IE83783B1 - A laser machining system and method - Google Patents
A laser machining system and method Download PDFInfo
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- IE83783B1 IE83783B1 IE2002/0203A IE20020203A IE83783B1 IE 83783 B1 IE83783 B1 IE 83783B1 IE 2002/0203 A IE2002/0203 A IE 2002/0203A IE 20020203 A IE20020203 A IE 20020203A IE 83783 B1 IE83783 B1 IE 83783B1
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- gaseous environment
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Description
INTRODUCTION
The invention relates to laser machining of substrates.
Laser drilling of micro-vias with pulsed lasersrnay be performed using two methods.
In the first method a stationary beam is used (pixel vias): Using this technique a
number of laser pulses are delivered to a single point on the substrate. The number of
pulses required to reach a certain depth depends on their energy. This technique is
suitable for vias smaller than approximately 100 microns diameter. The exact via
diameter depends on the laser beam diameter, optical and laser parameters and
material properties.
In another method a beam is scanned along the outer profile of the via. This
technique is suitable for vias larger than approximately 100 microns diameter. The
laser moves in a circular pattern, in one or more concentric circles. Several
repetitions might be required to reach the required depth. The via diameter is a
«function of the radius of the outer circle and the beam diameter. Such a via is.
referred to as a scannedor trepanned "via.
Typically, using a laser beam to machine vias results in several problems. These
problems result in the requirement for several post laser machining process steps.
Specifically the issues are:
Debris
During the laser drilling process debris on the topside of the Wafer is caused by the
accumulation of debris and molten material at the via outlet. This is represented in
Fig. A. It typically appears in two distinct forms. In oneforrn debris appears as a
“lip” of material surrounding the via. The height of the lip can be several tens of pm.
It is believed that one of the processes that may contribute to the formation of the lip
is the re-solidification of molten and gaseous material ejected fiom the via during
laser cutting. . Typically the debris cannot be removed by conventional washing
techniques. An ideal via should have no debris or lip structures.
The second form of debris appears as a “dust” that covers on the top surface of the
wafer. Typically this debris can be removed by a simple Wash process, however it is
more favourable to eliminate the presence of debris completely.
S idewall thermrzl damage zone
A second problem that occurs for via drilling is that the accumulated heat in the
material increases as the pulse rate, pulse energy and total number of pulses of laser
light into the material are increased. At some point the heat dissipated into the
V surrounding material can cause severe thermal damage to the internal walls of the
structure. The impact of thermal damage is to reduce structural integrity due to
rnicrocracking and crystal deformation. One technique to reduce this is to chop the
beam so as to reduce the total energy delivered to the via. This however reduces the
total energy delivered to the surface and is not an efficient technique.
Sidewall composition
Ultimately the objective of the via drilling process is to achieve an insulated rnicrovia
that can be metallized. It is essential that the via is structurally sound and that all
reliability criteria can be met. Using conventional techniques it is not possible to
control the sidewall material composition or conductivity. To better understand the
requirements of the via drilling process the full cycle is described in the next section.
Where an insulating layer is required on the internal walls of a microvia before
insertion of a metal interconnect, two additional processes subsequent to machining
of the via are required. The first is to clean and smooth the via walls followed by a
second process to grow an insulating layer. This three-step process is illustrated in
Fig. B. In step 1, a laser is used to machine 21 via structure with rough tapered Walls.
A taper within 21 via is defined as the “slope” of the side wall, which is arctanfa/b) as
shown in Fig. A. Step 2 is an cleaning step whereby the sidewalls of the Via structure
are cleaned. This preferably results in a smooth, high quality finish. In step 3, an
insulating layer is created on the internalvia walls. '
The objectives of this invention are:
To provide a technique for laser drilling of micro-vias where the amount of debris on
the topside of the via (the side from which the via is machined) is reduced.-
To provide a technique for laser drilling rnicro—vias where the extent of the internal
sidewall heat affected zone within the via is reduced.
To simplify the process of producing a via with desired side-wall morphology,
composition and optical and electrical properties.
To achieve greater versatility in use of laser-drilled vias.
To streamline the process of producing high quality via structures with desired
internal wall properties into a single step process
To reduce the equipment set required in conventional via manufacturing techniques
SUMJVLARY OF THE JNVENTION
According to a first aspect of the invention there is provided a laser
machining system comprising a laser source, and a beam delivery system comprising
means for controlling delivery of a laser beam generated by the laser source to a
substrate to machine the substrate, wherein
the system further comprises a gas handling system comprising means for
providing a controlled gaseous environment around a machining site and wherein
the beam delivery system further comprises means for controlling laser pulse
energy, laser pulse separation, and number of pulses according to optical, thermal, and
mechanical properties of material(s) being machined.
In one embodiment, the beam delivery system and the gas handling system comprise
means for controlling beam pulsing parameters and the gaseous environment to drill a
via in the substrate.
In one embodiment, the gas handling system comprises means for controlling the
gaseous environment to achieve a desired smoothness in a machined wall of the
substrate.
In one embodiment, the beam delivery system comprises means for controlling
pulsing parameters to minimise thermal damage to the substrate.
In one embodiment, the laser pulses are not evenly spaced in time.
In one embodiment, the gas handling system and the beam delivery system comprise
means for controlling laser pulsing and the gaseous environment to provide a
controlled insulating lining in a via drilled in a semiconductor substrate.
In one embodiment, the substrate is of Si material and the lining is SiO2.
In one embodiment, the beam delivery system and the gas handling system comprise
means for controlling beam pulsing parameters and the gaseous environment to drill a
via having a lining suitable for use as an electrical insulator.
In one embodiment, the beam delivery system and the gas handling system
comprise means for controlling beam pulsing parameters and the gaseous
environment to drill a via having a lining suitable for use as an optical waveguide
cladding.
The invention also provides a method in which any of the systems directed above is
used to machine a substrate or workpiece.
According to a second aspect of the invention there is provided a laser machining
method comprising the steps of delivering a laser beam onto a substrate to machine
the substrate, wherein
a gaseous environment is provided around a machining site;
the laser beam is pulsed; and
the laser beam and the gaseous environment are controlled to machine the
substrate to achieve desired properties in the substrate and
wherein laser pulse energy, laser pulse separation, and number of pulses are
controlled according to substrate optical, thermal, and mechanical properties.
In one embodiment, oxygen or nitrogen concentration in the gaseous environment is
controlled to control or prevent oxide or nitride growth as a via lining.
In one embodiment, a controlled amount of inert gas is introduced into the gaseous
environment.
In one embodiment, a gas having properties for dissociation in the presence of the
laser beam is introduced into the gaseous environment, and the dissociated gases etch
the substrate.
In one embodiment, the machining is to drill a via and laser beam parameters are
dynamically varied as a function of current via depth.
In one embodiment, the laser beam and the gaseous environment are controlled to
provide an electrically insulating lining, and the method comprises the further step of
filling the via with an electrically conducting material to provide an
electrical conductor in the substrate.
In one embodiment the laser beam and the gaseous environment are controlled to
provide an optically opaque lining, and the method comprises the further step of
filling the via with an optically transmissive material to provide an optical waveguide
in the substrate with the lining as a cladding.
In one embodiment, the laser beam and the gaseous environment are controlled to
provide a thermally conductive path, and the method comprises the further step of
filling the via with a thermally conductive material to provide a thermally conductive
path in the substrate.
ln one embodiment, the method comprises the further step of connecting a heat sink to
the thermally conductive material in the via.
DETAILED DESCRJ1°_TION OF THE ]NVENTIOH
Brief Description of the Drawings
The invention will be more clearly understood from the following description of
some embodiments thereof, given by Way of example only with reference to the
accompanying drawings in which:-
Fig. 1 is a diagram illustrating laser drilling of a through via according to the
invention;
Fig. 2 is a diagram illustratirig a laser machining system used for via drilling
in a controlled gaseous environment,
Fig. 3 is a diagram illustrating steps for drilling of multiple vias using a multi-
step laser process;
Fig. 4 is a plot of a laser pulse train illustrating the Variables Arm, NT_Ddfly and
T__Delay;
Fig. 5(a)‘ is a View of the wafer subdivided into rectangles of equal in area to
the galvanometer field of view, and Fig. 503) is an expanded View of Fig. 5(a)
showing a typical galvanorneter field of view where d;, cl; and d3 are the
distances between via structures;
Fig. 6 is a plot of ablation depth vs. number of laser pulses;
Fig. 7 is a diagram showing drilling through successive layers of a multi-
layered substrate using a multi-step laser process;
Fig. 8 is a diagram illustrating drilling of a substrate on both sides to complete
a through via;
Fig. 9 is a diagram showing a 1nulti~step laser machining process from both
sides of the substrate in order to achieve a profiled via internal sidewall;
Figs. 10, 11 and 12 are diagrams illustrating optical parameters for laser
drilling;
Fig. 13 is a diagram showing the area surrounding a via over which a
defocused laser beam is placed for laser cleaning;
Fig. 14 a set of views of a substrate of an opto-electronic component of the
invention;
Fig. 15 is a perspective view showing the top surface of the substrate in more
detail;
Figs. 16 and l7 are diagrams showing formation of a V—shaped groove for an
optical fibre;
Fig. 18 is a perspective view of an optical fibre;
Fig. 19 is a perspective View illustrating View in more detail; and
Fig. 20, 21, and 22 are diagrams illustrating drilling of angled vias;
Fig 23(A) is a diagram showing conventional location of pads and solder balls
on the same side of the die as the active device
Fig 23(B) is a diagram showing the relocation of pads and solder balls on the
back side of a wafer that is accessed using laser machined via structures; and
Fig 24 illustrates the connection of four 1/ 0 points of an active device to a
common ground or common power point on the bottom of a wafer
Descri tion of the Embodiments
A substrate is machined to provide a formation such as a via in the substrate. There
is excellent control over the sidewall physical morphology, structural integrity and
composition. Also, there is excellent control over the amount and location of debris
generated. Also, the invention provides improved versatility for via laser drilling,
whereby blind vias, multilayer vias, and angled vias may be drilled in a simple and
effective manner.
This parameter is defined as the number of vias that can be drilled in each second
The unit of machining speed is the number of vias per second. It is always the
intention to optimise this parameter to the highest value, while maintaining
acceptable cut quality.
Via Cutting Time (T Delay)
This parameter is defined as the time the galvo will remain at a particular via
position during a given laser machining step.’ The depth of the via drilled win be
proportional to T _,Delay. Normally this parameter is optimised to the lowest possible
value in order to obtain the highest machining speed (V,,m,,) with acceptable via
quality. The optimum T__Delczy for a via machining process will vary according to the
physical properties (such as thermal conductivity) of the material to be machined,
plasma expansion/relaxation time, distance between Vias, via quality, laser pulse
energy and other parameters. A higher T__DeIay will typically cause more thermal
damage to the via structure.
Single Pulse Period (Atgg)
This parameter is defined as the temporal distance between laser pulses in a pulse
train and is essentially the inverse of laser repetition rate:
A = —————-—-—-—
Repetirion Rate
F or example, for a laser repetition rate of 45 kHz, Arm ~ 22 its.
Number of Pulses NEW)
This parameter is defined as the number of pulses to be delivered to‘ a certain via
position during a given laser machining step. T_DeZay, Arm and NT M, are related to
each other by the following expression:
T*Delay = NT_D:ImJ >< Arm;
These parameters are depicted in Fig. 4.
This parameter is defined as the total number of steps required to machine a single
via structure to the required depth. The total number of steps will determine the
depth of the via that is drilled.
Densigg of Vias (NWM)
This parameter is defined as the total number of vias Within the permitted working
field of View of the galvonometer(galvanorneter) scanner. Np,-_,,,,—c,‘,is illustrated in Fig.
Distance between Vias (aim:
This parameter is defined as the distance between two vias, and normally is different
from via to via. dy,-, is illustrated in Fig. 3 and Fig. 5.
Galvo Jump Speed (J Speed)
This parameter is defined as the speed of the galvanometer movement between two
via structures and is measured in units of metres per second (ms"). A higher value of
J_Speea’ results in an increase in machining speed.
Jump Deiav (J Delay}
This parameter is defined as the settling time for the galvanometer after moving to a
new via position. If J_Delczy is too short this will result in a “spiking” laser line
following movement from one via to the next. However, a long J_DeZay will result in
a lower machining time. A higher galvo jump speed (J_.S‘peea') normally requires‘ a
higher jump delay (J_DeZay), and hence the optimisation of these two parameters is
essential in order to obtain the highest machining speed with acceptable cut quality.
Optical Parameters
Beam Diameter
Beam diameter refers to the 1/ e squared diameter of the spatial intensity profile of
the laser. In via machining, the diameter of the via is a function of the beam
diameter. Beam diameter is a variable that can be controlled through selection of the
focusing lens, scanlens or beam telescope. The effect of modifying beam diameter is
to modify the power density level at focus.
P eak Power Density (Intensity) .
This parameter is defined as the peak power per unit area, where the peak power is
the energy per second.
Peak Power Density = Ar XA
Where E is the energy in Joules, A2‘ is the pulse width in seconds and A is area in
centirnetres squared and the peak power density is in Watts per centimetre squared.
Energy Density
. E
Energy Density = E
This parameter defined as the energy (E) in Joules divided by the area (A) in
centimetres squared. The unit of energy density is Joules per centimetre squared.
Depth of Focus
The depth of focus for focused beams of second, third and fourth harmonic YAG,
YLF and Vanadate type lasers is larger than in multirnode lasers and in lasers with
large M squared values. This is primarily due to the fact that the spatial output from
YAG, YLF and Vanadate type laser systems is Gaussian. A large depth of focus is
highly advantageous for via machining in thick wafer substrates, as generally the
Wafer thickness is such that it can be placed at a fixed distance from the Working lens
without repositioning to compensate for defocusing efi°ects. However, under certain
circumstances, the beam may require certain defocusing in order to improve the wall
quality or taper angle Within the via.
Formation of through hole micro-vias
Referring to Fig. 1 a substrate 1 is drilled with an insulating layer on the via side-
walI2 by controlled laser pulses 3 and/or a controlled gas flow 4. A substrate is
defined as a workpiece to be machined,‘ which may include but is not limited to
Wafers and other semiconductor, electronic or optoelectronic device. This substrate
may consist of a single material, or a combination of materials including, but not
limited to, a layered structure consisting of a single material or multiple materials,
and these materials may or may not be patterned.
Combining the control of laser pulses and gas flow or using just one of these
individual processes will enable machining of vias with significantly reduced debris
and significantly reduced sidewall thermal damage. For example a smooth, high
quality internal Wall surface and a low degree of via taper can be produced using a
rnulti~step laser machining process whereby a controlled number of laser pulses (with
variable inter-pulse separation and pulse energy) are delivered to the substrate within
a certain time period. This reduces thermal damage to the via sidewalls, thus
resulting in smooth internal sidewalls.
A laser controller controls the laser pulse energy, inter—puIse separation and the
number of pulses per laser machining step on the basis of the optical, thermal and
mechanical properties of the material to be machined, the machining depth within
the substrate material and also the laser type to be used.
A gas handling system controls the gaseous environment. In one embodiment, the
environment includes photo—activated etchants of the substrate material. The oxygen
concentration in the environment is selected to remain constant or to vary during
machining of the via suucture, thus promoting growth of an oxide layer on the
internal via walls during laser machining. Control of the oxygen concentration or the
concentration of oxygen containing gases (e.g. CO2) permits control of the oxide
layer thickness required for electrical insulation of a typical metal interconnect
inserted into 3. via machined in a silicon substrate.
Referring to Fig. 2 a laser machining system 10 comprises a galvanometer 11 and a
telecentric lens 12 providing a Q-switched UV laser beam 1'3. The beatn 13 is
directed onto an access Window 14 of a cleanroom chamber 15 containing a wafer 16
to be drilled. The system 10 comprises gas inlets 17 and a vacuum pumping lines 18
for the chamber 15. The chamber 15 is mounted on an X~Y translation stage.
Laser light enters the window 14 of the gas reaction chamber 15, which is capable of
withstanding pressures of up to 10 bar. Alternatively, this chamber can be evacuated
to typical pressures in the region. of 0.1 .bar for machining at pressures below
atmosphere. Different gases are introduced into the chamber l5 by the series of mass
flow controllers 17, which permit control of the relative gas concentrations in the
chamberl5 for laser machining in a static gas environment. Also, the mass flow
Controllers 17 and 18 on the inlet and vacuum outlet lines permit control of the flow
rate of different gases for machining in a non—stat.ic gas environment.
The vacuum exhaust line 18 permits evacuation of the chamber 15 below
atmospheric pressure and is also used to vent Waste gaseous products produced
during the laser machining process. A filter connected to the vacuum line permits
filtering of unwanted waste products and recycling of unused gases. A detector
connected to the chamber 15 permits measurement of the relative and absolute
concentrations of parent gases and by-products produced during laser machining.
- Smooth internal via Walls are. achieved by varying the laserpulse energy and the
ir1ter—pu1se separation at different steps throughout the multi-step process used to
machine a single via structure. This permits control of thermal loading in the
substrate to be machined, thus preventing excess damage of the via side Walls due to
thermal stress. '
Also, introduction of fluorocarbon gases (e.g. CF4) into the chamber 15 during
machining of the via structure results in reduced debris machining as photo-
dissociated fluorine assists in silicon removal in a gaseous form. Other photo-
activated gases used in low debris machining of silicon the gas reaction chamber
include, but are not limited to, cblorofluorocarbons and halocarbons.
By combining the above two process, a superior quality via can be achieved.
Laser machining of via structures in an inert gas environment such as helium and
argon permits suppression ' of oxide growth on the internal via sidewalls for
applications requiring non—insulating side-walls, The introduction of nitrogen into
the gas reaction chamber during machining permits growth of the insulating material
silicon nitride on the internal via side-Walls for applications requiring non—insulating
side walls.
Following laser machining, a layer is formed on the internal side—wall of the via
structure. This layer is formed when substrate material, melted during laser
machining, re-solidifies upon cooling. Through suitable choice of gas mixes, their
flow rates and their relative concentrations the stoichiometry, microstructure and
other properties of this layer can be altered so as to produce a sidewall with electrical
and/ or optical properties best suited to the desired application. Gas mixes include:
Active (e.g. 0;, C02) 4- Inert gases (e.g. He, Ar) for control of oxide growth in the via
intemal side-walls, Nitrogen + Inert gases (e.g. He, Ar) for control of nitride growth
in the via internal side¥walls, Active (e.g. O2, CO2) or Nitrogen + Etcham gas (e,g_
chlorofluorocarbons, halocarbons) for control of oxide or nitride growth in the via
internal side-Walls with reduced surface roughness and reduced debris inside and
oumide the via structure. '
Multi~step micro via machining
Fig. 3 shows the basic operation of a multi step via machining strategy. In this
strategy, the laser beam stays at a single via position for a certain predetermined
period of time (i.e. T__Delay) and will then be moved to another via position. The
laser beam will then move back to the first via position after it has finished cutting
the last via. This procedure is repeated another (NM-1) times. One purpose of the
‘multi’ step approach is to reduce the heat affected zone (HAZ) within the via, which
is believed to contribute to debris and to downgrade the side-wall quality of the via.
Also, by changing the laser pulse and beam properties at different steps in a multi-
step process, mulitlayer structures consisting of diflerent materials can be efficiently
machined during each step.
Ma" ScedV
The machining speed as a function of other laser processing parameters can be
derived as follows:
For a given galvanometer field of View as depicted in Fig. 5.
Time required to cut a single step for a via = -T_DeZay
. : N T_ D510), X
Total time required to cut a single step for all was to be machined (t1)
= Nvza/new X T _DeIay
ti = Nwh/flew X Nr_pa.,y>< At”
The total jump distance of the ’ galvanometer for a single cutting step .—_ the total
distance (dvga) travelled by the galvanometer for N V,,,,,~_.1,, number cf viag
i=NvialޢM
= 2%
1'2]
The total galvanometer jump time required to machine a single step for NV,-,,,M
number of vias (tg)
"Wyn//tau
2%
.. i=1
—i J_S'peed
The total galvanometer setting time during machining of a single step for Ny,,,fi,,d
number of vias (t3)
= J-_D€[dy X ]\.r,.,'a/fig;
The total time required to complete a single via drilling step for NWM number of
vias '
= ix + t2 + is
Assuming all the beam parameters are remain unchanged throughout all the process
steps, the total time required to complete all via drilling steps in a multistep process
for a single galvanometer field ofview is
: .N;,q,X{t1+ tz ‘i’ 1:3)
The number of vias drilled per second (i.e. Machining Speed or V,,,,,c,,)
Nvia /field
NW ><(t1+t2 +r3)
N via I field
‘' “N vial /2.51
(NStep)X[([v'v1'a/field >
+(J_DelayXNv,a,fie,d)]
(1.1)
From the expression (1.1) above, it is clearly indicates that one of the critical
parameters that determines the machining speed is N34,, i.e. the lower the NM the
higher the machining speed. Since the total number of pulses that is required to
machine 21 via is constant at a certain thickness of the machined material, one of the
possible ways to reduce NM is to increase the number of pulses fired (NT_Dg]ay) in each
step. However, an increase in N734“, may intro_du_ceV thermal damage in the via
structure due to the fact that a higher number of pulses is directed on each via at each
instant These two parameters are optimised in order to obtain the highest machining
speed with the best via quality.
Expression (1.1) also shows that the machining speed is proportional to J_Speea' but
inversely proportional to ./'_Delay. A higher J_S_peea7 (hence higher machining speed)
requires a longer f__DeZay (i.e. higher galvanometer setting time), which will in turn
reduce the overall machining speed. For a given substrate ‘material and laser type,
these two parameters are optimised in order yield the highest machining speed.
As shown in expression (1.1), another parameters that determines the machining
speed is the laser repetition rate, i.e. the higher the repetition rate, the lower the At“
and hence the higher the rnachining speed. This can be understood as more pulses is
delivered to the via in higher repetition rate which will increase the overall
machining speed. However, a laser pulse operated in higher repetition rate may
result in a lower average power due to the natural characteristics of the laser.
Furthermore, a higher repetition rate also has the potential to cause more thermal
damage on the side wall as more pulses are delivered to the via in a shorter period.
For a given type of laser, this parameter is optimised to yield the highest machining
speed with the best via quality.
Expression (1.1) also shows the machining speed is inversely proportional to the
i=Nm/ field _. a
total distance between via (i.e. ‘ Zdm ), i.e. the longer the distance, the lower the
i=1
machining speed. As part of invention a software algorithm has been developed
to determine the most ‘elfective’ (i.e. shortest) distance travelled by galvanometer for
a given set of via distributions Within the galvo field‘ in order to obtain the highest
possible machining speed.
Example 1
In a process of drilling 1000 was in a galvo field of 15 mm X 15 mm at a laser
repetition rate 01°55 kl-Iz, using two pulses on each via and 50 machining steps, for a
jump speed of Sm/sec and a jump delay of 50 pus. The total distance between all the
vias is ~ 0.48 m.
The machining speed can be estimated as below:
——«— = 18.19 us = 18.19 X 10‘ sec.
55000
Afjgz :
Nmamgp = 1000
J_.S'peed= 5 In/sec
J_DeIay = 50 p5 = 50 X 10*‘ sec
NT_Ddgr = 2 (for‘2 pulse on each step)
NM = 50
V,,,ad,=
~ 110 vias/ sec.
Fonnation of blind microvias
Through-hole micro—vias enable connectivity between top and bottom surfaces of a
substrate However, in certain applications where it is not required that the via
structure is completely drilled through the substrate material, a blind Via is formed.
Ari example of an application requiring such 21 via type is where conductive layers lie
within a semiconductor or dielectric stack and it is necessary to drill partially through
the Wafer down to the conductor without defacing or darrraging the conductor. In
this specific example of the via drilling process the metal is unharmed and retains its
integrity, the debris produced during machining does not block the conductive path,
and the via is fully drilled through to the metal or conductor.
An example of a structure like this is shown in Fig. 7. As an example the top layer
may be crystalline silicon of thickness Dx+dz, below it is a copper layer of thickness
c and below it is another layer of silicon.
In one embodiment of the invention it is required to drill a via down to the level of
metal, without damaging the metal. In that instance, the via is drilled to depth Dx
using normal via machining parameters. To drill to the metal layer the laser power,
repetition frequency and pulse energy may be modified to remove the thickness dz at
a slower rate. This ensures that excess. energy is not dissipated in the conductive
metal layer and that a clean Contact is left to the top of the metal thin film.
Additionally, performing this machining process in the presence of aphoto-activated
etchant gas or / and multistep process will aid this process.
In another embodiment it is required to drill though the copper at this point, without
damaging the underlying silicon layer. In that case the above two steps are repeated,
and then the laser drilling parameters are altered again so to drill through the copper
without damaging the underlying silicon.
In a further embodiment the semiconductor structure may be a stacked structure of
semiconductor, dielectric (e.g. polymer, quartz, glass) and/ or metal materials.
Depending on the layer the via drilling parameters (pulse separation, pulse energy,
average power, laser foal spot size etc.) may be modified to ensure optimised
machining through each layer of the multilayer structure. Modifying the parameters
in this way ensures that defects such as delarnination, melting and debris are
minimised.
Control of via depth and shape
When drilling vias through a semiconductor, the ablation depth increases
logarithmically with the number of laser pulses. This is represented approximately
for silicon in Fig. 6. In summary, Inachirring‘ vias in thin wafers is exponentially
faster than in thicker Wafers. This data is true if the via drilling parameters are ‘held
COl1SlI3.I1t.
A second impact of the above observation is that the taper of the microvia is
dependent on the laser parameters used to machine the via. ‘Specifically, peak power
and average power.
To improve the removal of material at points deeper in the substrate it is beneficial to
modify the laser parameters so that pulses with higher peak power are used. This
approach enables more efficient removal of material in deeper vias and also enables
control of the micro via taper.
In a further embodiment, the via may be formed by first machining from one side of
the wafer to a depth dl and by flipping the Wafer and drilling to a dept d2 in
registration with the via on the other side of the wafer, the complete via of depth
dI+d2 may be drilled. This is represented in Fig. 8. The effect of this is to eliminate
taper and ensure that top and bottom diameter are identical and also, as the depth of
machining is faster for thinner wafers (or for reduced depth vias) the total number of
pulses required is significantly less than in the case of drilling from one side.
Another embodiment of the invention relates to scanning the beamin a circle or
series of concentric circles with a specific offset. This method is particularly useful for
larger diameter vias where the power density would be too low to be efficient in a
direct pixel via machining approach. Using the techniques involving gas ambient
control and correct delays the via quality of a scanned via is improved greatly. Also
the sidewall morphology and composition can be accurately controlled in this
manner. Finally, using multiple steps in the depthwise direction, the scanned via
taper can be controlled to fonn a nozzle. A schematic of this approach is illustrated
in Fig. 9. This is not limited to purely circular vias. Elliptical via profiles are also
possible through a scanning beam machining process.
Formation of angled vias
Aside from straight through vias and blind vias a further embodiment of the
invention is the formation of angled vias. The benefits of angled vias include moving
topside wafer bumpconnections to the backside of the wafer, reducing topside area
required for connectivity and the ability to connect from topside or bottom-side to a
point at the edge of a micromachined side wall structure to enable contact with an
embedded device.
The logical approach to laser drilling micro-vias at an angle is to tilt the substrate
with respect to the optic axis. In practice this is not easy to implement as it will be
difficult to maintain a constant Working distance between the lens and the work
surface. Only along the axis of tilt can this distance be maintained, with either side
either nearer or further from focus. While oscillating the tiltwill have the effect of
widening the via without giving it any preferred angle in space.
Over a 6" Wafer a tilt of as little as 10° will introduce a vertical displacement of
26min at the periphery and even over a 10mm square area the vertical displacement
will be l.74mm which is significantly outside the depth of focus of all but the longest
scanning lenses.
To be used in such an instance would require automatic refocusing of the beam over
the field of view to compensate for the different relative displacements of the Work
surface from the scan lens. This also can be achieved by moving the vertical position
of the wafer depending on the distance of the machining site from the axis of tilt.
In one embodiment of this invention a non-telecentric lens is used to form angled
rnicrovias. Such a lens is depicted in Fig. 10. The lens diameter is D and the
diagonal of the Field of View is L. Since the lens is non—teIecentric the emergent
beam is not orthogonal to the optic axis (except when incident along the optic axis)
and the largest available deviation angle is 9, which is arctan((L-D)/2*WD). For a
typical non-telecentric F-theta lens with a>Wo1'lcing Distance of 188m, a lens
diameter of 90mm and a Field of View diagonal of 140mm the maximum deviation
angle is 7l.6°..
The available range for angled vias is then, for this particular lens, between 0 and
7.6”. By employing appropriate lenses other ranges may be achieved between 1 and
°.
The range of angles available is determined by the scan angle of the galvanometer
and the lens specification. To achieve control of angle requires that the wafer
position can be controlled relative to the object field point at which the beam forms
that angle relative to the normal angle of the optical system. This can be achieved by
an X-Y table positioning system synchronised with galvanometer and laser.
Variable Focus During Via Drilling
Zoom Telescope
Due to the finite depth of focus of the F-theta lens the via diameter may change with
propagation depth through the substrate, something which may be undesirable. To
mitigate this a variable focus system may be employed that will have the effect of
modifying the focused spot position during the drilling process. Such a system
utilises a zoom telescope in conjunction with the scan lens. The telescope permits the
spot size at focus to be adjusted automatically between 5 and Sflum. An example of
the optical configuration is shown in the schematic of Fig. 11.
The telescope and scan lens system are constructed such that the collimation of the
beam upon exiting the telescope is tailored to permit the scan lens come to a focus at
any position Within 5mm of its nominal working distance. This is illustrated in the
graphic of Fig. 12. Automation and software control integrated into the machining
environment makes this methoda highly reliable and sophisticated solution to the
problem of maintaining precise control of via aspect ratios. It is significantly easier
than drilling some way into the vias, stopping, changing the focus manually by
adjusting the galvanometer/ scan lens position and then completing the via.
In one embodimentof the invention the available via diameters may be configured to
be between 5 and 200p.rn by adjusting the zoom telescope to an appropriate setting
permitting a Wide range of sizes to be achieved.
In a further embodiment the speed of drilling can be increased by adjusting the
collimation of the zoom telescope to bring about different focus positions Within the
workpiece, ensuring a consistent spot size through the body of the via.
In a multi—step process the diameter of the via may be adjusted at each processing
step permitting a precisely controlled via profile.
In a further embodiment the incident beam may, by defocusing the telescope, be
enlarged to a degree such that the incident intensity is less than the ablation threshold
for the workpiece but higher than the cleaning threshold permitting explosive
cleaning of the surface in the vicinity of the via entrance. This is depicted in Fig. 13.
In another version of the invention the sidewall of each via can be cleaned as a final
processing step by directing a beam of lower power into the via and traversing the
sidewalls, thereby removing debris and improving the quality of the finished surface.
Through appropriate adjustment of the main laser parameters such as the
pulsewidth, pulse energy and repetition rate, beam size and divergence the taper
angle of the via may be controlled.
Another embodiment of this invention is to control the wafer [vertical position
relative to the beam Waist at each step in the multi~step process. Variation of the
vertical position relative to the waist has the effect of modifying the beam size at each
step. This is analogous to controlling the plane of focus for a fixed beam diameter.
The following summarises features of the invention.
Producing high quality microviastructures in semiconductor and insulator materials
by means of a multistep laser machining process.
Producing high quality rnicrovia structures in semiconductor and insulator materials
by means of a controlled gaseous environment permitting control of the internal wall
morphology and material composition of the microvia structure.
Producing high quality microvia structures in semiconductor and insulator materials
by means of both a controlled gaseous environment and a multi-step laser machining
process.
The laser pulse energy, laser pulse separation and the number of laser pulses per
machining step are selected based on the substrate optical, thermal and mechanical
properties, the laser type and the depth within the substrate.
A controlled amount of oxygen or an oxygen-containing gas is used in a gas reaction
chamber during laser machining in order to promote controlled oxide growth on the
internal Walls of the microvia structure.
A controlled amount of an inert gas is used in a gas reaction chamber during laser
machining in order to suppress the growth of an oxide layer on the internal walls of
the rnicrovia structure.
A_ gas,~ upon photo- or thermal- dissociationvirr the presence of the machining laser
I beam, produces by-products that are etchants of the substrate to be machined, thus
permitting cleaner machining of the microvia structure and reduction of debris at the
rnicrovia entrance and exit and on the internal Walls.
A gas, which is naturally reactive with by-products of the laser induced material
removal preventing deposition of reacted species.
A multi-step laser machining process is used to control the thermal loading in the
substrate material thus preventing thermal shock leading to cracking and damage to
the internal microvia Walls.
A microvia structure with a diameter in excess of 100 microns is machined by
scanning of the laser beam in a pattern comprising of a series of concentric circles.
A rnicrovia'structure with a diameter less than 100 microns is machined using a laser
beam held in a fixed position on the substrate surface.
The focal spot size of the laser beam is altered using an automated beam telescope
during machining of the rnicrovia structure in order to define the internal side-wall
Contour.
Subsequent to machining of the microvia structure the focal spot size of the laser
beam is increased so as to permit laser cleaning of the debris field on the surface of
the substrate surrounding the microvia entrance aperture.
The laser focal spot is scanned through the substrate material during machining of
the microvia structure by adjusting the collimation of an automated beam telescope
resulting in control of the internal side-wall contour of the microvia structure.
A sloped microvia structure is drilled at an acute angle by tilting the wafer with
respect to the beam.
In one embodiment, the microvia is drilled at an acute angle with respect to a
substrate surfacewusing a non-telecentric lens thus permitting access to a device
within the substrate, thereby allowing a layered component architecture and also
allowing tighter geometric positioning of components
A series of sloped microvia structures are machined fiom a common entrance
aperture on one side of a substrate to the same or dilferent depths in the interior or to
the bottom of the_subst1'ate, thus permitting electrical connection of multiple points
in the substrate with a common point on the substrate surface following insertion of
conducting material into the microvia structures.
A series of sloped rnicrovia structures are machined from one side of a substrate to
connect with a common point in the interior or on the other side of the substrate,
thus permitting electrical connection of multiple points in the substrate surface with a
common point on the substrate interior or on the other site of the substrate following
insertion of conducting material into the microvia structures.
A via is drilled through the substrate to electrically interconnect opposed sides of the
sub strate .
A via is drilled as a blind via or through via to facilitate thermal contacting and heat
dissipation.
The substrate is of silicon material and the insulating lining is of SiO;.
The laser beam has a wavelength in the VIS—UV range.
Example of application of the invention .
In recent years there have been major developments in production of integrated
electronic components. However, because of the mechanical requirements for
positioning of items such as waveguides and optical sources and detectors
automation of production of optical components has not been achieved to any
significant extent.
This invention enables device structures that simplify manufacturability of such
Components.
For example an optical component may be produced by:-
providing a substrate;
etching a trench in the substrate using a laser beam; and
mounting an optical device in the trench.
The method comprises the further steps of laser drilling vias to provide electrical
conductors. Additionally, the capability to machine a slopedovia drilled at an angle
with respect to a substrate surface for access to a device within the substrate allows a
layered component architecture. The benefit of such a structure is that multiple
contact pads on one side of the wafer or substrate can be connected to a single pad or
trench on the opposing side of the wafer or substrate. As described previously the
system allows formation of an Si/SiO2 or alternative insulating lining on the via.
Referring to Figs. 14 and 15, a substrate 101 for an optical component is illustrated.
The substrate is a Silicon Optical Bench (SiOB). The substrate 101 comprises a V-
shaped groove 102 and trenches 103 and 104 for an optical source and associated
Peltier thermal control elements. Sloped microvias 105 are laser drilled to reach the
trenches for electrical connection. Laser drilling is also used to create mounting
holes 106. Also, through vias (not shown) are drilled through the substrate for
electrical access of one side to the other.
In the drawings, an optical fibre is indicated by the numeral 10, in which there is a
core 111 and cladding 112. The etching progress is illustrated in Fig. 17, in which a
spot pattern 116 is illustrated.
The use of angled (sloped) vias allows layering of layered components in which
components are placed one above the other, while still making required electrical
contacts. This allows increased component density. One architecture is placing of
electrical circuits on the substrate surface and optical devices within the substrate
body.
An aspect of the laser via drilling is that by appropriate control of the drilling
parameters and gases such as oxygen an insulating lining is inherently created. _'l'hus
the solder which is filled into the vias is insulated from the surrounding
semiconductor material without the need for adding a discrete insulating lining.
Thickness of the insulating lining can be controlled by suitable control of the laser
parameters and gas environment. In one embodiment, the laser beams are UV or
visible, for example 355 nm / 266 run UV systems and 532nm Green laser systems
and having a repetition frequency greater than 1 kHz.
Referring to Figs. 20 to 22, the methods by which the sloped vias are drilled is
illustrated. As shown in Figs. 20 and 21, a non-telecentric lens is used to direct the
output from a galvanotneter laser head.
Referring to 22, an alternative method is to tilt at an appropriate angle using a
tilt stage. It may be necessary topadjust the height of the Wafer to ensure the depth of
focus is correct. However if the wafer can be tilted about the particular point where
the via is to be formed this adjustment of height may not be necessary. i.e. the
required change in depth is simply the distance of the via from the tilt point times the
sine of 90°minus the required angle for the via.
Referring again to Figs. 14 to 19, there are angled vias Whose entrances are on the
top of the wafer and which exit in the chamber where the laser will be placed The
laser is on its side allowing for the electrical connects. A chamber beneath the laser is
created for a thermistor and the whole system is placed on a peltier. The laser is
placed in the chamber and then appropriate electrical material is placed in the trench
ensuring that safe clean contacts are made with the laser Without causing damage to
it. The ‘ground’ trench could be placed to the right of the chamber with the
appropriate electrical material helping to keep the laser chip stable.
The laser sources for drilling and etching are second and higher order harmonic
frequency solid state lasers operating with high pulse energies and nanosecond pulse
widths. Implementation of the invention is possible with shorter pulse width lasers
but central to the high throughput aspect of this invention is that the laser must
operate at a high repetition rate.
The following are advantageous features of the invention.
The formation of V-grooves on an integrated optical chip (IO C) used for optical
fibre alignment by laser ablation, using, for example an N d:Yag frequency tripled or
quadrupled laser. Where SiOB is the objective, 35 Snrn and 266mm are particularly
useful. Where LiNbO3 and insulating substrates are used 266nm ( or fourth
harmonic) lasers are the preferred option.
The formation, by laser ablation, of component slots or trenches for alignment
and/or housing ofi for example laser diode chips or pliotodiode chips represent
examples of blind vias.
The formation , by laser ablation of through—vias on an integrated optical chip (IOC)
to allow electrical contacts from one side through to the other side of a Wafer.
The formation, by laser ablation, of sloped or angled through—vias where typically the
vias start at the top (or bottom or side) of the Wafer and will exit in a trench allowing
for precise electrical connections to a particular component to be placed in the
trench. In the case of silicon, with a specific set of laser parameters, the via side wall
composition may be controlled to consist of glass like silicon dioxide. The
composition and structure of this oxide may further be controlled by the presence or
absence of a gas and the laser parameters. For example removal of oxygen from the
ablation reaction will reduce the amount of SiO2.
In another embodiment, the microvia enables formation of an optical waveguide
through connection. The waveguide structure is, defined by a cladding and a core
region. In this example the cladding is formed partially by control of the laser
machining parameters and gas environment so that a glassy layer is formed on the
internal via side—Wall. The optical waveguide can then be formed through filling the
via with appropriate plastic or glass material with sufficiently high transmission to
form an optical waveguide through via.
Exarnnle of auolication of the invention
Angled vias can be of use for shriiiking the size of a die, especially in the case of die
which have active devices and pads on the same side and which have comparable
areas of the die utilized by the pads and the devices. This is of particular interest for
these cases where the devices have a low number of I/ 0 connections, but can still
hold true for die with a larger number of 1/ 0 connections where the edges of the die
contain the majority of the pads (edge-leaded die). A
In one embodiment of the invention, the device would exist on one side of the die
with its position roughly centered on the die face (see Fig. 23). The 1/03 are
distributed around this device. Vias are drilled from the contact locations on the
i device to a series of pads. Instead of using pads which extend outward from the
device to the edge of the die as in Fig. 23A, the vias are metallized (or otherwise
made into conduction paths), and pads are now formed on the back of the die as in
Fig. 23B. The angle of the vias allows for the I/ Os to be positioned all the way in to
the center of the die backside, allowing for uniform population of the surface of the
back of the die.
Now that the device is the only feature on the front side of the wafer, the die size
may be significantly reduced (by the area which was previously taken up by the bond
pads). This not only allows for smaller die and smaller package sizes, but also allows
more die to be created on the same wafer during Wafer fabrication. While the first of
these benefits is primarily a product feature benefit (smaller die, less real estate
wasted in the final device), the latter one has the potential to significantly reduce the
manufacturing costs incurred in wafer fabrication for making these devices (the same
number of die can be made on fewer wafers).
In a further embodiment of the invenfion that is specific to this example the vias
don't necessarily have to be angled, particularly when there is a small number of 1/ 0
connections from the active device. ln this instance, straight vias can be machined
from the contact locations on the device through to pads located on the backside of
the die. Vias at the edge of the die will connect to pads on the bottom side of the die
that can be patterned in towards the centre of the die, thus maintaining the footprint
of the active device from the top to the bottom of the die and thereby still permitting
an increased number of active devices on the front side of the wafer.
In a further embodiment of the invention, angled vias can be useful for redistribution
of 1/ 0 connections on a die. In a typical bumped die, approximately a third of the
1/ 0 connections are for either power or ground. When these leads go out to the
package, all the grounds are connected together and all the power leads are also
connected. Only the signal leadsneed to remain independent. As such, angled vias
allow for an altogether new approach. The grounds and/ or the power leads can be
- Connected at the pads themselves when the wafer is bumped on the back. In this
technique, angled vias would point in different directions from the bump with respect
to the in-plane component of their trajectory (see 24). As such, one bump on the
back of the wafer can be connected to multiple contact points on the front of the
wafer [e. g. one pad for four ground contacts is feasible).
The invention is not limited to the embodiments described but may be varied in
’ construction and detail.
Claims (8)
1. A laser machining system comprising a laser source, and a beam delivery system comprising means for controlling delivery of a laser beam generated by the laser source to a substrate to machine the substrate, wherein the system further comprises a gas handling system comprising means for providing a controlled gaseous environment around a machining site and wherein the beam delivery system comprises means for controlling laser pulse energy, laser pulse separation, and number of pulses according to optical, thermal, and mechanical properties of material(s) being machined.
2. A system as claimed in claim 1, wherein the beam delivery system and the gas handling system comprise means for controlling beam pulsing parameters and the gaseous environment to drill a via in the substrate.
3. A system as claimed in claims l or 2, wherein the gas handling system comprises means for controlling the gaseous environment to achieve a desired smoothness in a machined wall of the substrate.
4. A system as claimed in any preceding claim, wherein the beam delivery system comprises means for controlling pulsing parameters to minimise thermal damage to the substrate.
5. A system as claimed in claim 4 wherein the laser pulses are not evenly spaced in time.
6. A system as claimed in any preceding claim, wherein the gas handling system and the beam delivery system comprise means for controlling laser pulsing and the gaseous environment to provide a controlled insulating lining in a via drilled in a semiconductor substrate.
7. A system as claimed in claim 6, wherein the substrate is of Si material and the lining is SiO2.
8. A system as claimed in any preceding claim, wherein the beam delivery system and the gas handling system comprise means for controlling beam pulsing parameters and the gaseous environment to drill a via having a lining suitable for use as an electrical insulator. A system as claimed in any of claims 1 to 7, wherein the beam delivery system and the gas handling system comprise means for controlling beam pulsing parameters and the gaseous environment to drill a via having a lining suitable for use as an optical waveguide cladding. A method in which any of the systems claimed in any of the preceding claims is used to machine a substrate or workpiece. A laser machining method comprising the steps of delivering a laser beam onto a substrate to machine the substrate, wherein a gaseous environment is provided around a machining site; the laser beam is pulsed; and the laser beam and the gaseous environment are controlled to machine the substrate to achieve desired properties in the substrate and wherein laser pulse energy, laser pulse separation, and number of pulses are controlled according to substrate optical, thermal, and mechanical properties. A method as claimed in claim 11, wherein oxygen or nitrogen concentration in the gaseous environment is controlled to control or prevent oxide or nitride growth as a via lining. A method as claimed in claim 12, wherein a controlled amount of inert gas is introduced into the gaseous environment. A method as claimed in any of claims 11 to 13, wherein a gas having properties for dissociation in the presence of the laser beam is introduced into the gaseous environment, and the dissociated gases etch the substrate. A method as claimed in any of claims 11 to 14, wherein the machining is to drill a via and laser beam parameters are dynamically Varied as a function of current via depth. A method as claimed in any of claims 11 to 15, wherein the laser beam and the gaseous environment are controlled to provide an electrically insulating lining, and the method comprises the further step of filling the via with an electrically conducting material to provide an electrical conductor in the substrate. A method as claimed in any of claims 11 to 15, wherein the laser beam and the gaseous environment are controlled to provide an optically opaque lining, and the method comprises the further step of filling the via with an optically transmissive material to provide an optical waveguide in the substrate with the lining as a cladding. A method as claimed in any of claims 11 to 15, wherein the laser beam and the gaseous environment are controlled to provide a thermally conductive path, and the method comprises the further step of filling the via with a thermally conductive material to provide a thermally conductive path in the substrate. A method as claimed in claim 18 comprising the further step of connecting a heat sink to the thermally conductive material in the via.
Priority Applications (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
IE2002/0203A IE83783B1 (en) | 2002-03-22 | A laser machining system and method |
Applications Claiming Priority (4)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
EPEUROPEANPATENTOFFICE(EPO)22/03/2 | |||
EP01650029 | 2001-03-22 | ||
IE20010298 | 2001-03-22 | ||
IE2002/0203A IE83783B1 (en) | 2002-03-22 | A laser machining system and method |
Publications (2)
Publication Number | Publication Date |
---|---|
IE20020203A1 IE20020203A1 (en) | 2002-10-02 |
IE83783B1 true IE83783B1 (en) | 2005-01-26 |
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