacitor for storing the video signal as described above.

Claims (12)

 1. A method of manufacturing a plate for an active matrix liquid crystal display screen, the plate forming one face of the display screen and being equipped with a plurality of conductive armatures (124, 158) of a transparent material defining image points and arranged along lines and columns of armatures, of a plurality of control transistors (130) connected respectively by a first channel electrode to each armature, of tracks (122) electrically conductive, called addressing columns, connected respectively to a second channel electrode of each control transistor (130) respectively of an armature column, the addressing columns (122) being respectively arranged between the columns conductive armatures, and electrically conductive tracks (138), said address lines respectively connected to a control electrode of the control transistors of a line of armatures, the method com with the following steps - formation of the first blocks (124) and columns  addressing (122) in a first layer of  transparent material, electrical conductor  covering a transparent substrate plate (110),  the first blocks being arranged in lines and  columns of pavers, and a spacing (125) being  respectively preserved between each column  addressing (122) and the blocks (124) of each  column of cobblestones, - formation of address lines (138) and  control transistors (130), characterized in that the method further comprises - forming electrical insulation (148) of the  addressing columns (122) of the addressing lines  (138) and control transistors (130),  the electrical insulation (148) extending in a  part of said spacing (125) around and along the  addressing lines and columns, - the formation of second blocks (158) larger than  the first paving stones (124), covering the first  paved and extending to electrical insulation  (148) addressing columns, the second blocks  forming, with the first paving stones, the reinforcements  conductive of the image points.  2. A method of manufacturing a plate for a display screen according to claim 1, the plate being further equipped with an optical mask (112) for protecting the control transistors (130), disposed on the substrate ( 110), covering the transistors and the addressing columns and comprising openings (114) corresponding to the image points, characterized in that the second blocks (158) are formed in self-alignment with the openings (114) of the optical mask (112 ).  3. A method of manufacturing a plate for a display screen according to claim 1, the plate being further equipped with an optical mask (112) for protecting the control transistors (130), covering the transistors (130 ) and the addressing columns (122) and comprising openings (114) corresponding to the image points, characterized in that the electrical insulation (148) of the addressing columns (122) is formed according to a photolithography process and in self-alignment with the optical mask (112).  4. A method of manufacturing a plate for a display screen according to claim 3, characterized in that it comprises the following successive steps - formation of the optical mask (112) on a substrate  transparent (110), - deposition on the optical mask of a first layer of  passivation (118) then of the first layer  transparent conductive (120), - etching of the first conductive layer  transparent (120) to form the columns  addressing (122) and the first blocks (124), - forming the addressing lines (138) of the  control transistors (130), - deposition on the entire plate of a second  passivation layer (140), and a layer of  positive photosensitive resin (142), - exposure of the resin (142) through the mask  optics (112) and elimination of exposed parts of  the resin to form a first etching mask  (146) on the second passivation layer (140), the  engraving mask (146), self-aligned with the mask  optical (112), defining the location of said  electrical insulation of the addressing columns, - etching of the second passivation layer (140)  according to said first etching mask (146) for  forming said electrical insulation (148), - forming the second blocks (158) by depositing and placing  in the form of a second conductive layer  transparent (150).  5. A method of manufacturing a plate of a display screen according to claim 4, characterized in that, to form the second blocks (158), is deposited on the plate a second layer (4) transparent conductive (150 ), covering the whole of the plate and the first etching mask (146), then the first etching mask is eliminated and, by peeling, part of the second transparent conductive layer (150) covering the first etching mask ( 146).  6. A method of manufacturing a plate for a display screen according to claim 4, characterized in that, to form the second blocks (158), a second transparent conductive layer is deposited over the whole of the plate ( 150) after elimination of the first etching mask (146), a second etching mask (156) is defined on this second layer defining the location of the second blocks, the second transparent conductive layer (150) is etched according to the second mask ( 156) to form the second blocks (158), then the second etching mask is eliminated.  7. A method of manufacturing a plate of a display screen according to claim 6, characterized in that to form the second etching mask (156), a layer of negative photosensitive resin (152) is deposited on the second conductive layer (150), the negative photosensitive resin layer is exposed through the optical mask (112) and non-insolated parts of this layer are removed.  8. A method of manufacturing a plate of a display screen according to any one of the preceding claims, characterized in that a passivation of the plate is carried out after the formation of the second blocks.  9. A method of manufacturing a plate of a display screen according to any one of the preceding claims, characterized in that the first and second transparent conductive layers are made of indium tin oxide (ITO).  10. A method of manufacturing a plate of a display screen according to any one of the preceding claims, characterized in that the first and second passivation layers are respectively of silicon oxide and silicon nitride.  11. A method of manufacturing a plate of a display screen according to claim 7, characterized in that during the exposure of the positive photosensitive resin and / or during the second exposure of the negative photosensitive resin, overexposure.  12. Structure of a liquid crystal display screen plate, forming a face of the screen and comprising a plurality of conductive frames (124, 158) defining image points and arranged in rows and columns of frames , a plurality of control transistors (130) connected respectively by a first channel electrode to each armature and electrically conductive tracks (122), called address columns, connected respectively to a second channel electrode of each control transistor ( 130) respectively of a column of reinforcements, the addressing columns (122) being respectively arranged between the columns of conductive reinforcements, characterized in that each reinforcement comprises a first and a second block (124, 158) of material transparent, superimposed and mutually in contact, the first block (124) of each frame and the addressing columns (122) being in the same plane of the structure, and the second block (158) of each frame having, parallel to said plane, an area greater than the area, respectively of the first block of each frame.