US20060216860A1

US20060216860A1 - Flip chip interconnection having narrow interconnection sites on the substrate

Info

Publication number: US20060216860A1
Application number: US11/388,755
Authority: US
Inventors: Rajendra Pendse
Original assignee: Stats Chippac Pte Ltd
Current assignee: Stats Chippac Pte Ltd
Priority date: 2005-03-25
Filing date: 2006-03-24
Publication date: 2006-09-28
Also published as: TW200723355A; JP2012064980A; US20130026628A1; US20100193947A1; JP5565875B2; TWI404114B; KR20070107154A; WO2006105015A2; US9159665B2; WO2006105015A3; US20120211880A9; JP2008535225A; US8318537B2

Abstract

A flip chip interconnect of a die on a substrate is made by mating the interconnect bump onto a narrow interconnect pad on a lead or trace, rather than onto a capture pad. The width of the narrow interconnect pad is less than a base diameter of bumps on the die to be attached. Also, a flip chip package includes a die having solder bumps attached to interconnect pads in an active surface, and a substrate having narrow interconnect pads on electrically conductive traces in a die attach surface, in which the bumps are mated onto the narrow pads on the traces.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Provisional Application No. 60/665,208, filed Mar. 25, 2005, titled “Flip chip interconnection having narrow interconnection sites on the substrate”, which is hereby incorporated herein by reference.
This application is related to U.S. application Ser. No. 10/985,654, filed Nov. 10, 2004, titled “Bump-on-lead flip chip interconnection”.

BACKGROUND

This invention relates to semiconductor packaging and, particularly, to flip chip interconnection.
Flip chip packages include a semiconductor die mounted onto a package substrate with the active side of the die facing the substrate. The substrate is made up of a dielectric layer and at least one metal layer, patterned to provide substrate circuitry, which includes among other features traces (“leads”) leading to interconnect pads. The metal layer may be patterned by, for example, a mask-and etch process. Conventionally, interconnection of the circuitry in the die with circuitry in the substrate is made by way of bumps which are attached to an array of interconnect pads on the die, and bonded to a corresponding (complementary) array of interconnect pads (often referred to as “capture pads”) on the substrate. The capture pads are typically much wider than the leads, and can be as wide as, for example, about 2 to 4 times the nominal or design width of the leads.
The areal density of electronic features on integrated circuits has increased enormously, and chips having a greater density of circuit features also may have a greater density of sites (“die pads”) for interconnection with the circuitry on a package substrate.
The package is connected to underlying circuitry, such as a printed circuit board (e.g., a “motherboard”), in the device in which the package is employed, by way of second level interconnects (e.g., pins, secondary interconnect solder balls) between the package and the underlying circuit. The second level interconnects have a greater pitch than the flip chip interconnects, and so the routing on the substrate conventionally “fans out”. Significant technological advances in patterning the metal layer on the substrate have enabled construction of fine lines and spaces; but in the conventional arrangement space between adjacent pads limits the number of traces than can escape from the more inward capture pads in the array, and the fan out routing between the capture pads beneath the die and the external pins of the package is conventionally formed on multiple metal layers within the package substrate. For a complex interconnect array, substrates having multiple layers may be required to achieve routing between the die pads and the second level interconnects on the package.
Multiple layer substrates are expensive, and in conventional flip chip constructs the substrate alone typically accounts for more than half the package cost (about 60% in some typical instances). The high cost of multilayer substrates has been a factor in limiting proliferation of flip chip technology in mainstream products.
In conventional flip chip constructs the escape routing pattern typically introduces additional electrical parasitics, because the routing includes short runs of unshielded wiring and vias between wiring layers in the signal transmission path. Electrical parasitics can significantly limit package performance.

SUMMARY

According to the invention flip chip interconnect is accomplished by connecting the interconnect bump directly onto a narrow interconnection pad, or narrow pad, rather than onto a conventional capture pad. The width of the narrow pad according to the invention is selected according to the base diameter of the interconnect bump that is to be connected onto the narrow pad. Particularly, the width of the narrow pad is less (such as in a range about 20% to about 80%) than the base diameter of the interconnect bump. The invention provides more efficient routing of traces on the substrate. Particularly, the signal routing can be formed entirely in a single metal layer of the substrate. This reduces the number of layers in the substrate, and forming the signal traces in a single layer also permits relaxation of some of the via, line and space design rules that the substrate must meet. This simplification of the substrate greatly reduces the overall cost of the flip chip package. The bump-on-narrow-pad architecture also helps eliminate such features as vias and “stubs” from the substrate design, and enables a microstrip controlled impedance electrical environment for signal transmission, thereby greatly improving performance.
In one general aspect the invention features a flip chip interconnection having solder bumps attached to interconnect pads on a die and mated onto corresponding narrow interconnection pads on a substrate.
In another general aspect the invention features a flip chip package including a die having solder bumps attached to interconnect pads in an active surface, and a substrate having narrow interconnection pads in a die attach surface, in which the bumps are mated onto the narrow pads.
In some embodiments the bump-on-narrow-pad interconnection is formed according to methods of the invention without use of a solder mask to confine the molten solder during a re-melt stage in the process. Avoiding the need for a solder mask allows for finer interconnection geometry.
In some embodiments the substrate is further provided with a solder mask having openings over the narrow interconnection pads. In some embodiments the substrate is further provided with solder paste on the narrow interconnection pads.
In another general aspect the invention features a method for forming flip chip interconnection, by providing a substrate having narrow interconnection pads formed in a die attach surface and a die having bumps attached to interconnect pads in an active surface; supporting the substrate and the die; dispensing a quantity of a curable adhesive on the substrate (covering at least the narrow interconnection pads) or on the active side of the die (covering at least the bumps); positioning the die with the active side of the die toward the die attach surface of the substrate, and aligning the die and substrate and moving one toward the other so that the bumps contact the corresponding narrow interconnection pads on the substrate; applying a force to press the bumps onto the mating narrow pads, sufficient to displace the adhesive from between the bump and the mating narrow pad; at least partially curing the adhesive; melting and then re-solidifying the solder, forming a metallurgical interconnection between the bump and the narrow pad.
In another general aspect the invention features a method for forming flip chip interconnection, by providing a substrate having narrow interconnection pads formed in a die attach surface and having a solder mask having openings over the narrow pads, and a die having bumps attached to interconnect pads in an active surface; supporting the substrate and the die; positioning the die with the active side of the die toward the die attach surface of the substrate, and aligning the die and substrate and moving one toward the other so that the bumps contact the corresponding narrow pads on the substrate; melting and then re-solidifying to form the interconnection between the bump and the narrow pad.
In some embodiments the solder bump includes a collapsible solder portion, and the melt and solidifying step melts the bump to form the interconnection on the narrow pad. In some embodiments the substrate is provided with a solder paste on the narrow pads, and the step of moving the die and the substrate toward one another effects a contact between the bumps and the solder on the narrow pads, and the melt and solidifying step melts the solder on the narrow pad to form the interconnection.
In another general aspect the invention features a method for forming flip chip interconnection, by providing a substrate having narrow interconnection pads formed in a die attach surface and having a solder mask having openings over the narrow pads and having solder paste on the narrow pads, and a die having bumps attached to interconnect pads in an active surface; supporting the substrate and the die; positioning the die with the active side of the die toward the die attach surface of the substrate, and aligning the die and substrate and moving one toward the other so that the bumps contact the solder paste on the corresponding narrow pads on the substrate; melting and then re-solidifying the solder paste, forming a metallurgical interconnection between the bump and the narrow pad.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic sketch of a portion of a conventional bump-on-capture pad flip chip interconnection, in a sectional view parallel to the plane of the package substrate surface, as indicated by the arrows 1-1′ in FIG. 2.
FIG. 2 is a diagrammatic sketch showing a portion of a conventional bump-on-capture pad flip chip interconnection, in a sectional view perpendicular to the plane of the package substrate surface, as indicated by the arrows 2-2′ in FIG. 1.
FIG. 3 is a diagrammatic sketch showing a portion of another conventional bump-on-capture pad flip chip interconnection, in a sectional view perpendicular to the plane of the package substrate surface.
FIG. 4 is a diagrammatic sketch of a portion of an embodiment of a bump-on-narrow-pad flip chip interconnection according to an embodiment of the invention, in a sectional view parallel to the plane of the package substrate surface.
FIG. 5 is a diagrammatic sketch showing a portion of an embodiment of a bump-on-narrow-pad flip chip interconnection according to an embodiment of the invention as in FIG. 4, in a sectional view perpendicular to the plane of the package substrate surface, as indicated by the arrows 5-5′ in FIG. 4.
FIG. 6 is a diagrammatic sketch of a portion of another embodiment of a bump-on-narrow-pad flip chip interconnection according to an embodiment of the invention, in a sectional view parallel to the plane of the package substrate surface.
FIG. 7 is a diagrammatic sketch showing a portion of an embodiment of a bump-on-narrow-pad flip chip interconnection according to an embodiment of the invention as in FIG. 6, in a sectional view perpendicular to the plane of the package substrate surface, as indicated by the arrows 7-7′ in FIG. 6.
FIGS. 8 and 9 are diagrammatic sketches, each of a portion of another embodiment of a bump-on-narrow-pad flip chip interconnection according to an embodiment of the invention, in a sectional view parallel to the plane of the package substrate surface.
FIGS. 10A-10C are diagrammatic sketches in a sectional view illustrating steps in a process for making a flip chip interconnection according to an embodiment of the invention.
FIGS. 11A-11D are diagrammatic sketches in a sectional view illustrating steps in a process for making a flip chip interconnection according to an embodiment of the invention.
FIG. 12 is a diagrammatic sketch showing a force and temperature schedule for a process for making a flip chip interconnection according to an embodiment of the invention.
FIG. 13 is a diagrammatic sketch in a sectional view showing a bump-on-narrow-pad flip chip interconnection according to an embodiment of the invention.
FIGS. 14A-14E are diagrammatic sketches in plan view illustrating various interconnect pad shapes according to embodiments of the invention.
FIGS. 15A-15C are diagrammatic sketches in plan view illustrating various interconnect pad configurations according to embodiments of the invention.
FIGS. 16A and 16B are diagrammatic sketches in plan view illustrating solder mask openings according to embodiments of the invention.
FIG. 17 is a diagrammatic sketch in plan view illustrating details of various interconnect pad configurations in relation to a solder mask opening according to embodiments of the invention.
FIG. 18 is a diagrammatic sketch in plan view illustrating details of various solder mask configurations in relation to an interconnect pad according to embodiments of the invention.

DETAILED DESCRIPTION

The invention will now be described in further detail by reference to the drawings, which illustrate alternative embodiments of the invention. The drawings are diagrammatic, showing features of the invention and their relation to other features and structures, and are not made to scale. For improved clarity of presentation, in the FIGs. illustrating embodiments of the invention, elements corresponding to elements shown in other drawings are not all particularly renumbered, although they are all readily identifiable in all the FIGs.
The conventional flip chip interconnection is made by using a melting process to join the bumps (conventionally, solder bumps) onto the mating surfaces of the corresponding capture pads and, accordingly, this is known as a “bump-on-capture pad” (“BOC”) interconnect. Two features are evident in the BOC design: first, a comparatively large capture pad is required to mate with the bump on the die; second, an insulating material, typically known as a “solder mask” is required to confine the flow of solder during the interconnection process. The solder mask opening may define the contour of the melted solder at the capture pad (“solder mask defined”), or the solder contour may not be defined by the mask opening (“non-solder mask defined”); in the latter case—as in the example of FIG. 1, described in more detail below—the solder mask opening may be significantly larger than the capture pad. The techniques for defining solder mask openings have wide tolerance ranges. Consequently, for a solder mask defined bump configuration, the capture pad must be large (typically considerably larger than the design size for the mask opening), to ensure that the mask opening will be located on the mating surface of the pad; and for a non-solder mask defined bump configuration, the solder mask opening must be larger than the capture pad. The width of capture pads (or diameter, for circular pads) is typically about the same as the ball (or bump) diameter, and can be as much as two to four times wider than the trace width. This results in considerable loss of routing space on the top substrate layer. In particular, for example, the “escape routing pitch” is much bigger than the finest trace pitch that the substrate technology can offer. This means that a significant number of pads must be routed on lower substrate layers by means of short stubs and vias, often beneath the footprint of the die, emanating from the pads in question.
FIGS. 1 and 2 show portions 10, 20 of a conventional flip chip package, in diagrammatic sectional views; the partial sectional view in FIG. 1 is taken in a plane parallel to the package substrate surface, along the line 1-1′ in FIG. 2; and the partial sectional view in FIG. 2 is taken in a plane perpendicular to the package substrate surface, along the line 2-2′ in FIG. 1. Certain features are shown as if transparent, but many of the features in FIG. 1 are shown at least partly obscured by overlying features. Referring now to both FIG. 1 and FIG. 2, a die attach surface of the package substrate includes a metal layer formed on a dielectric layer 12. The metal layer is patterned to form leads 13 and capture pads 14. An insulating layer 16 , typically termed a “solder mask”, covers the die attach surface of the substrate; the solder mask is usually constructed of a photodefinable material, and is patterned by conventional photoresist patterning techniques to leave the mating surfaces of the capture pads 14 exposed. Interconnect bumps 15 attached to pads on the active side of the die 18 are joined to the mating surfaces of corresponding capture pads 14 on the substrate to form appropriate electrical interconnection between the circuitry on the die and the leads on the substrate. After the reflowed solder is cooled to establish the electrical connection, an underfill material 17 is introduced into the space between the die 18 and the substrate 12 , mechanically stabilizing the interconnects and protecting the features between the die and the substrate.
As FIG. 1 shows by way of example, signal escape traces in the upper metal layer of the substrate (leads 13), lead from their respective capture pads 14 across the die edge location, indicated by the broken line 11, and away from the die footprint. In a typical example the signal traces may have an escape pitch P_Eabout 112 um. A 30 um/30 um design rule is typical for the traces themselves in a configuration as shown in FIG. 1; that is, the traces are nominally 30 um wide, and they can be spaced as close together as 30 um. The capture pads are typically three times greater than the trace width and, accordingly in this example the capture pads have a width (or diameter, as they are roughly circular in this example) nominally 90 um. And, in this example, the openings in the solder mask are larger than the pads, having a nominal width (diameter) of 135 um.
FIGS. 1 and 2 show a non-solder mask defined solder contour. As the fusible material of the bumps on the die melt, the molten solder tends to “wet” the metal of the leads and capture pads, and the solder tends to “run out” over any contiguous metal surfaces that are not masked. The solder tends to flow along the contiguous lead 13, and here the solder flow is limited by the solder mask, for example at 19 in FIG. 1. A non-solder mask defined solder contour at the pad is apparent in FIG. 2, in which the material of the bumps 15 is shown as having flowed, 29, over the sides of the capture pads 14 and down to the surface of the dielectric layer of the substrate 12. This is referred to as a non-solder mask defined contour because the solder mask does not limit the flow of solder over the surface and down over the sides of the capture pads, and—unless there is a substantial excess of solder at the pad—the flow of solder is limited by the fact that the dielectric surface of the substrate is typically not wettable by the molten solder. A lower limit on the density of the capture pads in a conventional arrangement, as in FIG. 1, is determined by, among other factors, limits on the capacity of the mask forming technology to make reliable narrow mask structures, and the need to provide mask structures between adjacent mask openings. A lower limit on the escape density is additionally determined by, among other factors, the need for escape lines from more centrally located capture pads to be routed between more peripherally located capture pads.
FIG. 3 shows a conventional solder mask defined solder contour, in a sectional view similar to that in FIG. 2. A die 38 is shown affixed by way of bumps 35 onto the mating surfaces of capture pads 34 formed along with traces (leads 33) by patterning a metal layer on the die attach side of a dielectric layer of the substrate 32 . After the reflowed solder is cooled to establish the electrical connection, an underfill material 37 is introduced into the space between the die 38 and the substrate 32 , mechanically stabilizing the interconnects and protecting the features between the die and the substrate. Here the capture pads 34 are wider than in the example of FIGS. 1 and 2, and the solder mask openings are smaller than the capture pads, so that the solder mask material covers the sides and part of the mating surface of each capture pad, as shown at 39, as well as the leads 33. When the bumps 35 are brought into contact with the mating surfaces of the respective capture pads 34, and then melted, the solder mask material 39 restricts the flow of the molten solder, so that the shapes of the solder contours are defined by the shapes and dimensions of the mask openings over the capture pads 34. In a typical example of a conventional solder mask defined bump-on-capture pad interconnection, the capture pad has a diameter about 140 um, and the solder mask opening has a diameter about 90 um, and the routing traces are about 25-30 um wide. The diameter of the mating surface for attachment of the bump to the die pad (not shown in FIG. 2 or 3), that is, the place of interface between the bump and the die pad, is defined by the solder mask opening as having a diameter about 90 um, in this example.
FIGS. 4 and 6 each show a portion of a bump-on-narrow-pad (“BONP”) flip chip interconnection according to an embodiment of the invention, in a diagrammatic partial sectional view taken in a plane parallel to the substrate surface, along the lines 4-4′ and 6-6′ in FIGS. 5 and 7, respectively. Certain features are shown as if transparent. According to the invention the interconnection is achieved by mating the bumps onto respective narrow interconnection pads on the substrate and, accordingly, this is referred to herein as a “bump-on-narrow-pad” (“BONP”) interconnect. Solder mask materials typically cannot be resolved at such fine geometries and, according to these embodiments of the invention, no solder mask is used. Instead the function of confining molten solder flow is accomplished without a solder mask in the course of the assembly process (as described below). FIG. 5 shows a partial sectional view of a package as in FIG. 4, taken in a plane perpendicular to the plane of the package substrate surface, along the line 5-5′ in FIG. 4; and FIG. 7 shows a partial sectional view of a package as in FIG. 6, taken in a plane perpendicular to the plane of the package substrate surface, along the line 7-7′ in FIG. 6.
Escape routing patterns for bump-on-narrow-pad (“BONP”) substrates according to the invention are shown by way of example in FIGS. 4 and 6: in FIG. 4, arranged for a die on which the die attach pads for the interconnect balls are in a row near the die perimeter, the bumps 45 are mated onto corresponding narrow interconnection pads on the escape traces 43 in a row near the edge of the die footprint, indicated by the broken line 41; in FIG. 6, arranged for a die on which the die attach pads are in an array of parallel rows near the die perimeter, the bumps 65 are mated onto corresponding narrow interconnection pads on the escape traces 63 in a complementary array near the edge of the die footprint, indicated by the broken line 61.
As FIGS. 4 and 6 illustrate, the routing density achievable using bump-on-narrow-pad interconnect according to the invention can equal the finest trace pitch offered by the substrate technology. In the specific case illustrated, this constitutes a routing density which is approximately 90% higher than is achieved in a conventional bump-on-capture pad arrangement. In the perimeter row embodiments of BONP (e.g., FIG. 4), the bumps are placed at a fine pitch, which can equal the finest trace pitch of the substrate. This arrangement poses a challenge for the assembly process, because the bumping and bonding pitch must be very fine. In the perimeter array version of BONP (e.g., FIG. 6), the bumps are arranged on an area array, providing greater space for a larger bumping and bonding pitch, and relieving the technological challenges for the assembly process. Even in the array embodiments, the routing traces on the substrate are at the same effective pitch as in the perimeter row arrangement, and an arrangement as in FIG. 6 relieves the burden of fine pitch bumping and bonding without sacrificing the fine escape routing pitch advantage.
Referring particularly now to FIGS. 4 and 5, leads 43 and narrow interconnection pads 46 are formed by patterning a metal layer on a die attach surface of a substrate dielectric layer 42 . The narrow pads 46 are formed as a widening of the traces 43 at the interconnection sites. The “width” of an interconnection pad (W_bin FIG. 5) is the nominal or design dimension across the widened part of the trace at the interconnection site. According to the invention, the width of the narrow interconnection pad on a substrate is established according to the bump base width (“base diameter”) of the bumps on the die that is to be connected to the substrate. The “bump base width” (W_pin FIG. 5) is the nominal or design diameter of the generally round (approximately circular) contact interface between the bump 45 and the die pad 49. (As may be appreciated, the diameter of the bump, taken in a plane parallel to the bump-pad interface, may be greater than the bump base width, as illustrated diagrammatically in FIGS. 2, 3, 5 and 7, for example.) Particularly according to the invention, the interconnection pad width W_bis smaller than the bump base width W_p, and the narrow interconnection pad width may be as small as 20% of the bump base width. In many embodiments the narrow pad width is in a range about 20% to about 80% of the bump base width. In some embodiments the narrow interconnection pad width is less than the bump base width and greater than about 25% of the bump base width. In some embodiments the narrow pad width is less than about 60% of the bump base width.
According to the invention, electrical interconnection of the die 48 is made by joining the bumps 45 on the die onto the narrow interconnection pads 46 on the leads 43. The conventional comparatively wide capture pad is unnecessary according to the invention and, in embodiments as in FIGS. 4 and 5, no solder mask is required; the process is described in detail below.
Conventional capture pads typically are about the same width (diameter) as the bumps, and are typically two to four times as wide as the trace or lead width. As will be appreciated, some variation in the width of leads is expected. As used herein, a narrow interconnection pad has a nominal or design width at least about 120% of the nominal or trace design rule width, and bump-on-narrow-lead interconnection according to the invention includes bumps connected to widened parts of traces that are greater than about 120% of the nominal or trace design rule width, and less than the bump base diameter. An interconnection site that has a width less than about 120% does not constitute a narrow interconnect pad, and interconnection made by connecting bumps onto portions of leads that are less than about 120% of the nominal or trace design rule width is referred to as a “bump-on-lead” interconnection.
Similarly, referring to FIGS. 6 and 7, leads 63 and narrow interconnection pads 66 are formed by patterning a metal layer on a die attach surface of a substrate dielectric layer 62 . The signal escape traces lead across the die edge location, indicated by the broken line 61, and away from the die footprint. The narrow pads 66 are formed as a widening of the traces 63 at the interconnection sites. The “width” of an interconnection pad (W_bin FIG. 7) is the nominal or design dimension across the widened part of the trace at the interconnection site. In this example, as in the example shown in FIGS. 4 and 5, according to the invention, the width of the narrow interconnection pad on a substrate is established according to the bump base width of the bumps on the die that is to be connected to the substrate. The “bump base width” (W_pin FIG. 7) is the nominal or design diameter of the generally round (approximately circular) contact interface between the bump 65 and the die pad 69. Particularly according to the invention, the interconnection pad width W_bis smaller than the bump base width W_p, and the narrow interconnection pad width may be as small as 20% of the bump base width. In many embodiments the narrow pad width is in a range about 20% to about 80% of the bump base width. In some embodiments the narrow interconnection pad width is less than the bump base width and greater than about 25% of the bump base width. In some embodiments the narrow pad width is less than about 60% of the bump base width.
According to the invention, electrical interconnection of the die 68 is made by joining the bumps 65 on the die narrow interconnection pads 66 on the leads 63. Certain of the escape traces, e.g. 66, leading across the die edge location from interconnect sites in rows toward the interior of the die footprint, pass between the bumps 65 on more peripheral rows of interconnect sites. No capture pads are required according to the invention and, in embodiments as in FIGS. 6 and 7, no solder mask is required; the process is described in detail below.
According to the invention, as the techniques for forming the traces improves, it is possible to reliably form traces having nominal or design rule widths less than about 25 um. The reduced trace widths can provide for increased routing density. However, the mechanical reliability of a “bump-on-lead” flip chip interconnect on leads less than about 25 um may be unsatisfactory, because the dimensions of the interface between the bump and the lead are small, and may not provide sufficient bonding strength to provide a good electrical interconnection. The invention provides for reliable mechanical connection (and good electrical interconnection) by forming a narrow interconnect pad by widening the lead to an extent dimensionally related to the bump base diameter, and limited to less than the bump base diameter.
The narrow interconnect pad according to the invention may be shaped in any of a variety of ways. Some such shapes may be more readily manufacturable, and some may provide other process advantages. For example, the narrow pad may be generally rectangular, either square or elongated, as shown for example in FIGS. 14A and 14B; or, it may be generally round, either circular or elliptical, as shown for example in FIGS. 14C and 14D. Other shapes may be employed; one particularly useful shape is shown by way of example in FIG. 14E, having semicircular portions separated lengthwise the lead or trace by a square or rectangular portion. Also, the narrow pad can be formed as a symmetrical or an asymmetrical widening in the lead or trace, as shown in FIGS. 15A and 15B (showing a generally rectangular pad as an example). Also, the narrow pad need not be situated at, or near, the end of the lead or trace, but may be formed at any point where interconnection is specified, as illustrated in FIG. 15C (showing a generally rectangular pad as an example). Forming the pad longer than wide increases the wettable mating surface of the narrow pad (planar surface plus the exposed parts of the sides), and can improve the mechanical strength of the interconnection. Also, where the pad is longer than wide, the tolerance for misalignment of solder mask openings (or bump) is increased; particularly where the pad is at the end of the trace, an elongated pad can reduce the likelihood that a solder mask opening (or bump) will be situated off the end of the pad.
The solder mask openings shown by way of example in FIGS. 4, 6, 8 and 9 are generally round (circular or elliptical), but according to the invention the solder mask opening may be shaped in any of a variety of ways. It may be useful for example, to provide a generally rectangular solder mask opening, either square or elongated, as shown in FIGS. 16A, 16B, (showing a generally rectangular pad as an example). A square or rectangle of a given width has a greater area than a circle or ellipse having the same width (diameter, short axis). For this reason a square or rectangular mask opening has a capacity to hold a greater quantity of solder paste (or other fusible material), and accordingly this may provide an advantage where a fusible material such as a solder paste is to be applied to the mating surfaces on the narrow pads prior to mating with the bumps (described in more detail below). Also, it may be easier to print a fusible material into a square or rectangular mask opening than into a circular or elliptical mask opening, because there is greater tolerance for misalignment in the printing process. Also, given a width limitation for the mask opening, a square or rectangular mask opening provides a greater open area for mounting a large bump on the pad during the interconnection process.
Various narrow pad configurations according to embodiments of the invention are shown in FIG. 17 by way of example in relation to a circular mask opening 174 in a solder mask 176 . The mask opening in each example has a width (diameter) Wm, which may be, for example, about 90 um. A bump-on-lead configuration is shown at 173 . The lead or trace 172 has a nominal (design) width WL, which may be, for example, about 30 um. A narrow pad having a rectangular shape is shown at 175 . In this example the lead or trace at which the narrow pad is formed has a nominal (design) width WL′, which may be, for example, about 30 um. The rectangular narrow pad has a width WP′, which may be, for example, about 45 um. A narrow pad having an oval shape is shown at 177 formed at a wider lead or trace, having a nominal (design) width WL″, which may be, for example, about 50 um. A narrow pad having a rectangular shape expanded with an oval shape is shown at 179 . In this example the narrower lead or trace at which the narrow pad is formed has a nominal (design) width WL′″, which may be, for example, about 30 um. The rectangular portion of the narrow pad 179 has a width WP″, which may be, for example, about 45 um; and the oval expanded portion has a width WPE, which may be for example, about 50 um.
Various solder mask opening configurations according to embodiments of the invention are shown in FIG. 18 by way of example in relation to a lead (or trace) or narrow pad 182. In these examples the lead or narrow pad at the interconnect site has a width WL, which may be, for example, about 40 um. In a first example, a circular solder mask opening 185 having a width (diameter) Wm, which may be, for example, about 90 um, exposes an interconnect site portion 183 . In a second example a rectangular solder mask opening 187 having a width (across the lead or narrow pad) Wm′, which may be, for example, about 80 um, and a length Lm′, which may be, for example, about 120 um., exposes an interconnect site portion 183′. In a third example an elliptical solder mask opening 189 having a width (across the lead or narrow pad) Wm″, which may be, for example, about 80 um, and a length Lm″, which may be, for example, about 120 um., exposes an interconnect site portion 183″. Both the rectangular opening 187 and the oval opening 189 expose a greater length (hence, area) of the lead or pad at the site 183″, 183″ than does the circular solder mask opening 185, even though the circular opening in this example has a greater diameter. This provides a greater area for solder reflow during the interconnect process, and can result in a more robust interconnection. The area exposed by the rectangular opening 187 is slightly greater than that provided by the elliptical opening 189 having the same width and length; and moreover, the area would be reduced if there were a slight misalignment of the elliptical opening, but not by a slight misalignment of the rectangular opening. As a practical matter, however, an design rectangular opening may have more or less rounded corners because of resolution limitations in processes for patterning openings in the solder mask dielectric.
In some illustrative examples according to the invention, the diameter of the bump base on the die to be mounted may be about 90 um, and the narrow interconnect pad is formed on the substrate to a width in a range about 25 um (where the trace width is less than about 25 um) to about 50 um. This provides a significant improvement in routing density, as compared with a substrate having a conventional capture pad having a much larger diameter, which may be typically two to four times as great as the trace width.
As FIGS. 4 and 6 illustrate, bump-on-narrow-pad interconnect according to the invention can provide a significantly higher signal trace escape routing density. Also, as FIGS. 4 and 6 illustrate, the BONP interconnect according to this aspect of the invention does not require use of a solder mask to define the solder contour at the interconnect site.
The BONP interconnection structure of embodiments such as are shown by way of example in FIGS. 4, 5, 6 and 7 can be produced according to the invention by any of several methods, not requiring a solder mask. In general, interconnect bumps (typically solder bumps) are affixed onto interconnect pads on the active side of the die. A die attach surface of the substrate (termed the “upper” surface) has an upper metal layer patterned to provide the traces and narrow pads at interconnect sites as appropriate for interconnection with the arrangement of bumps on the particular die. In a preferred method of the invention, an encapsulating resin adhesive is employed to confine the solder flow during a melt phase of the interconnection process.
FIGS. 8 and 9 show two examples of a portion of a bump-on-narrow-pad flip chip interconnection according to other embodiments of the invention, in a diagrammatic sectional view taken in a plane parallel to the substrate surface. Certain features are shown as if transparent. According to this aspect of the invention a solder mask is provided, which may have a nominal mask opening diameter in the range about 80 um to 90 um. Solder mask materials can be resolved at such pitches and, particularly, substrates can be made comparatively inexpensively with solder masks having 90 um openings and having alignment tolerances plus or minus 25 um. In some embodiments laminate substrates (such as 4 metal layer laminates), made according to standard design rules, are used. In the embodiments of FIGS. 8 and 9, for example, the traces may be at ˜90 um pitch and the narrow pads may be in a 270 um area array, providing an effective escape pitch ˜90 um across the edge of the die footprint, indicated by the broken line 81.
In embodiments as in FIGS. 8 and 9 a no-flow underfill is not required; a conventional capillary underfill can be employed.
In embodiments as in FIG. 8 the interconnection is achieved by mating the bumps directly onto an narrow interconnect pad 84 on a narrow lead or trace 83 patterned on a dielectric layer on the die attach surface of the substrate 82; the solder mask 86 serves to limit flow of solder within the bounds of the mask openings 88, preventing solder flow away from the interconnect site along the solder-wettable lead. The solder mask may additionally confine flow of molten solder between leads, or this may be accomplished in the course of the assembly process.
In embodiments as in FIG. 9, as in FIG. 8, narrow pads on traces 93 are patterned on a dielectric layer on the die attach surface of the substrate 92. Solder paste is provided at the interconnect sites (narrow pads) 94 on the leads 93, to provide a fusible medium for the interconnect. The openings 98 in the solder mask 96 serve to define the paste. The paste is dispensed, for example by a standard printing process, then is reflowed, and then may be coined if necessary to provide uniform surfaces to meet the balls. The solder paste can be applied in the course of assembly using a substrate as described above with reference to FIG. 8; or, a substrate may be provided with paste suitably patterned prior to assembly. Other approaches to applying solder selectively to the interconnect sites may be employed in the solder-on-narrow-pad embodiments of the invention, including electroless plating and electroplating techniques. The solder-on-narrow-pad configuration provides additional solder volume for the interconnect, and can accordingly provide higher product yield, and can also provide a higher die standoff.
Accordingly, in some embodiments the solder-on-narrow-pad configuration according to the invention is employed for interconnection of a die having high-melting temperature solder bumps (such as a high-lead [high Pb] solder, conventionally used for interconnection with ceramic substrates) onto an organic substrate. The solder paste can be selected to have a melting temperature low enough that the organic substrate is not damaged during reflow. To form the interconnect in such embodiments the high-melting interconnect bumps are contacted with the solder-on-narrow-pad sites, and the remelt fuses the solder-on-narrow-pad to the bumps. Where a noncollapsible bump is used, together with a solder-on-narrow-pad process, no preapplied adhesive is required, as the displacement or flow of the solder is limited by the fact that only a small quantity of solder is present at each interconnect, and the noncollapsible bump prevents collapse of the assembly.
In other embodiments the solder-on-narrow-pad configuration according to the invention is employed for interconnection of a die having eutectic solder bumps.
One embodiment of a preferred method for making a bump-on-narrow-pad interconnection is shown diagrammatically in FIGS. 10A-10C.
Referring to the FIGs., a substrate 112 is provided, having at least one dielectric layer and having a metal layer on a die attach surface 113, the metal layer being patterned to provide circuitry, particularly narrow interconnection pads 114 on traces or leads, on the die attach surface. The substrate 112 is supported, for example on a carrier or stage 116, with a substrate surface opposite the die attach surface 113 facing the support. A quantity of an encapsulating resin 122 is dispensed over the die attach surface 113 of the substrate, covering at least the narrow interconnection pads 114 on the leads. A die 102 is provided, having bumps 104 attached to die pads (not shown in the FIG.) on the active side 103. The bumps include a fusible material which contacts the mating surfaces of the narrow pads. A pick-and-place tool 108 including a chuck 106 picks up the die by contact of the chuck 106 with the backside 101 of the die. Using the pick-and-place tool, the die is positioned facing the substrate with the active side of the die toward the die attach surface of the substrate, as shown in FIG. 10A; and the die and substrate are aligned and moved one toward the other (arrow M) so that the bumps 104 contact the corresponding narrow interconnection pads 114 on the traces (leads) on the substrate. Then a force is applied (arrow F) to press the bumps 105 onto the mating surfaces 134 at the narrow pads 115 on the leads, as shown in FIG. 10B. The force must be sufficient at least to displace the adhesive 122 from between the bumps and the mating surfaces 134 at the narrow interconnection pads 115. The bumps may be deformed by the force, breaking the oxide film on the contacting surface of the bumps and/or on the mating surface of narrow pads. The deformation of the bumps may result in the fusible material of the bumps being pressed onto the top and over the edges of the narrow pads. The adhesive is caused to cure at least partially, as shown at 132, as for example by heating to a selected temperature. At this stage the adhesive need only be partially cured, that is, only to an extent sufficient subsequently to prevent flow of molten solder along an interface between the adhesive and the conductive traces. Then the fusible material of the bumps 105 is melted and then is re-solidified, forming a metallurgical interconnection between the bump 105 and narrow pad 115, and the adhesive curing is completed, to complete the die mount and to secure the electrical interconnection at the mating surface (now an interconnect interface) 144, as shown generally at 140 in FIG. 10C. In the plane of the sectional view shown in FIG. 1C, interconnection is formed between certain of the bumps 145 and corresponding narrow interconnection pads 155 on certain of the leads, as for example in a configuration as in FIG. 6. Other leads 156 are interconnected on narrow interconnection pads at other localities, which would be visible in other sectional views. A comparatively high trace density is shown. The curing of the adhesive may be completed prior to, or concurrently with, or following melting the solder. Typically, the adhesive is a thermally curable adhesive, and the extent of curing at any phase in the process is controlled by regulating the temperature. The components can be heated and cured by raising the temperature of the chuck on the pick and place tool, or by raising the temperature of the substrate support, for example.
The process is shown in further detail in FIGS. 11A-11D. In FIG. 1A, a substrate 212 is provided on a die attach surface with conductive (metal) traces, and narrow interconnection pads 214 at interconnect sites on the traces are covered with an adhesive 222. The die 202 is positioned in relation to the substrate 212 such that the active side of the die faces the die attach side of the substrate, and is aligned (arrows A) such that bumps 204 on the die are aligned with corresponding mating surfaces on narrow pads 214. The die and the substrate are moved toward one another so that the bumps contact the respective mating surfaces on the narrow pads. Then as shown in FIG. 11B a force is applied to move the bumps 205 and narrow pads 215 against one another, displacing the adhesive as shown at 232 in FIG. 1I B, and deforming the bumps onto the mating surfaces 234 and over the edges of the narrow pads. Deformation of the bumps on the narrow pads breaks the oxide film on the contact surfaces of the bumps and the mating surfaces of the narrow pads, establishing a good electrical connection, and deformation of the bumps over the edges of the narrow pads helps establish a good temporary mechanical connection. As in the example of FIG. 10A-10C, the narrow interconnection pads of certain of the traces 216 are out of the plane of FIG. 11B. Heat is applied to partially cure the adhesive as shown at 236 in FIG. 11C. Then heat is applied to raise the temperature of the bumps sufficiently to cause the fusible material of the bumps to melt, as shown in FIG. 11D. This substantially (though not necessarily fully) completes the cure of the adhesive 246 and completes the metallurgical interconnection of the bumps 245 onto the mating surfaces 244 at the narrow interconnection pads 215. The cured adhesive stabilizes the die mount.
In an alternative embodiment of a preferred method, the adhesive can be pre-applied to the die surface, or at least to the bumps on the die surface, rather than to the substrate. The adhesive can, for example, be pooled in a reservoir, and the active side of the die can be dipped in the pool and removed, so that a quantity of the adhesive is carried on the bumps; then, using a pick-and-place tool, the die is positioned facing a supported substrate with the active side of the die toward the die attach surface of the substrate, and the die and substrate are aligned and moved one toward the other so that the bumps contact the corresponding traces (leads) on the substrate. Such a method is described in U.S. Pat. No. 6,780,682, Aug. 24, 2004, which is hereby incorporated by reference. Then forcing, curing, and melting are carried out as described above.
A force and temperature schedule for a process according to the invention is shown diagrammatically by way of example in FIG. 12. In this chart, time runs from left to right on the horizontal axis; a force profile 310 is shown as a thick solid line, and a temperature profile 320 is shown as a dotted line. The temperature profile begins at a temperature in the range about 80° C.-about 90° C. The force profile begins at essentially zero force. Beginning at an initial time t_ithe force is rapidly (nearly instantaneously) raised 312 from F_ito a displacement/deformation force F_dand held 314 at that force for a time, as discussed below. F_dis a force sufficiently great to displace the adhesive away from between the bumps and the mating surfaces of the narrow interconnection pads; and, preferably, sufficient to deform the fusible (narrow pad-contacting) portion of the bumps onto the mating surface, breaking the oxide films and forming a good metal-to-metal (metallurgical) contact, and, in some embodiments, over the edges of the narrow pads to establish a mechanical interlock of the bumps and the narrow pads (“creep” deformation). The total amount of force required will depend upon the bump material and dimensions and upon the number of bumps, and can be determined without undue experimentation. As the force is raised, the temperature is also rapidly raised 322 from an initial temperature T_ito a gel temperature Tg. The gel temperature Tg is a temperature sufficient to partially cure the adhesive (to a “gel”). Preferably, the force and temperature ramps are set so that there is a short lag time t_def, following the moment when F_dis reached and before T_gis reached, at least long enough to permit the elevated force to displace the adhesive and to deform the bumps before the partial cure of the adhesive commences. The assembly is held 314, 324 at the displacement/deformation pressure F_dand at the gel temperature T_gfor a time t_gelsufficient to effect the partial cure of the adhesive. The adhesive should become sufficiently firm that it can subsequently maintain a good bump profile during the solder remelt phase—that is, sufficiently firm to prevent undesirable displacement of the molten fusible material of the bump, or flow of the molten fusible material along the narrow pads and leads. Once the adhesive has partially cured to a sufficient extent, the pressure may be ramped down rapidly 318 to substantially no force (weight of the components). The temperature is then rapidly raised further 323 to a temperature T_msufficient to remelt the fusible portions (solder) of the bumps, and the assembly is held 325 at the remelt temperature T_mfor a time t_melt/cureat least sufficient to fully form the solder remelt on the narrow pads, and preferably sufficient to substantially (though not necessarily fully) cure the adhesive. Then the temperature is ramped down 328 to the initial temperature T_i, and eventually to ambient. The process outlined in FIG. 12 can run its course over a time period of 5-10 seconds.
The adhesive in embodiments as in FIG. 12 may be referred to as a “no-flow underfill”. In some approaches to flip chip interconnection, the metallurgical interconnection is formed first, and then an underfill material is flowed into the space between the die and the substrate. The “no-flow underfill” according to the invention is applied before the die and the substrate are brought together, and the no-flow underfill is displaced by the approach of the bumps onto the narrow pads, and by the opposed surfaces of the die and the substrate. The adhesive for the no-flow underfill adhesive according to the invention is preferably a fast-gelling adhesive—that is, a material that gels sufficiently at the gel temperature in a time period in the order of 1-2 seconds. Preferred materials for the no-flow underfill adhesive include, for example, so-called non-conductive pastes, such as those marketed by Toshiba Chemicals and by Loktite-Henkel, for example.
Alternative bump structures may be employed in the bump-on-narrow-pad interconnects according to the invention. Particularly, for example, so-called composite solder bumps may be used. Composite solder bumps have at least two bump portions, made of different bump materials, including one which is collapsible under reflow conditions, and one which is substantially non-collapsible under reflow conditions. The non-collapsible portion is attached to the interconnect site on the die; typical conventional materials for the non-collapsible portion include various solders having a high lead (Pd) content, for example; and gold (Au), for example. The collapsible portion is joined to the non-collapsible portion, and it is the collapsible portion that makes the connection with the narrow interconnect pad according to the invention. Typical conventional materials for the collapsible portion of the composite bump include eutectic solders, for example.
An example of a bump-on-narrow-pad interconnect employing a composite bump is shown in a diagrammatic sectional view in FIG. 13. Referring now to FIG. 13, die 302 is provided on die pads in the active side of the die with composite bumps that include a noncollapsible portion 345 and a collapsible portion 347. The collapsible portion may be, for example, a eutectic solder or a relatively low temperature melt solder). The collapsible portion contacts the mating surface of the narrow pad and, where deformation of the fusible portion of the bump over the narrow pad is desired, the collapsible portion of the bump is deformable under the conditions of force employed. The noncollapsible portion may be, for example, a solder having a high lead (Pb) content. The noncollapsible portion does not deform when the die is moved under pressure against the substrate during processing, and does not melt during the reflow phase of the process. Accordingly the noncollapsible portion can be dimensioned to provide a standoff distance between the active surface of the die and the die attach surface of the substrate.
As may be appreciated, the bumps in embodiments as shown in, for example, FIGS. 4, 5, 6 and 7 need not necessarily be fully collapsible bumps. The structures shown in those FIGs. may alternatively be made using composite bumps, or using non-collapsible bumps (high-Pb, or Au) in a solder-on-narrow-pad method, as described above.
And, as may be appreciated in view of the foregoing, an interconnect as appears for example in FIG. 13 can be formed by bringing a non-composite non-collapsible bump (high-Pb, Au) into contact with a narrow interconnect pad provided on the mating surface with a fusible material (such as, for example, a eutectic solder or a relatively low temperature melt solder, which may be provided as a solder paste). Or, the narrow interconnect pad may be provided on the mating surface with a fusible material and the bumps may be composite bumps, also provided with a collapsible (fusible) portion. Where the narrow interconnect pads are provided on the mating surface with a fusible material, it may be preferred to employ a solder mask, followed by a capillary underfill, in the process.
Other embodiments are within the following claims.

Claims

1. A flip chip interconnection, comprising a solder bump attached to an interconnect pad on a die and mated onto a pad at an interconnect site on a substrate, wherein a width of the pad at the interconnect site is less than a nominal width of the contact interface between the bump and the die pad.

2. The flip chip interconnection of claim 1 wherein the width of the pad at the interconnect site is at least about 20% as great as the nominal width of the contact interface between the bump and the die pad.

3. The flip chip interconnection of claim 1 wherein the width of the pad at the interconnect site is at least about 25% as great as the nominal width of the contact interface between the bump and the die pad.

4. The flip chip interconnection of claim 2 wherein the width of the pad at the interconnect site is less than about 80% as great as the nominal width of the contact interface between the bump and the die pad.

5. The flip chip interconnection of claim 2 wherein the width of the pad at the interconnect site is less than about 60% as great as the nominal width of the contact interface between the bump and the die pad.

6. The flip chip interconnection of claim 1 wherein the width of the pad at the interconnect site is greater than about 120% of a trace design rule width.

7. A flip chip interconnection, comprising a solder bump attached to an interconnect pad on a die and mated onto a pad at an interconnect site on a substrate, wherein a width of the pad at the interconnect site is less than a design width of the contact interface between the bump and the die pad.

8. The flip chip interconnection of claim 7 wherein the width of the pad at the interconnect site is at least about 20% as great as the design width of the contact interface between the bump and the die pad.

9. The flip chip interconnection of claim 7 wherein the width of the pad at the interconnect site is at least about 25% as great as the design width of the contact interface between the bump and the die pad.

10. The flip chip interconnection of claim 8 wherein the width of the pad at the interconnect site is less than about 80% as great as the design width of the contact interface between the bump and the die pad.

11. The flip chip interconnection of claim 8 wherein the width of the pad at the interconnect site is less than about 60% as great as the design width of the contact interface between the bump and the die pad.

12. The flip chip interconnection of claim 1 wherein the width of the pad at the interconnect site is greater than about 120% of a nominal trace width.

13. A flip chip semiconductor package comprising a plurality of solder bumps each attached to an interconnect pad on a die and mated onto a corresponding a pad at an interconnect site on a substrate, wherein a width of the pad at the interconnect site is less than a nominal width of the contact interface between the bump and the die pad.

14. The flip chip semiconductor package of claim 13 wherein the width of the pad at the interconnect site is at least about 20% as great as the nominal width of the contact interface between the bump and the die pad.

15. The flip chip semiconductor package of claim 13 wherein the width of the pad at the interconnect site is at least about 25% as great as the nominal width of the contact interface between the bump and the die pad.

16. The flip chip semiconductor package of claim 14 wherein the width of the pad at the interconnect site is less than about 80% as great as the nominal width of the contact interface between the bump and the die pad.

17. The flip chip semiconductor package of claim 14 wherein the width of the pad at the interconnect site is less than about 60% as great as the nominal width of the contact interface between the bump and the die pad.

18. The flip chip semiconductor package of claim 14 wherein the width of the pad at the interconnect site is greater than about 120% of a nominal trace width.

19. A flip chip semiconductor package comprising a plurality of solder bumps each attached to an interconnect pad on a die and mated onto a corresponding a pad at an interconnect site on a substrate, wherein a width of the pad at the interconnect site is less than a design width of the contact interface between the bump and the die pad.

20. The flip chip semiconductor package of claim 19 wherein the width of the pad at the interconnect site is at least about 20% as great as the design width of the contact interface between the bump and the die pad.

21. The flip chip semiconductor package of claim 19 wherein the width of the pad at the interconnect site is at least about 25% as great as the design width of the contact interface between the bump and the die pad.

22. The flip chip semiconductor package of claim 20 wherein the width of the pad at the interconnect site is less than about 80% as great as the design width of the contact interface between the bump and the die pad.

23. The flip chip semiconductor package of claim 20 wherein the width of the pad at the interconnect site is less than about 60% as great as the design width of the contact interface between the bump and the die pad.

24. The flip chip semiconductor package of claim 20 wherein the width of the pad at the interconnect site is greater than about 120% of a nominal trace width.

25. The flip chip interconnection of claim 1, the interconnection being non-solder mask defined.

26. The flip chip interconnection of claim 1, the interconnection being solder mask defined.

27. A substrate for flip chip interconnection of a die, the die having a bumps each attached on a die pad, the substrate comprising a patterned metal layer having pads at interconnect sites, wherein a width of a said pad at a said interconnect site is less than a nominal width of a contact interface between a said bump and the die pad.

28. The substrate of claim 27 wherein the width of the pad at the interconnect site is at least about 20% as great as the nominal width of the contact interface between the bump and the die pad.

29. The substrate of claim 27 wherein the width of the pad at the interconnect site is at least about 25% as great as the nominal width of the contact interface between the bump and the die pad.

30. The substrate of claim 28 wherein the width of the pad at the interconnect site is less than about 80% as great as the nominal width of the contact interface between the bump and the die pad.

31. The substrate of claim 28 wherein the width of the pad at the interconnect site is less than about 60% as great as the nominal width of the contact interface between the bump and the die pad.

32. The substrate of claim 28 wherein the width of the pad at the interconnect site is greater than about 120% of a trace design rule width.

33. A method for forming a flip chip interconnection, comprising providing a substrate as recited in claim 27; supporting the substrate and the die; dispensing a quantity of a curable adhesive on the substrate; positioning the die with the active side of the die toward the die attach surface of the substrate, and aligning the die and substrate and moving one toward the other so that the bumps contact the corresponding narrow interconnection pads on the substrate; applying a force to press the bumps onto the mating narrow pads, sufficient to displace the adhesive from between the bump and the mating narrow pad; at least partially curing the adhesive; melting and then re-solidifying the solder, forming a metallurgical interconnection between the bump and the pad.

34. The method of claim 33 wherein dispensing a quantity of a curable adhesive on the substrate covers at least the interconnection pads on the substrate.