TW200413836A - Method for repairing mask by using multiple phase steps - Google Patents

Abstract

A method for repairing a mask by using multiple phase steps is disclosed. The method forms multiple phase steps beside a pit defect on a transparent substrate of a mask by removing part of the transparent substrate. With the multiple phase steps, the light destructive interference on the edge of the pit defect can be decreased, and the light intensity of the pit defect can be increased, thereby obtaining a purpose of effectively repairing the pit defect of the mask.

Description

200413836

The technical field to which the invention belongs: A photomask repair method ', in particular, a method for repairing Pit defects on a light-transmitting substrate in a manner that benefits from Multlple Phase Steps. Prior art: Generally, a mask is manufactured by first covering a substrate made of a flat and transparent glass insulating material such as quartz with an opaque film having a thickness of about several hundred A, such as chromium (Cr). Next, a layer of photoresist is formed to cover the opaque film, and exposure is performed using, for example, an exposure technique with an electron beam resolution, and the pattern of each layer of the integrated circuit element is transferred to the photoresist. After development, the desired circuit pattern is formed in the photoresist. Then, the pattern is transferred to the above-mentioned opaque film by an etching method, and a light-transmitting area and an opaque area are formed on the photomask. After the required circuit pattern is transferred to the opaque film, an additional layer of chromium dioxide film with a thickness of about 200 A can be added to the surface of the chromium film to prevent the reflection of the metal chromium film during exposure. The post exposure step is performed. During the manufacture of the reticle, or during the use of the reticle, it is highly likely that defects will form on the reticle. Therefore, when using a photomask for pattern replication, it is usually necessary to perform repairs on defects on the photomask. There are generally two types of mask defects. One is Transparent Defect, which is caused by, for example, the formation of a pattern that is supposed to be an opaque area due to an error in the pattern formed by the metal chromium film or the phase shift layer. Light transmission area. The other is Opaque Defect, which is caused by, for example, excessive coating of the metal chromium film, so that the portion of the photomask that should be the light-transmitting area forms an opaque area.

However, in addition to the damage to the substrate, pitting defects are generated. The height is lower than that of the untreated and undamaged positive and transparent substrates and the normal surface, which causes a phase shift. Later, the quality will be transferred on file. For example, the laser. This kind of loss of light transmission of the normal substrate table normal surface path is different. Therefore, due to the phase shift, the defect-type repair on the wafer is highly effective between the surfaces of the photomask substrate, and the generation of depressions is not the type of the light-transmitting photoresist layer. The surface of the light-transmitting substrate is low, and the height difference is poor, so that the induced light trap will not only create a depression in the recessed substrate, but also a serious shadow mask may be damaged by the light-transmitting substrate, and the upper depression Defects that lead to pitting defects. Due to the change of the phase of the recessed defect light through the recessed defect, the trapped edge of the light-transmitting substrate causes the trapped defect to pass through the photolithography process. Most of them use sodium hydroxide (NaOH) to repair the light. Concave defects on the light-transmitting substrate of the mask 'It is to immerse the photomask in a sodium hydroxide solution, and use sodium hydroxide to rot the light-transmitting substrate to remove the pitting defects or reduce the pitting defects and undamaged The height difference on the surface of the light-transmitting substrate can achieve the purpose of repairing the recessed defects. However, when the depth of the recessed defects is too large, the sodium hydroxide solution cannot effectively remove the recessed defects on the light-transmitting substrate. At this time, the phase shift phenomenon at the edge of the recessed defect still exists, which will also cause the light intensity of the recessed defect area to decrease. For example, when the recessed defects on the transparent substrate of the photomask produce the worst phase shift, if the critical size of the recessed defects is less than 1.2 micrometers (em), after the exposure step can be performed through this photomask, Will produce a photoresist layer on the wafer

200413836

Image lines; if the critical size of the recessed defect is greater than 丨 · 8 microns, #light stepping through this mask may result in two image lines on the photoresist layer of the wafer. As a result, the reliability of the lithography process will be reduced. In view of the above, when using the sodium hydroxide solution to repair the recessed defects on the light-transmitting substrate of the photomask, deeper recesses cannot be effectively removed, but light will still be generated on the edges of the recessed defects of the light-transmitting substrate. Destructive interference, and it is difficult to increase the light intensity of the recessed defect area to a certain intensity, so that the recessed defect on the transparent substrate of the photomask cannot be effectively repaired. Therefore, one of the main objects of the present invention is to provide a method for repairing a photomask, which uses step-like multiple phase steps to reduce the edges of the recessed defects on the transparent substrate of the photomask and the absence of the transparent substrate. Optically destructive interference caused by phase differences between defective surfaces. In this way, the light intensity of the depression can be effectively improved. Another object of the present invention is to provide a method for repairing a recessed defect on a light-transmitting substrate of a photomask by using multiple phase steps. The transparent substrate and the recessed defects form multiple phase steps. Therefore, the phase difference effect at the edges of the recessed defects can be effectively reduced, and the light intensity of the recessed defects can be improved, making it difficult to image the recessed defects on the photoresist of the wafer. According to the above-mentioned object, the present invention further provides a multi-phase step mask repair method, which enables the above-mentioned mask to include at least a light-transmitting substrate and a recess defect on one surface of the light-transmitting substrate. Moreover, the surface of the light-transmitting substrate has a first phase, and the multi-phase step mask repair method of the present invention 200413836 V. Description of the invention (4) At least includes the following steps: A second phase; and removing a portion of the light-transmitting substrate around the above-mentioned recessed defects, thereby forming a stepped groove having a plurality of layers in the light-transmitting substrate, wherein a pre- It is assumed that a phase difference is provided, and a layer immediately adjacent to the above-mentioned depression defect also has the preset phase difference between the depression defect and the depression defect. In addition, in these layers, the phase is gradually decreased from the second phase of the above-mentioned depressions to the first phase of the surface of the light-transmitting substrate. When making each step of the stepped groove described above, it is formed by using a photolithography and an etching process to define a light-transmitting substrate around a recessed defect, respectively. Among them, the height of each layer depends on the phase of the layer, and in the present invention, the preset phase difference between two adjacent layers is preferably less than 60 degrees, and more preferably 45 degrees. In addition, the number of levels of the stepped grooves depends on the phase difference between the surface of the transparent substrate and the recessed defect and the selected predetermined phase difference. By removing a part of the light-transmitting substrate around the recessed defect, a phase step can be formed between the surface of the light-transmitting substrate and the recessed defect. Therefore, the phase difference between the surface of the light-transmitting substrate and the recessed defects can be reduced in a decreasing manner, and the destructive interference of light between the surface of the light-transmitting substrate and the edges of the recessed defects can be reduced, and the light intensity of the recessed defects can be increased to achieve repair Purpose of pitting defects in light substrates. Embodiments: The present invention discloses a mask repair method with multiple phase steps, which forms a stepped phase between a recessed defect on a light-transmitting substrate of the mask and a surrounding non-defective surface to reduce light at the edges of the recessed defect. Interference, increase the light intensity of the recessed defects' and effectively repair the recessed defects on the transparent substrate of the photomask, and

200413836 V. Description of the invention (5) The purpose of improving the reliability of the lithography process is achieved. In order to make the description of the present invention more detailed and complete, you can refer to the following description and cooperate with the diagrams of the fla chart to FIG. 6. 0 β Please refer to FIG. 1 and FIG. 1b, which shows that the photomask has a concave defect. A top view of the light-transmitting substrate, and FIG. 1b is a cross-sectional view obtained along the line “-I” of the & The photomask 100 includes at least a light-transmitting substrate 102 ', wherein the material of the light-transmitting substrate may be a light-transmissive glass material such as quartz. The light-transmitting substrate 102 has a recessed defect 104, and it can be seen from FIG. Ib that the 'dent-defect 104' and the surface 103 of the light-transmitting substrate 102 have a gap of 105. When repairing the recessed defect 104 on the light-transmitting substrate 102, please refer to FIG. 2 to FIG. 5 together. FIG. 2 to FIG. 5 are repair masks showing a preferred embodiment of the present invention. Process cross-sectional view of a recessed defect on a transparent substrate. When using multiple phase steps to repair this recessed defect 104, the phase of the surface 103 of the transparent substrate 102 is assumed to be 0 degrees, and then the phase of the surface 103 of the transparent substrate 102 is used as a reference to measure the recessed defect. 1 〇4 phase. After the phase of the recessed defect 104 is measured, a calculation step is performed, and the phase of the measured recessed defect 104 is subtracted from the phase of the surface 1 0 3 of the transparent substrate 102 to obtain a phase difference, and then selected. A predetermined phase difference between two adjacent phases. Among them, a predetermined phase difference between two adjacent phase steps may be selected first, and then the measurement of the phase of the recessed defect 104 and the calculation of the phase difference between the recessed defect 104 and the surface 103 of the transparent substrate 102 may be performed. Not in this limit. In the present invention, the predetermined phase difference between two adjacent phase steps is preferably less than 60 degrees, and may be, for example, 45 degrees. Then, the calculated phase difference between the recessed defect 104 and the surface 103 of the transparent substrate 102 is divided by

Page 9 200413836 V. Description of the invention (6) After selecting the predetermined phase difference between two adjacent phase steps, subtract the value obtained by 1 to obtain the number of phase steps. For example, if the measured phase of the recessed defect 104 is 180 degrees, the wavelength of the exposure light is 248 meters, and the height difference between the recessed defect 104 and the surface 103 of the transparent substrate 102 is 1 〇5 is 238.5 nanometers (nm), and the predetermined phase difference between two adjacent phase steps is 45 degrees, the phase difference between the recessed defect 104 and the surface 103 of the transparent substrate 102 is 180 degrees minus A degree difference of 180 degrees is obtained by removing 0 degrees. Divide the phase difference of 180 degrees by the predetermined phase difference of 45 degrees to get the value 4. At this time, subtract 1 from the value 4, and the obtained value 3 represents the number of phase orders, that is, it needs to be between the recessed defect 104 The light-transmitting substrate 102 between the surface 10 and the surface 10 of the light-transmitting substrate 102 is additionally provided with three phase steps, as shown in FIG. 5, a layer 108, a layer 112, and a layer 116. Since 3 additional phase steps are added between the concave defect 104 and the surface 103 of the light-transmitting substrate 102, the 180-degree phase difference between the concave defect 104 and the surface 103 of the light-transmitting substrate 102 is common. It is shared by 4 phase differences, that is, the depression defects 104 and 108 as shown in FIG. 5, between the level 108 and the level 112, between the level 112 and the level 116, and between the level 116 and the surface 130. between. Therefore, the difference in height between two adjacent layers is the height difference between the recessed defect 104 and the surface ^ 103 by 2385 nm divided by 4, and the difference in the degrees between the two adjacent layers is approximately 59.6 nm. At this time, multiple phase steps can be made in the transparent substrate 10 according to the calculated height difference between two adjacent layers and the number of layers. Please refer to FIG. 1b and FIG. 2 together. First, for example, a photoresist layer 10 is formed by coating (c0atin) to cover the light-transmitting substrate 102 and the defect UH in FIG. 11}. For example, light exposure methods, ion beams (⑹

V. Description of the invention (7) Exposure method, 4, stomach, #, difficult ^ Exposure of the photoresist layer 106 in the sub-beam (ara) exposure method ^ ^ Transferring the pattern of layer 108 to the photoresist method to remove a part The photoresist is fine, and the second one will be; ΗΠ. After transferring to the photoresist layer 106 and exposing a part of the surface, the remaining photoresist layer 10 is used as a mask, such as an etched & I® half-light-transmitting substrate 102, and is immediately adjacent to the recess Defect 104 is formed, as shown in FIG. 2. Among them, the height of the level 108 and the recessed defect is about 59.6 nm, so that the phase difference between the level 108 and the recessed defect is 1 θ as the value calculated previously. In addition, the above-mentioned light-transmitting substrate # 7 can be performed by, for example, dry etching of a plasma (Dry Etchind ion beam (FOCUS Ion Beani; F 〖B). After completing step ^ 8, The remaining photoresist layer 10 may be peeled off to expose the surface 103 of the earth material 102, the depression defect 104, and the formed layer 10 8 °. Then, the coating method may be used in the same manner. A photoresist layer 11 () is formed to cover the surface 103 of the light-transmitting substrate 102, the recessed defect 104, and the layer 108. Then, for example, a light exposure method, an ion beam exposure method, or an electron beam is used. The exposure method exposes the photoresist layer 110 and transfers the pattern of the layer 112 to the photoresist layer 11 ^. Then, 'for example, the development method is used to remove a part of the photoresist layer 丨 i 〇, thereby transferring the pattern of the layer 112 Into the photoresist layer 11 (), and expose a part of the surface 103 of the light-transmitting substrate 102. The remaining photoresist layer 11 is used as a mask, and part of the light is removed by, for example, etching. The substrate 丨 02 is used to form a layer 112 around the layer 108, as shown in Fig. 3. Among them, the step The height difference between 11 2 and stratum 1 008 is also about 5 9 · 6 nanometers, which also makes stratum 112 200413836. V. Description of invention (8) The phase difference between stratum 108 and 45 is 45 degrees, as the value calculated previously. As with the above-mentioned “money-cutting light-transmitting substrate” 〇2, for example, dry plasma engraving technology or focused ion beam technology using plasma can be selected. After the layer 丨 2 is formed, the remaining photoresist layer 11 can be peeled In addition, the surface 10 of the light-transmitting substrate 10, the depression defect 104, the layer 108, and the layer 112 are exposed. After the layer 112 is completed, the photoresist layer can also be formed by, for example, coating. 114 is covered on the surface 10 of the light-transmitting substrate 10, the recessed defect 104, the layer 108, and the layer 1 and 2. For example, light exposure technology, ion beam exposure technology, or electron beam exposure technology is used. The photoresist layer 114 is exposed, and the pattern of the layer 116 is transferred to the photoresist layer i. 4. Then, for example, a developing technology is used to remove a part of the photoresist layer 114 to transfer the pattern of the layer 116 to the photoresist layer 114. The surface of the light-transmitting substrate 10 below the photoresist layer 114 of the hafnium portion is exposed. Surface 103: At this time, the remaining photoresist layer 丨 4 can be used as a mask to remove part of the light-transmitting substrate 10 by, for example, etching, and form a layer around the layer 12 6 'as shown in Figure 4. Similarly, the same degree difference between layer 丨 丨 6 and layer 11 2 is about 5 9 · 6 nm, so that the phase difference between layer 丨 丨 6 and layer 丨 i 2 It is 45 degrees, as calculated previously. Among them, dry etching technology or focused ion beam technology can be used to etch the transparent substrate 10, and dry etching technology can be performed using plasma. It is worth noting that the above-mentioned light-transmitting substrates around the pit defect 104 can be removed without coating a photoresist, and each layer can be directly fabricated by using an etching technique with accurate positioning force. Formation of steps is not limited to the method described above. Then, the remaining photoresist layer 114 can be peeled off, and the surface of the transparent substrate can be peeled off.

200413836 V. Description of the invention (9) 1 0 3, Defect defect 1 0 4, Level 1 008, Level 11 2, and Level 1 16 are exposed ', and Level 108, Level 112, and Level 116 are on the transparent substrate 1 〇2 is formed in a stepped groove, as shown in Figure 5. Among them, the phase of the layer 〇0, the phase of the layer 11 2 and the phase of the layer 116 are gradually decreased in a stepwise manner with a predetermined phase difference, and then the surface 103 of the transparent substrate 10 and the recessed defect 104 There are multiple phases. In addition, please refer to FIG. 6. FIG. 6 is a top view of the transparent substrate of the photomask after repairing according to a preferred embodiment of the present invention, that is, a top view of the cross-sectional view shown in FIG. 5. . In Figure 6, it can be seen that 'depression defect 104 is surrounded by level 1 () 8, level 108 is surrounded by level 112, level 112 is surrounded by level 丨 丨 6, and level 116 is adjacent. On the surface of the transparent substrate 102, 103. That is to say, each position of the recessed defect 104 is surrounded by a multi-order consisting of a layer 丨 〇8, a layer 丨 1 2, and a layer 116 in order, and the pit defect 104 and the light-transmitting substrate 102 Among the surfaces 103, there are multiple phase steps composed of layers 108, 2, and 11, and 11 to alleviate the phase difference between the surfaces 103 and the recessed defects 04. In the above-mentioned preferred embodiment, the phase difference between the recessed defect 104 and the surface 103 of the light-transmitting substrate 10 is 80 degrees, which may be a predetermined phase adopted between two adjacent layers. Divided by 45 degrees. Therefore, the phase difference between the recessed defect 104 and the level 108, the phase difference between the level 108 and the level 丨 2, the phase difference between the level 112 and the level 116, and the level 丨 丨The phase difference between 6 and the surface 103 of the light-transmitting substrate 〇 02 can be equal to 45 degrees, that is, the phase appears stepwise from 0 degrees to 45 degrees to 90 degrees to 135 degrees to 180 degrees. However, the number of levels between the recessed defect 104 and the surface 103 depends on the phase difference between the recessed defect 104 and the surface 103 of the light-transmitting substrate 10 and the predetermined phase between two adjacent levels used.

Page 13 200413836 V. Description of the Invention (ίο) Poor. Therefore, if the phase difference between the recessed defect 104 and the surface 103 of the light-transmitting substrate 102 cannot be divisible by a predetermined phase difference between two adjacent levels, the phase difference between the recessed defect 104 and the level 1 08, the level 1 The phase difference between 〇8 and layer 11 2 and the phase difference between layer 11 2 and layer 116 is preferably kept the same, while the phase difference between layer 11 6 and the surface 1 0 3 of the transparent substrate 1 0 Resentment will change according to the actual process. Due to the addition of multiple phase steps composed of the stratum 108, the stratum 112, and the stratum 116, the phase between the recessed defect 104 and the surface 103 of the light-transmitting substrate 102 is stepped and gradually decreases toward the surface 103. Therefore, the light destructive interference between the edge of the recessed defect 104 and the surface 103 of the light-transmitting substrate 102 can be effectively reduced, and the light intensity of the recessed defect 104 can be improved, and the light intensity of the recessed defect 104 can be even reduced from a relatively low level · 1 increased to a level of 0.9. In this way, the purpose of repairing the concave defect 104 on the light-transmitting substrate 102 can be achieved. In summary, one of the advantages of the present invention is that the use of the stepped multiple phase steps can alleviate the light-damaging interference caused by the concave defect edges on the transparent substrate of the photomask. Therefore, the purpose of increasing the light intensity of the recessed defects can be achieved. Another advantage of the present invention is that by adding multiple phase steps by using etching technology between the surface of the non-defective light-transmitting substrate and the recessed defects, the light intensity of the recessed defects can be greatly improved, so that the recessed defects can be prevented from being imaged on the wafer. On the photoresist, it can effectively repair the concave defects of the light-transmitting substrate of the photomask, thereby achieving the purpose of improving the reliability of pattern transfer. As will be understood by those familiar with this technology, the above are only preferred embodiments of the present invention and are not intended to limit the scope of patent application for the present invention;

Page 14 200413836 V. Description of the invention (11) Other equivalent changes or modifications made without departing from the spirit disclosed in the present invention shall be included in the scope of patent application described below. 200413836 Brief description of the drawings " The preferred embodiment of the present invention has been described in more detail in the foregoing explanatory text with the following figures, in which: Fig. La shows the light-transmitting base with recessed defects of the photomask The top view of the material, Figure lb is a cross-sectional view taken along the line I-I of Figure la, and Figures 2 to 5 are the repair light of a preferred embodiment of the present invention. A process cross-sectional view of a recessed defect on the transparent substrate of the mask; and FIG. 6 is a top view of the transparent substrate of the mask after repairing according to a preferred embodiment of the present invention. Comparative description of drawing numbers: 100 mask 102 light-transmitting base material 103 surface 104 depressions 105 height 106 photoresist layer 108 step layer 110 photoresist layer 112 step layer 114 photoresist layer 116 step layer

Claims (1)

200413836 6. Scope of Patent Application 1. A mask repair method of multiple phase steps' wherein the mask includes at least a light-transmitting substrate and a recessed defect is located on a surface of the light-transmitting substrate, and The mask repair method for the surface of the light-transmitting substrate with a first phase 'the multiple phase steps at least includes measuring a second phase of the recessed defect; and forming a plurality of phase steps of the transparent layer around the recessed defect The phase of the phase steps in the light substrate increases from the first phase of the surface of the transparent substrate to the second phase of the recessed defect, and every two adjacent 誃 = between the phase steps A predetermined phase difference, and the predetermined phase difference is between the second phase step adjacent to the recessed defect and the recessed defect. Λ2. The mask repair of multiple phase steps as described in item J of the scope of the patent application, wherein the material of the transparent substrate is quartz. The described "㈣ ... repairing method and method 4 of the above-mentioned ... order of light single repair I Shenguang patent encirclement! The multiple phase order of the" mask repair method 2 "steps to form these phase steps Including at least a portion of the light-transmitting substrate from which the recessed defect is removed, thereby forming a tool in the light-transmitting substrate. " 200413836 VI. Scope of patent application Low-level One of a plurality of levels is a stepped groove, and the stepped depression is next to the depression defect. Lingyi take 6. The step of forming multiple layers of light as described in item 5 of the scope of the patent application, wherein the steps of forming each of these layers include at least: > a method of forming a photoresist layer to cover the transparent substrate On the surface; the exposure step, in which the layers of each of the layers are resisted; and Wang Tongguang performs a surname engraving step. 7. The mask repair of multiple phase steps as described in item 6 of the scope of patent application, wherein the exposure step is performed using light exposure exposure method ^ '4 * 8. The multiple phase steps as described in item 6 of scope of patent application Photomask repair method, in which the exposure step is performed using an ion beam (ion-beam) exposure method 9. The multi-phase mask repair method in accordance with item 6 of the patent application scope, wherein the exposure step is performed using an electron beam ( E-beam) exposure method. 1 〇 The multi-phase mask repair method according to item 6 of the scope of patent application, wherein the etching step is performed by using a dry etching method (Dry Etching) 〇 11 Multi-phase mask repair 200413836 VI. Scope of Patent Application Law 'The order to carry out the engraving of the last name does not use plasma. The mask repair method of multiple phase steps described in item 6 of the patent scope = j 八 中 进 This etching step is etched using a focused ion beam (FIB) 1 3 · The multiple phases described in item 5 of the scope of patent application Step mask repair method, wherein the number of layers is subtracted from the second phase by the second phase, and the phase difference of the second phase minus the first phase is divided by the phase difference and then subtracted by 1. And get. 14. A method for repairing a photomask with multiple phases, wherein the photomask includes at least a light-transmitting substrate and a recessed defect on a surface of the light-transmitting substrate, and the surface of the light-transmitting substrate has a first A phase, the multi-phase mask repair method includes at least: measuring a second phase of the recessed defect; and removing a portion of the light-transmitting substrate around the recessed defect, thereby forming a layer having A stepped groove of a plurality of layers, wherein each two adjacent layers have a preset phase difference, and the steps adjacent to the concave defect and the concave defect have a predetermined phase difference. And the phases of the layers are decreasing from the second phase of the recessed defect toward the first phase of the surface of the transparent substrate. 1 5 · Multi-phase mask repair method as described in item 14 of the patent application page 19 200413836 6. The scope of patent application method, wherein the material of the light-transmitting substrate is quartz 16. The multi-phase mask repair method as described in item 14 of the scope of patent application, wherein the preset phase difference is less than 60 degrees . 17 • The multi-phase mask repair method according to item 14 of the scope of patent application, wherein the preset phase difference is 45 degrees. 18. The multi-phase step mask repair method as described in item 14 of the scope of the patent application, wherein the steps of forming each of these layers include at least ... forming a photoresist layer covering the surface of the light-transmitting substrate Above; performing an exposure step to transfer the patterns of each of the layers into the photoresist layer; and performing an etching step. 1 9. The multi-phase step mask repair method according to item 8 of the scope of the patent application, wherein performing the exposure step uses a light irradiation exposure method. 20. The multiple-phase mask repair method according to item 18 of the patent application park, wherein the exposure step is performed using an ion beam exposure method. 21. The multi-phase mask repair method according to item 8 of the patent application range, wherein the exposure step is performed using an electron beam exposure method. Page 20 200413836 VI. Scope of Patent Application " " ^ ---- 2 2 · As for the multi-phase mask repair method according to item 18 of the scope of patent application, the etching step is performed by using A dry etching method. Supplement 23: The multiple phase step mask repair method as described in item 22 of the scope of patent application, wherein the etching step is performed using a plasma. -Supplement 2 4 _ The mask repair method of multiple phases as described in the first patent application scope item ′, wherein the etching step is performed by a focused ion beam etching method. 25. The multi-phase step photomask repair method described in item 14 of the scope of the patent application, wherein the number of layers is the second phase minus the first phase and the first phase minus the first phase The phase difference of a phase is divided by the preset phase difference and then subtracted by 1 to obtain. Page 21