Methods of dicing semiconductor wafer into chips, and structure of groove formed in dicing area

A method for dicing a semiconductor wafer into chips is provided, in which the peeling-off of a hard protective film on the surface of a semiconductor substrate may be avoided. Two parallel grooves are formed at a dicing area around a chip by an etching process. Then, SiO.sub.2 film is deposited on the GaAs substrate as a protective film. At this time, a bending portion at the interface between the protective films on the inner surface of the groove and the surface of the substrate. When the part between two grooves is cut by a dicing blade, a stress to the protective film caused by the edge of the blade is concentrated to the bending portion, resulting in a crack along the bending portion.

TECHNICAL FIELD
 The present invention relates to a method for dicing a semiconductor wafer
 into chips, particularly to a method for dicing a semiconductor wafer
 provided with grooves in dicing areas. The present invention further
 relates to the structure of such grooves.
 BACKGROUND ART
 When a semiconductor wafer (or a semiconductor substrate) on which
 semiconductor elements are built in is diced into chips, if the surface of
 the semiconductor wafer is covered by an electrically insulating hard
 protective film such as oxide film and nitride film, the peeling-off of
 the protective film is caused in the edge part of a cut line.
 With reference to FIG. 1, the protective film 4 provided on the substrate 2
 is peeled off during a dicing step by a blade 6. A reference numeral 8
 denotes a part of the film peeled off.
 In order to prevent the peeling-off of the protective film conventionally,
 a dicing step is conducted after the protective film is etched away along
 a dicing line as shown in FIG. 2, reference numeral 10 denoting the part
 of the protective film etched away.
 According to the conventional dicing method in FIG. 2, the step for etching
 away the protective film is needed to be conducted other than the step for
 fabricating semiconductor elements, resulting in the problems of the
 increase of manufacturing steps and cost.
 When the semiconductor wafer is diced after the protective film is peeled
 off along dicing lines, the substrate is laid bare in the edge part of
 each diced chip. Accordingly, in the case where a semiconductor element 12
 is mounted on a printed circuit board 14 and the element 12 is connected
 to the board 14 through a lead wire 16 as shown in FIG. 3, there is a
 possibility that the substrate 12 and the wire 16 are electrically shorted
 around the position 18, resulting in the malfunction of the semiconductor
 element 12.
 DISCLOSURE OF INVENTION
 An object of the present invention is to provide a method for dicing a
 semiconductor wafer which is covered by an electrically insulating hard
 protective film such as oxide film or nitride film to protect the surface
 of the wafer.
 Another object of the present invention is to provide a structure of
 grooves formed in dicing areas of the wafer.
 Still another object of the present invention is to provide a structure of
 grooves for preventing the displacement of the dicing position from being
 caused during a dicing step by an full automatic dicing apparatus.
 A further object of the present invention is to provide a method for
 detecting the edge of each cut line accurately.
 According to the present invention, grooves are formed in dicing area
 around each chip by utilizing etching steps during the fabrication of
 semiconductor elements. Then, a hard protective film is deposited on the
 inner surface the groove and the surface of the substrate. An edge of a
 dicing blade is aligned in such a manner that the edge passes through the
 bottom of the groove. A stress is applied upward or downward to the
 portion of the protective film to which the edge of the dicing blade is
 contacted. This stress is propagated from the protective film on the
 groove to the protective film on the substrate. At this time, the stress
 is concentrated to the bending portion at the interface between the
 protective films on the inner surface of the groove and the surface of the
 substrate, so that the crack is caused along the bending portion. The
 bending portion where such stress is caused is herein referred to as a
 crack caused portion.
 In order to cause such crack, it is required that the radius of the bending
 portion is substantially smaller than the thickness of the protective
 film. For example, if the radius of the bending portion is one-half the
 thickness of the protective film, a bending stress caused at the bending
 portion having a bending angle of 0.degree.-120.degree. is 1.5 times that
 caused at another part of the film. Also, if the radius of the bending
 portion is one-tenth the thickness of the protective film, the pending
 stress caused at the pending portion is increased by 2.5 times that of a
 peripheral part of the film. If the radius of the bending portion is
 one-twentieth the thickness of the protective film, the bending stress is
 increased by 3.4 times that of a peripheral part of the film. While an
 acute angle is preferable for a bending angle of the bending portion, the
 angle of 90.degree. is comparable to an acute angle.
 As a protective film is disrupted by a crack caused at the bending portion,
 the stress caused by the edge of the blade is not propagated to the area
 of a semiconductor element. Therefore, the peeling-off of the protective
 film is not caused at the area of a semiconductor element.
 The width of a groove is preferably about 1-20 .mu.m. While a groove is
 preferably formed at both-side edges of a dicing line, respectively, a
 groove may be formed only at one-side edge of a dicing line in the case
 where only the area neighboring said one-side edge is required to be
 protected by the film. Alternatively, only one groove which has a width
 larger than the that of a dicing line may be formed in place of providing
 groove in both-side edges of a dicing line, respectively.
 When one or more etching steps are conducted in the fabricating process of
 semiconductor elements, an additional groove may be formed in the bottom
 of an already provided groove to increase the number of crack caused
 portions. In this case, if a stress has been propagated through a first
 crack caused portion, the propagation of the stress may be blocked at a
 second crack caused portion. As a result, the peeling-off of the
 protective film at the area of a semiconductor element may necessarily be
 avoided.
 Furthermore, according to the present invention, a groove non-formed part
 may be provided for grooves to be formed at a dicing area. An accurate
 position of the edge of a cut line may be detected at the groove
 non-formed part by means of a CCD camera. In this manner, the position of
 the edge of the cut line may be accurately recognized, so that a next
 dicing position estimated based on the accurate position of the edge of
 the cut line may also precisely recognized, i.e. an erroneous recognition
 for a dicing position is not caused. AS a result, a misregistration of a
 dicing position may be avoided.
 In the present invention, a groove non-formed part is needed to be provided
 at one or more positions for one dicing line which is required to have a
 more accurate dicing position. In most cases, as a plurality of chips each
 having the same configuration are arranged repeatedly on a wafer, it is
 easy to provide only one groove non-formed part at the peripheral part of
 each chip. As a result, a groove non-formed part may be easily searched by
 a CCD camera after dicing.
 Where the length of a groove non-formed part is too long, a chipping or a
 peeling-off of a protective film is caused, and where is too short, it
 becomes difficult to detect a accurate position of a cut line. When
 monitoring a groove non-formed part on a CRT with a magnification of 400,
 the length of a groove non-formed part is preferably about 10-100 .mu.m.
 Also, a dicing method of the present invention comprising the steps of: a)
 detecting an edge of a cut line after cutting a first dicing area of the
 semiconductor wafer by picking up an image of an area of the cut line
 including the groove non-formed part by an image pick-up device; b)
 correcting a next dicing position stored in the full automatic dicing
 apparatus based on the edge of the cut line detected; c) cutting a next
 dicing area based on the next dicing position corrected; d) detecting an
 edge of a cut line after cutting the next dicing area in the step c) by
 picking up an image of an area of the cut line including the groove
 non-formed part by an image pick-up device; e) correcting a further next
 dicing position stored in the full automatic dicing apparatus based on the
 edge of the cut line detected in the step d); f) cutting a further next
 dicing area based on the further next dicing position corrected in the
 step e); and g) repeating the steps d)-f).
 It should be noted that the dicing method of the present invention is
 applicable to not only a semiconductor wafer provided with a protective
 film thereon but also a semiconductor wafer not provided with a protective
 film thereon.

BEST MODE FOR CARRYING OUT THE INVENTION
 Embodiments of the present invention will now be described in detail with
 reference to the drawings. A first embodiment
 FIGS. 4A, 4B and 4C are cross sectional views for illustrating each step of
 a first embodiment. In general, the fabrication process of semiconductor
 elements includes etching processes. Utilizing an etching process, for a
 dicing area 20 in a semiconductor substrate 2 of GaAs having 300 .mu.m
 thickness as shown in FIG. 4A, two parallel grooves 22 and 24 each having
 10 .mu.m width and 0.7 .mu.m depth are formed by an etching process as
 shown in FIG. 4B. In FIG. 4B, "w" denotes the width and "t" the depth of
 each groove. The distance between the centers of grooves 22 and 24 is the
 same as the thickness of a dicing blade (not shown), 25 .mu.m for example.
 Then, as shown in FIG. 4C, an SiO.sub.2 film 26 of 0.4 .mu.m thickness is
 deposited on the inner surface of the grooves 22, 24 and the surface of
 the semiconductor substrate 2 as a surface protective film. At this time,
 a bending portion (or a crack caused portion) 28 is formed at the
 interface between the inner surface of the groove and the surface of the
 substrate. As material for the surface protective film, SiN, Al.sub.2
 O.sub.3, TiO.sub.2, Ta.sub.2 O.sub.5 and the like may be used other than
 SiO.sub.2.
 FIG. 5 shows the bending portion 28 in expanded manner. The bending radius
 of the portion 28 is denoted by "R" and the bending angle ".theta.". In
 this embodiment, the thickness of the protective film 26 is two times or
 more the radius R of the bending portion 28, and .theta. is about
 90.degree..
 Using a dicing blade (not shown) of 25 .mu.m thickness, the part between
 the two grooves 23 and 24 is diced to form a cut line. During a dicing
 step, the both-side edges of the dicing blade pass through the bottoms of
 the two grooves, respectively. At this time, a stress caused in the
 protective film 26 by the edges of the blade during a dicing step is
 concentrated to the bending portion 28, causing a crack along the bending
 portion. The protective film 26 is disrupted at the bending portion, so
 that the stress caused by the edges of a dicing blade is not propagated to
 the area of semiconductor elements. Therefore, the peeling-off of the
 protective film is not caused at the area of the semiconductor elements. A
 second embodiment
 FIGS. 6A, 6B and 6C are cross sectional views for illustrating each step of
 a second embodiment. For a dicing area 20 in a semiconductor substrate 2
 of GaAs having 300 .mu.m thickness as shown in FIG. 6A, one groove 30
 having the width larger than the width (25 .mu.m) of the dicing area is
 formed by an etching process as shown in FIG. 6B. The groove 30 has 35
 .mu.m width and 0.7 .mu.m depth. Then, as shown in FIG. 6C, an SiO.sub.2
 film 26 of 0.4 .mu.m thickness is deposited on the inner surface of the
 groove 30 and the surface of the semiconductor substrate 2 as a surface
 protective film.
 Using a dicing blade (not shown) of 25 .mu.m thickness, the central part of
 the groove 30 is diced to form a cut line. During a dicing step, the
 both-side edges of the dicing blade pass through the bottom of one groove
 30. At this time, a stress caused in the protective film 26 by the edges
 of the blade during a dicing step is concentrated to the bending portion
 31 at the interface between the inner surface of the groove and the
 surface of the substrate, causing a crack along the bending portion. The
 protective film 26 is disrupted at the bending portion, so that the stress
 caused by the edges of a dicing blade is not propagated to the area of
 semiconductor elements. Therefore, the peeling-off of the protective film
 is not caused at the area of the semiconductor elements. A third
 embodiment
 FIGS. 7A, 7B and 7C are cross sectional views for illustrating each step of
 a third embodiment. According to this embodiment, for a dicing area 20 in
 a semiconductor substrate 2 of GaAs having 300 .mu.m thickness as shown in
 FIG. 7A, two parallel first grooves 22 and 24 each having 10 .mu.m width
 and 0.7 .mu.m depth are formed as shown in FIG. 7B by an etching process
 in the same way as the first embodiment.
 Additional second grooves each having 5 .mu.m width and 1.3 .mu.m depth are
 formed in the bottom of the first grooves 22 and 24, respectively, by an
 etching process in such a manner that the second groove is parallel to the
 first groove in which the second one is provided. Therefore, the
 configuration of each groove consisted of the first and second ones has
 two-step shape. The distance between the centers of two grooves each
 having two-step shape is 25 .mu.m as in the first embodiment.
 Then, as shown in FIG. 7D, an SiO.sub.2 film 26 having 0.4 .mu.m thickness
 is deposited on the inner surface of the grooves and the surface of the
 substrate as a surface protective film.
 Using a dicing blade of 25 .mu.m thickness, the part between the two
 grooves is diced to form a cut line. During a dicing step, the both-side
 edges of the dicing blade pass through the bottoms of the two grooves,
 respectively. In this case, the peeling-off of the protective film 26 is
 substantially prevented at the bending portion 36 between the first groove
 and the second groove. If the peeling-off of the protective film in part
 proceeds over the bending portion 36, the peeling-off will be completely
 stopped at the bending portion 38 between the inner surface of the first
 groove and the surface of the substrate and then will not be caused at the
 area of the semiconductor element.
 In this manner, the propagation of the peeling-off of the protective film
 to the element area may be completely prevented by providing a plurality
 of crack caused portions. It is understood for those who skilled in the
 art that the method in this embodiment may be applicable to the second
 embodiment.
 While three embodiment have described heretofore, the depth of each groove
 formed around a chip generally do not exceed the depth of the
 semiconductor element. This is because the grooves are formed by utilizing
 the etching step in the fabrication of semiconductor elements. The present
 invention, however, is useful for grooves each having a depth larger than
 that of the element area, these grooves being formed by a etching process
 other than the etching process during the fabrication step of the
 elements. A fourth embodiment
 A full automatic dicing apparatus is generally used for dicing a
 semiconductor wafer into chips. According to the full automatic dicing
 apparatus, the alignment of a wafer is carried out at first. This is
 automatically conducted by utilizing unique patterns on the wafer. Then, a
 dicing blade is moved to the position where a first dicing line seems to
 be in order to cut only one line. Then, a CCD camera is moved to the
 position where the cut line seems to be formed in order to search the cut
 line. The image of the cut line picked up by the CCD camera is darker than
 that of a peripheral part of the substrate, so that the cut line is
 recognized as "black" by processing the CCD image into binary.
 FIG. 8 shows the substrate 2 after cutting, the substrate provided with
 grooves such as described in the first, second and third embodiments. In
 the figure, the edge 40 of the groove and the edge 42 of the cut line 43
 are parallel each other.
 It should be noted that the present embodiment is applicable to a
 semiconductor not having a protective film thereon, then a dicing step
 will be explained hereinafter for a wafer not provided with a protective
 film.
 Returning to FIG. 8, monitoring these edges by a display (for example CRT)
 connected to the CCD camera, it is impossible to distinguish the edge 42
 of the cut line 43 formed by a dicing step from the edge 40 of the groove
 formed by an etching step. That is, the edge 40 of the groove looks as
 "black" as the edge 42 of the cut line on the display. Therefore, the full
 automatic dicing apparatus erroneously recognizes the groove edges as the
 cut line edges. In such a case, an erroneous feedback is conducted in a
 next dicing, resulting in a displacement of a dicing position. Such
 displacements are accumulated until a cut line is correctly recognized,
 then the dicing position will have a large misregistration with respect to
 the target position.
 In order to prevent such misregistration of the dicing position, a part at
 which grooves are not formed, i.e. a groove non-formed part is left
 without forming grooves across the overall length according to the present
 embodiment.
 FIG. 9 shows the structure of such grooves including non-formed part, the
 grooves being provided in a dicing area 44 in a semiconductor wafer 2 of
 300 .mu.m thickness which is diced by a blade of 25 .mu.m thickness. Two
 grooves 46 and 48 each having 5 .mu.m width and 2 .mu.m depth are formed
 in parallel by an etching process. The distance between the centers of two
 grooves is 25 .mu.m, and the distance between the outer edges of two
 grooves is 30 .mu.m. A groove non-formed part 50 having the length "L" of
 50 .mu.m is left near a chip alignment mark or a unique pattern (not
 shown).
 FIG. 10 shows the substrate 2 after cutting the part between two grooves 46
 and 48 in the substrate shown in FIG. 9, the cut line being designated by
 reference numeral 51. When the cut line 51 is picked by a CCD camera and
 is monitored by a display (CRT) having a magnification of 400, the length
 "L" of the groove non-formed part 50 is 20 mm on the display. This shows
 that the length "L" is long enough to check the shape of the cut line 51
 on the display.
 Monitoring a groove formed area of the wafer 2 on the display, the width of
 "black" region is 12 mm, thereby it is recognized that the real width of
 "black" region on the wafer is 30 .mu.m. This dimension (i.e. 30 .mu.m) is
 recognized to be matched with the distance between the outer edges of the
 grooves 20 and 22. This dimension is also unchanged even if a dicing
 position is moved across 2.5 .mu.m to the direction perpendicular to the
 longitudinal direction of grooves. This means that it is impossible to
 detect the edge of the cut line 51 accurately at the groove formed region.
 In contrast to this, the width of the cut line 51 is recognized to be 10 mm
 on the display and 25 .mu.m in real dimension at the groove non-formed
 part 50. This shows that the both-side edges of the cut line are inwardly
 apart by 2.5 .mu.m from the outer edges of the grooves 46 and 48,
 respectively. Even if the dicing position deviates by 1 .mu.m from the
 target position, the recognition of the edges of the cut line is enough
 possible. Based on the accurate position of the edges of the cut line, the
 dicing position may be corrected in the next dicing step.
 FIG. 11 shows a wafer 62 in which grooves of the present invention are
 formed around chips 60 to be diced. Two parallel grooves 62 and 64 formed
 at the peripheral area of the wafer 62 is provided with only one groove
 non-formed part 66. Two parallel grooves 72 and 74 formed at the inward
 area of the wafer 62 is also provided with only one groove non-formed part
 76.
 In a full automatic dicing apparatus, the positions of all the dicing areas
 are estimated by searching unique patterns 80 on chips. Based on this
 estimation, a dicing blade is moved to a dicing area where the grooves 62
 and 64 are formed to cut only one line. Then, a CCD camera is moved to the
 position where a cut line is estimated to be in order to search the cut
 line. Furthermore, the position of a groove non-formed part 66 is
 estimated by the position of the unique pattern 80 and the groove
 non-formed part is picked up by a CCD camera. In the image picked up by
 the CCD camera, the cut line is necessarily darker than that of a
 peripheral part of the substrate. By processing the CCD image into binary,
 it is possible to recognize the portion of "black" as a cut line and the
 interface portion between "black" and "white" as the edge of the cut line.
 As shown in FIG. 10, the edge in the groove non-formed part 50 monitored
 by the CCD camera designates a correct cut line edge. Therefore, a correct
 cut line edge may be detected at the area of the groove non-formed part 66
 in FIG. 11.
 Based on the position of the cut line edge thus obtained, an estimated
 position of dicing area where the grooves 72 and 74 are formed is
 corrected and an area at the corrected position is then diced. And, the
 correct cut line edge is detected at the groove non-formed part 76 to
 correct a next dicing position. The same processing steps may be
 hereinafter repeated to dice a plurality of chips accurately from the
 wafer.
 In the case where only one groove non-formed part is provided for each
 dicing area, if the position of a wafer mounted to the full automatic
 dicing apparatus is deviated from the correct position, the unique pattern
 82 may be recognized as the unique pattern 80, so that the groove
 non-formed part 66 may be erroneously decided to be at the position 86. In
 order to prevent such error, the groove non-formed parts 66 and 67 can be
 provided for all of chips, respectively, as shown in FIG. 12. In this
 manner, a groove non-formed part may easily be searched from any unique
 pattern 80 which is neighbored thereto.
 Industrial Applicability
 According to the present invention described above, a groove is formed at a
 dicing area to provide a surface protective film with a bending portion,
 so that the propagation of the peeling-off of the film toward an area of
 semiconductor elements is blocked. As this method is conducted by
 utilizing etching steps during the fabrication of the elements, the step
 of etching away the film in the conventional method is not needed. As a
 result, the number of steps is not increased in the present invention.
 Furthermore, according to the present invention, the groove non-formed part
 is provided for grooves to be formed at a dicing area, so that the
 position of a cut line may be accurately detected. As a result, a
 misregistration of a dicing position is not caused in a full automatic
 dicing apparatus.