WO2003059627A1

WO2003059627A1 - Droplet deposition apparatus

Info

Publication number: WO2003059627A1
Application number: PCT/GB2003/000140
Authority: WO
Inventors: Werner Zapka; Lars Levin
Original assignee: Xaar Technology Limited
Priority date: 2002-01-15
Filing date: 2003-01-15
Publication date: 2003-07-24
Also published as: AU2003201678A1; CN1310761C; CN1612809A; GB0200852D0

Abstract

Droplet deposition apparatus comprises a plurality of parallel channels each defined at least in part by facing side walls of piezoelectric material, and electrode means for the application of an electric field to selected ones of the side walls. The electrode means are connected to an electrical drive circuit by connection tracks located within grooves extending along the side walls.

Description

DROPLET DEPOSITION APPARATUS

The present invention relates to droplet deposition apparatus, and a method of manufacturing the same.

Figures 1 to 3 illustrate, in perspective and sectional views respectively, a typical ink jet printhead 8 incorporating piezoelectric wall actuators operating in shear mode and known, for example, from US-A-5 016 028. It comprises a base 10 of piezoelectric material mounted on a circuit board 12, of which only a section showing connection tracks 14 is illustrated.

A multiplicity of parallel grooves are formed in the base 10 extending into the layer of piezoelectric (PZT) material, as is described, forexample, in US-A-5016028. Each groove comprises a forward part which is comparatively deep to provide ink channels 20 separated by opposing actuator walls 22 having uniformly co-planar top surfaces, and a rearward part which is comparatively shallow to provide locations 23 for connection tracks. Forward and rearward parts are connected by a "runout" section R of the channel, the radius of which is determined by the radius of the cutting disc used to form the channels.

After forming the grooves, the walls are 'pre-passivated' with silicon nitride. The purpose of this is to cover the non-active parts of the walls with a thin (1 μm) layer of low dielectric material, prior to application of the electrodes. The parts of the walls intended to be active are masked from the pre-passivation. The silicon nitride is applied by electron cyclotron resonance chemical vapour deposition, using a plasma of silane, nitrogen and argon. The cyclotron radiation excites the plasma in such a way as to produce deposition of silicon nitride with very low inclusion of hydrogen, without producing excessive heating in order not to exceed the Curie temperature of the PZT.

As illustrated in Figure 3, after pre-passivation, metallized plating is deposited in the grooves to provide in the forward part electrodes 26 on the opposing faces of the ink channels 20. The electrodes extend approximately one half of the channel height from the tops of the walls (Figure 3(a) and (b)). The plating also provides in the rearward part connection tracks 24 connected to the electrodes 26 in each channel 20 (Figure 3(c)). The tops of the walls are kept free of plating metal so that the electrodes 26 form isolated actuating electrodes for each channel.

Following deposition and coating of the base 10 with a passivant layer of silicon nitride, acting as an ionic and electron barrier for electrical isolation of the electrode parts from ink to prevent corrosion, the base 10 is mounted as shown in Figure 1 on the circuit board 12 and bonded wire connections 28 are made to connect the connection tracks 24 on the base part 10 to the connection tracks 14 on the circuit board 12.

A cover 16 is attached to the tops of the actuator walls 22 thereby forming a multiplicity of "closed" channels 20. At one end, each channel has access to a supply of replenishment ink via window 27 in the cover 16. At the other end of each channel is located a nozzle 30 which may be formed (advantageously by UV excimer laser ablation) in a nozzle plate 17 bonded to the printhead.

In the example, the printhead is operated by delivering ink from an ink supply via the window 27, from where it is drawn into the ink channels 20 to the nozzles 30. As is known, for example, from EP-A-0 277 703, appropriate application of voltage waveforms to the electrodes on either side of a channel wall results in a potential difference being set up across the wall which in turn causes the poled piezoelectric material of the channel walls to deform in shear mode and the wall to deflect transversely relative to the respective channel. One or both of the walls bounding an ink channel can be thus deflected. Movement of the walls establishes an acoustic pressure wave in the ink along that length of the channel closed on the top and bottom by the base and cover respectively and closed on both sides by respective channel walls. This length is known as the "active" length of the channel and is denoted in figures 1 and 2 by "L". The acoustic pressure wave travels along the length of the channel and eject an ink droplet therefrom. The rationale for pre-passivation is as follows: because PZT has an extremely high dielectric constant, most of the voltage difference across the walls appears on the silicon nitride layer, and very little on the PZT. As a result, the stray capacitance of the non-active parts of the wall is minimised, which reduces currents and makes it easier for the drive chips to achieve a fast rise-time. In addition, the open-topped walls are not very stiff, and without pre-passivation they would cause unwanted acoustic waves in the manifold region of the printhead.

Furthermore, pre-passivation prevents excess build-up of heat in the printhead by preventing actuation of unnecessary portions of PZT, and thus reduces the total amount of heat generated during the printing operation. As disclosed in our co- pending International patent application no. PCT/GB01/00652, a heat sink may be attached to the top of the cover component 16 for dissipating heat generated in the channels during droplet ejection. However, in the arrangement shown in Figure 1 the requirement to provide ink supply via the cover component 16 limits the extent to which the heat sink may overlie the channels, thereby restricting the amount of heat which may be transferred from the ink channels to the heat sink.

The present invention seeks to solve this and other problems.

In one aspect the present invention provides droplet deposition apparatus comprising a plurality of parallel ejection channels each defined at least in part by facing side walls of piezoelectric material, and electrode means for the application of an electric field to selected ones of the side walls, wherein the electrode means are connected to a drive circuit by connection tracks located within grooves extending along the edges of the side walls defining the ejection channels.

In order to remove a greater amount of heat generated in the ink channels during deposition, the present inventors have realised the desirability to provide a heat sink extending over substantially the entire surface of the cover component, and thus relocate the ink supply from the cover component. However, in the prior arrangement shown in Figures 1 to 3, the need to provide the circuit board 12 and the bonded wire connections 28 at the rear of the ink channels means that the ink supply can not be conveniently relocated to the rear of the ink channels without interfering with these connections.

As the connection tracks of the present invention are located within grooves extending along the edges of the side walls of the ink channels, that is, not along the bottom of the rearward part of the channels as shown in Figure 3(c), connection of these tracks to the drive circuit can be performed by an alternative method, for example, using tape bonding to the rear surfaces of the side walls of the ink channels. This bonding is somewhat easier to perform than the bonding of the wire connections 28 described above to connect the connection tracks 24 on the base part 10 to the connection tracks 14 on the circuit board 12, and would not interfere with the provision of an ink supply at the rear of the channels. Accordingly, the entire upper surface of the cover component can now become available for contact to a heat sink for efficient cooling of the apparatus.

Preferably, each groove comprises a substantially L-shaped (that is, having a bottom surface and a single side wall) groove or notch extending along a respective edge of a side wall. In one embodiment, the connection track extends along the bottom surface only of an L-shaped groove. As the connection tracks no longer extend along the side walls of the channels, but are provided on the bottom surfaces of grooves extending from the edges of the side walls of the ink channels, there is no longer the need to pre-passivate the walls of the channels prior to electrode deposition. Accordingly, a manifold can be provided in the cover component to enable ink to be supplied to the channels; as the connection tracks do not lie on the facing side walls of the open-topped portions of the channels, there are no problems associated with the generation of acoustic waves in the ink in these portions of the channels.

In one embodiment, each wall has a groove extending along on one side thereof, a connection track located within the groove connecting the drive circuit to electrode means provided on that side of the wall. In another embodiment, each wall has grooves extending along opposite sides thereof, connection tracks located within the grooves connecting the drive circuit to respective electrode means provided on the opposite sides of that wall.

Preferably, each channel is defined by the facing side walls and a bottom surface extending between the side walls, and wherein the open side of the channel is closed over an active channel length by a cover secured to the side walls. Preferably, the bottom surface is defined by a base, the base and the side walls being integral.

The apparatus preferably comprises liquid supply means for supplying liquid to the channels for replenishment of droplets ejected from the channels. In one embodiment a nozzle plate defining an ejection nozzle for each channel is located at one end of the channels, the liquid supply means being located at the other end of the channels.

The connection tracks may be conveniently formed from electrically conductive paste.

In another aspect the present invention provides a method of manufacturing droplet deposition apparatus, the method comprising the steps of: forming in a sheet of piezoelectric material a plurality of first parallel channels having a first depth; depositing electrically conductive material within the first channels; forming in the sheet a plurality of second parallel channels extending parallel to the first channels and having a second depth greater than the first depth, at least one of the facing side walls of each of the second channels being located within a respective first channel and displaced from a side wall of that first channel; and selectively depositing electrode material on at least a portion of the facing side walls of each of the second channels so as to contact the electrically conductive material within the remainder of the first channel and thereby enable electrical signals to be conveyed to the electrodes in a second channel wall for actuation thereof.

Preferably the step of depositing the electrically conductive material comprises depositing the material over the entire surface of the sheet, and subsequently removing material deposited on the top surfaces of the walls of the first channels. In the preferred embodiments the material is removed by machining.

Preferably a cover is bonded to the sheet to close portions of the channels over an active length, and a heat sink may be attached to the cover.

In one embodiment, prior to depositing electrically conductive material within the first channels, a plurality of third parallel channels extending perpendicularly to and intersecting the first channels are formed in the sheet, the electrically conductive material being subsequently deposited within both the first and third channels.

The method may comprise the steps of sectioning the sheet along first section lines parallel to the third channels and intersecting the electrodes, and sectioning the sheet along second section lines parallel to the third channels and intersecting respective third channels such that segments of the material deposited in the third channel provide sites for electrically connecting the electrodes to external drive circuitry. Preferably a nozzle plate is applied at the location of a first section line to define nozzles for droplet ejection. Preferably liquid replenishment means are provided at the location of a second section line.

In preferred embodiments the channels are formed by the removal of material from the sheet, for example by sawing.

Features described above relating to method aspects of the present invention can be applied to apparatus aspects, and vice versa.

The invention is further illustrated, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of one form of conventional inkjet printhead;

Figure 2 is a sectional view of the printhead of Figure 1 taken along the line A-A of Figure 1 ;

Figures 3(a), (b) and (c) are sectional views of part of the printhead of Figure 1 taken along lines B-B, C-C and D-D respectively;

Figures 4 to 9 are perspective views illustrating steps in the formation of a first embodiment of a printhead in accordance with the present invention;

Figure 10 is a sectional view of the printhead of Figure 9 taken along the line E-E of Figure 9;

Figures 11 (a) and (b) are sectional views of part of the printhead of Figure 10 taken along section lines F-F and G-G respectively;

Figures 11 (c) and (d) are sectional views similar to those of Figures 11 (a) and 11 (b) respectively except that conductive material fills the grooves formed along the channel walls;

Figures 12 to 16 are perspective views illustrating steps in the formation of a second embodiment of a printhead in accordance with the present invention;

Figure 17 is a sectional view of the printhead of Figure 16 taken along the line H-H of Figure 16 with cover component attached;

Figure 18 is a perspective view of the printhead of Figure 17 following sectioning (with cover removed); and

Figure 19 is sectional view illustrating steps in the formation of a third embodiment of a printhead according to the present invention. Steps in the formation of a printhead according to a first embodiment of the present invention will now be described with reference to Figures 4 to 11.

With reference first to Figure 4, a sheet 100 of piezoelectric material, such as lead zirconium titanate (PZT) is provided. The PZT sheet 100 comprises two layers of PZT, one layer poled in the direction Z and the other poled in the opposite direction. A plurality of substantially parallel-sided shallow first channels 102 are formed in the PZT sheet 100, for example, by cutting or otherwise forming grooves of rectangular cross-section through the PZT sheet 100 (only eight first channels are shown in Figure 4 for simplicity only). Cutting may be performed using parallel, diamond dust impregnated disks mounted on a common shaft, or by laser cutting. In this embodiment the first channels 102 have a width of 65μm and a depth of 30μm, with a spacing between the first channels 102 substantially equal to the spacing of the ink channels to be subsequently formed in the PZT sheet 100 (approximately 140μm).

With reference to Figure 5, following formation of the first channels 102 a plurality of substantially parallel-sided cross-channels 104 are similarly formed in the PZT sheet 100 substantially orthogonal to the first channels 102. In this embodiment the cross-channels 104 also have a width of 65μm, but extend to a depth of 10Oμm, with a spacing between adjacent cross-channels 104 of 3mm.

The entire top surface of the PZT sheet 100 is subsequently coated in electrically conductive material 106 so as to completely fill the channels. In order not to depolarize the PZT sheet 100, a low temperature, preferably room temperature, curing paste, as described, for example, in a brochure entitled "Product Data Sheet on Screen-Printable Ag, Ag/AgCI and C pastes" from Acheson Coloiden BV, Scheemda, Netherlands, is used. Following curing, the paste is removed from the top surfaces 108 of the sheet 100 lying between the channels, for example, by micro-machining. In one embodiment the resultant structure is illustrated in Figure 6, with the paste effectively coating the walls of the first channels 102 and cross- channels 104. In another embodiment, the paste effectively fills the first channels 102.

Other suitable methods for depositing the electrically conductive material 106 includes electroless nickel electroplating, sputtering and the like.

With reference to Figure 7, a plurality of substantially parallel-sided second channels 110 of uniform depth are formed in the PZT sheet 100. These second channels 110 provide the ink ejection channels. The ink channels 110 are formed substantially in parallel with the first channels 102, have the same width (approximately 65μm) as the first channels 102, and are slightly displaced relative to the first channels 102 by a distance of approximately 20μm such that one wall 112 of each ink channel 110 is formed within a first channel 102 and the other wall 118 of each ink channel is formed between first channels 102. This results in each ink channel 110 having a width of 85μm to a depth of 20μm, and a width of 65μm for the remainder thereof, with one wall 112 containing a groove 114 extending the entire length thereof, a track 116 of electrically conductive material lying within the groove 114, and the other channel wall 118 being planar.

As shown in Figure 8, metal electrodes 120 are subsequently deposited on the front, active area of each wall 112, 118 of the ink channels 110. Prior to metallization, a mask is formed in the places where electrode material, such as aluminium, is not required; a liftoff subsequently removes the mask and aluminium where electrode material is not required. As an alternative, a line of sight metal vapour deposition may be used to deposit the metal electrodes. As the PZT sheet 100 comprises two oppositely poled layers of piezoelectric material, the walls 112, 118 of the ink channels 110 serve as wall actuators of the so called "chevron" type, such as are the subject of European Patents No. 0277703 and No. 0278590, the disclosures of which are incorporated herein by reference. These actuators are known to be advantageous because they require a lower actuating voltage to establish the same pressure in ink channels during operation. In order to connect the electrode material deposited on the side walls 112,118 to the track 116, the electrode material extends the full depth of the side walls 112, 118 and over the bottom surface of the active part of the ink channel 110.

After metallization, silicon nitride passivation is applied to provide an ionic and electron barrier to prevent corrosion of the tracks 116 and electrodes 120 by the ink. With reference to Figures 9 and 10, a cover component 122 is then adhered to the top surfaces 108 of the channel walls. The cover 122 is formed with a series of parallel undercuts 124 which serve to define the active length L (approximately 1 mm) of each ink channel 110.

All of the processes so far are carried out at wafer scale, so that a number of printheads are made all at once. At this stage the printheads are diced apart along first section lines F-F intersecting electrodes 120 and second section lines G-G intersecting cross-channels 104. With reference to Figure 11(a), sectioning along line F-F provides a planar front surface 126 devoid of metallic plating or paste to which a nozzle plate, similar to that shown in Figure 1 , is attached with a thin layer of adhesive. With reference to Figure 11(b), sectioning along line G-G exposes the back surfaces 128 of the side walls of the ink channels 110, each surface 128 having a bond pad 130 formed thereon (formed during the deposition of the electrically conductive paste within the cross-channels 104) which is electrically connected to a respective track 116. The bond pads 130 provide convenient sites for electrically connecting, for example, by tape electrical (anisotropic) bonding, the tracks 116 to a drive circuit for supplying the actuating voltages to the electrodes 120 via, for example, connections 28 shown in Figure 1. An ink supply (not shown) is then connected to the rear of the channels 110 for supplying ink to the channels, and a heat sink (not shown) is mounted on the top of the cover 122.

Figures 11 (c) and 11(d) are equivalent to Figures 11(a) and 11(b) for the embodiment where the paste effectively fills the groove 114, as mentioned earlier.

By locating the connection tracks 116 along shallow grooves 114 formed along the substantially fully-closed top edges of the walls 112 of the ink channels 110, there is no need to pre-passivate the walls 112, 118 of the ink channels. In addition, as the ink supply is attached to the rear of the channels, the entire top surface of the cover 122 becomes available for bonding to a heat sink, thereby improving cooling of the printhead. In comparison to the prior art printhead illustrated in Figures 1 and 2, there is no requirement to provide any "run out" of piezoelectric material rearward from the rear surface of the cover to provide a site for electrical connection of the tracks 116 to a drive circuit, thereby reducing the amount of piezoelectric material required to form the printhead.

Steps in the formation of a printhead according to a second embodiment of the present invention will now be described with reference to Figures 12 to 17.

Shallow first channels 102 having a depth of approximately 20μm and a width of 85μm are formed in the PZT sheet 100. Unlike the first embodiment, recessed cross-channels 104 having inclined side walls and a depth of 40μm are subsequently formed substantially orthogonal to the first channels 102, as shown in Figure 12. Metallization by vacuum deposition at normal incidence (90°) to the top surface 106 of the sheet is then performed, during which metal, such as aluminium, is deposited over the inclined and bottom surfaces of the recessed cross-channels 104, the bottom surfaces only of the first channels 102 and the top surfaces 106 of the sheet 100. As in the first embodiment, metal is then removed from the top surfaces 106 of the sheet 100 by micro-machining so that metal remains only within the channels 102 and 104, as shown in Figure 13. Alternatively, the top surfaces 106 may be masked during the metallization. Also as in the first embodiment, during the metallization the first channels 102 may be filled with metal.

With reference to Figure 14, a plurality of substantially parallel-sided second channels 110 of varying depth are formed in the PZT sheet 100. Each channel 110 is of width 65μm and is aligned centrally within a respective first channel 102, such that a groove 114 of width 10μm and containing a metal track 116 is formed on either side of a channel 110. Each channel is formed in one pass by sawing or dicing with a diamond/metal dicing blade, the blade depth being varied during its passage along the channel such that each channel 110 has a comparatively deep central portion to provide a pair of back-to-back ink channels each with relatively shallow end portions, similar to the grooves 20 described above with reference to Figures 1 and 2.

With reference to Figure 15, electrodes 120 are formed on the facing side walls of the ink channels using a combination of masking and metallization as described above. In this embodiment, the electrodes 120 on the facing side walls of each ink channel 110 are not connected across the bottom surface of the channel, the electrodes 120 extending to approximately half of the depth of the ink channels. As shown in Figure 16, grooves 140 are subsequently formed parallel to the first channels 102 and extending between the top surfaces 106 across the cross- channels 104 in order to define a series of bond pads 130 within the cross-channels 104.

As shown in Figure 17, a cover component 122 is subsequently attached to the tops 106 of the channels. Unlike the first embodiment, and similar to the prior art printhead described with reference to Figures 1 and 2, the cover component 122 has windows 142 formed therein to enable ink to be supplied to the ink channels 110. Sectioning is then performed along first section lines l-l each passing through the centre of a respective cross-channel 104 and second section lines J-J each positioned through the centre of the electrodes 120. As shown in Figure 18, each sectioned bond pad 130 provides an electrical connection point for individually connecting both of the connection tracks 116 and electrodes 120 associated with an ink channel 112 to a drive circuit (not shown).

In this second embodiment, unlike the first embodiment the ink supply is retained through the cover component, as in the prior art printhead described earlier. However, this second embodiment shares with the first embodiment the feature that there is no requirement to perform pre-passivation of the channels prior to formation of the connection tracks 116, as the connection tracks do not lie on the facing side walls of the ink channels 110. It will be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention.

For example, with reference to Figure 19, the electrodes 120 ofthe first embodiment may be formed centrally on the facing side walls of the ink channels 110, as in the second embodiment. With pairs of grooves 124 now formed in the cover 122, and sectioning performed along a line F-F passing through the centre of the deposited electrodes 120 so as to divide the ink channels 110 in two and along a line G-G passing through the groove 104, the assembly is sectioned into facing printheads, as in the second embodiment, the active length ofthe ink channel of each printhead being controlled, as in the first embodiment, by the positioning of the grooves 124 in the cover 122.

Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features.

Claims

1. Droplet deposition apparatus comprising a plurality of parallel ejection channels each defined at least in part by facing side walls of piezoelectric material, and electrode means for the application of an electric field to selected ones of the side walls, wherein the electrode means are connected to a drive circuit by connection tracks located within grooves extending along the edges ofthe side walls defining the ejection channels.

2. Droplet deposition apparatus according to Claim 1 , wherein each groove comprises a substantially L-shaped groove extending along a respective edge of a side wall.

3. Droplet deposition apparatus according to Claim 1 or 2, wherein each wall has a groove extending along on one side thereof, a connection track located within the groove connecting the drive circuit to electrode means provided on that side of the wall.

4. Droplet deposition apparatus according to any preceding claim, wherein each wall has grooves extending along opposite sides thereof, connection tracks located within the grooves connecting the drive circuit to respective electrode means provided on the opposite sides of that wall.

5. Droplet deposition apparatus according to any preceding claim, wherein each connection track terminates at an electrical termination located at an end surface of a side wall.

6. Droplet deposition apparatus according to any preceding claim, wherein each channel is defined by the facing side walls and a bottom surface extending between the side walls, and wherein the open side of the channel is closed over an active channel length by a cover secured to the side walls.

7. Droplet deposition apparatus according to Claim 6, wherein the bottom surface is defined by a base, the base and the side walls being integral.

8. Droplet deposition apparatus according to any of Claims 6 to 7, wherein a heat sink is attached to the coverto dissipate heat generated during droplet ejection.

9. Droplet deposition apparatus according to any preceding claim, comprising liquid supply means for supplying liquid to the channels for replenishment of droplets ejected from the channels.

10. Droplet deposition apparatus according to Claim 9, wherein a nozzle plate defining an ejection nozzle for each channel is located at one end of the channels, the liquid supply means being located at the other end of the channels.

11. Droplet deposition apparatus according to any preceding claim, wherein the connection tracks are formed from electrically conductive paste.

12. A method of manufacturing droplet deposition apparatus, the method comprising the steps of: forming in a sheet of piezoelectric material a plurality of first parallel channels having a first depth; depositing electrically conductive material within the first channels; forming in the sheet a plurality of second parallel channels extending parallel to the first channels and having a second depth greater than the first depth, at least one of the facing side walls of each of the second channels being located within a respective first channel and displaced from a side wall of that first channel; and selectively depositing electrode material on at least a portion of the facing side walls of each of the second channels so as to contact the electrically conductive material within the remainder of the first channel and thereby enable electrical signals to be conveyed to the electrodes in a second channel wall for actuation thereof.

13. A method according to Claim 12, wherein the step of depositing the electrically conductive material comprises depositing the material over the entire surface of the sheet, and subsequently removing material deposited on the top surfaces of the walls of the first channels.

14. A method according to Claim 13, wherein the material is removed by machining.

15. A method according to any of Claims 12 to 14, wherein a cover is bonded to the sheet to close portions of the channels over an active length.

16. A method according to Claim 15, wherein a heat sink is attached to the cover.

17. A method according to any of Claims 12 to 16, wherein, prior to depositing electrically conductive material within the first channels, a plurality of third parallel channels extending perpendicularly to and intersecting the first channels are formed in the sheet, the electrically conductive material being subsequently deposited within both the first and third channels.

18. A method according to Claim 17, comprising the steps of sectioning the sheet along first section lines parallel to the third channels and intersecting the electrodes, and sectioning the sheet along second section lines parallel to the third channels and intersecting respective third channels such that segments of the material deposited in the third channel provide sites for electrically connecting the electrodes to external drive circuitry.

19. A method according to Claim 18, wherein a nozzle plate is applied at the location of a first section line to define nozzles for droplet ejection.

20. A method according to Claim 18 or 19, wherein liquid replenishment means are provided at the location of a second section line.

21. A method according to any of Claims 12 to 20, wherein the channels are formed by the removal of material from the sheet.

22. A method according to Claim 21 , wherein the channels are formed by sawing.

23. Droplet deposition apparatus substantially as herein described with reference to Figure 10 or 17 of the accompanying drawings.

24. A method of manufacturing droplet deposition apparatus substantially as herein described with reference to Figures 4 to 11 or 12 to 17 of the accompanying drawings.