Blind via formation in a photoimageable dielectric material

Details Blind via formation in a photoimageable dielectric material Blind via formation in a photoimageable dielectric material

1999-06-30

2003-05-27

Baxter, Janet (Department: 1752)

Radiation imagery chemistry: process, composition, or product th

Imaging affecting physical property of radiation sensitive...

Making electrical device

C430S313000, C430S319000, C430S320000, C430S322000

Reexamination Certificate

active

06569604

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a blind via structure, and associated laser ablation methods of formation, that includes a blind via within a photoimageable dielectric (PID) layer on a substrate, such that the sidewall of the blind via makes an obtuse angle with the blind end of the blind via.
2. Related Art
Electronic structures commonly include blind vias coupled to a dielectric substrate. A blind via is characterized by an open end and a closed end (“blind end”). If the sidewall of a blind via is plated with metal and subsequently treated with solder, the blind via may serve to electrically couple a circuitization layer on the substrate with such electrical interconnects as a ball grid array (BGA) or a controlled collapse chip connection (C
4
) solder ball. The BGA may be coupled to an electronic carrier such as a circuit card. The C
4
solder ball, which may have any utilizable geometrical shape such as a spherical or columnar structure, may be coupled to an electronic assembly such as a chip. A blind via may be formed within a dielectric substrate by mechanical drilling or by laser ablation. With either method of blind via formation, each blind via must be individually and separately formed. This is costly and time-consuming for electronic structures that require numerous blind vias on the same dielectric substrate.
A known method of forming blind vias simultaneously includes use of a photoimageable dielectric (PID) material. If a PID layer is formed on a substrate, blind vias may be formed within the PID layer by a known photoimaging process that cross links (i.e., polymerizes) photoexposed PID material and allows multiple blind vias within the PID layer to be formed simultaneously. This known process is illustrated in
FIGS. 1-4
.
FIG. 1
shows a front cross-sectional view of a PID layer
12
on a substrate
10
for the purpose of forming one or more blind vias within the depth of the PID layer
12
. In particular,
FIGS. 1-4
depict the formation of two blind vias of different widths. The PID layer
12
may be formed on the substrate
10
by any known method in the art. A preliminary step in the process of blind via formation is baking the PID layer
12
(on the substrate
10
) at a temperature, such as between 85 and 95° C., that will evaporatively remove residual solvents within the PID material. The residual solvents are impurities that may interfere with cross linking of the subsequently photoexposed PID material.
In
FIG. 2
, a mask layer
14
has been placed on the PID layer
12
. The mask layer
14
includes an opaque region
16
and an opaque region
18
. The opaque region
16
and the opaque region
18
, which may be formed inter alia by emulsions on a glass-plate representation of the mask layer
14
, are located above portions in the PID layer
12
where blind vias will be subsequently formed. The mask layer
14
also includes a transparent region
30
. The PID layer
12
includes a blocked region
26
under the opaque region
16
, a blocked region
28
under the opaque region
18
, and an unblocked region
34
under the transparent region
30
.
Referring to
FIG. 3
, blind vias will be subsequently formed from the blocked region
26
, the first blocked region
28
, and portions (called “soft” regions) of the unblocked region
34
. A soft region includes a portion of the unblocked region
34
which contacts a blocked region and is geometrically positioned to be subsequently etched away, as will be explained infra. As shown in
FIG. 3
, the unblocked region
34
includes a soft region
27
which contacts the blocked region
26
, a soft region
29
which contacts the blocked region
28
, and a hard region
32
which include all portions of the unblocked region
34
which are not soft regions. As will be described infra and depicted in
FIG. 4
, a first blind via
46
will be subsequently formed from the blocked region
26
and the soft region
27
, and a second blind via
48
will be subsequently formed from the blocked region
28
and the soft region
29
.
FIG. 3
depicts an “exposure” step in which a light source
20
directs light
22
of a suitable wave length, such as ultraviolet radiation, onto the mask layer
14
. The light
22
passes through the transparent region
30
of the mask layer
14
into the unblocked region
34
of the PID layer
12
. The light
22
initiates a cross-linking process which polymerizes the PID-exposed material in the unblocked region
34
. The rate and magnitude of cross linking decreases with increasing distance in the direction
60
as measured from the top surface
36
of the unblocked region
34
. PID material that is not cross linked (i.e., PID material in the blocked region
26
and in the blocked region
28
) tends to dissolve, or be washed away, if contacted by a developer solution. PID material that is cross linked (i.e., PID material in the unblocked region
34
, especially in the hard region
32
) tends to resist being dissolved if contacted by a developer solution. After the preceding exposure step, the mask layer
14
is removed.
After the exposure step, the PID layer
12
may optionally be allowed to stand at room temperature for a period of time, typically between a half-hour and two hours. This optional “standing” step relates to the fact that the cross linking process in the unblocked region
34
may continue to occur even after exposure of the mask layer
14
to the light
22
has been terminated. The time duration of the standing step, and whether the standing step should be utilized, depends on the cross-linking characteristics of the PID material in relation to the light
22
; i.e., on the rate of crosslinking that occurs both during and after exposure of the mask layer
14
to the light
22
in consideration of the amount of cross linking that prevents light-exposed PID material from being washed away by a subsequently-applied developer solution.
After the mask layer
14
is removed, and after the exposure step (or after the standing step if elected), the PID layer
12
is exposed to a developer solution, such as by spraying, which washes away the PID material that has not been cross linked or sufficiently cross linked. In this “developing” step, the washing away of the non-polymerized PID material in the blocked region
26
, and of the insufficiently polymerized PID material in the soft region
27
, results in formation of the first blind via
46
as shown in FIG.
4
. In
FIG. 3
, the PID material in the soft region
27
is insufficiently cross linked to resist being washed away because of the attenuation of cross linking with increasing distance in the direction
60
, as stated previously. In effect, the less cross-linked PID material at the bottom of the soft region
27
is more susceptible to being washed away by developer solution than is the more cross-linked PID material at the top of the soft region
27
. Thus, after the developer solution washes away the non-polymerized PID material in the blocked region
26
, the residual developer solution within the blocked region
26
contacts and washes away the insufficiently cross linked material in the soft region
27
. Similarly, the washing away of the non-polymerized PID material in the blocked region
28
, and of the insufficiently polymerized PID material in the soft region
29
, results in formation of the second blind via
48
as shown in
FIG. 4. A
blind via formed as described above, such as the first blind via
46
or the second blind via
48
, is called a “photovia”. After the preceding developing step, cleaning may be accomplished, if necessary, by any known method such as by rinsing the PID layer
12
with water.
FIG. 4
depicts a sidewall
56
of the first blind via
46
as making an angle &thgr;
1
with respect to the blind end (i.e., bottom bounding surface)
52
of the first blind via
46
. Similarly, the sidewall
58
of the second blind via
48
makes an angle &thgr;
2
with respect to the blind end
54
of the second blind via
48
. &thgr;
1
and &thgr;
2
are acute angles (i.e., less

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