The phenomenon of zero surface shrinkage is called Zero-Shrinkage.

The big disadvantage of LTCC substrate is the volume shrinkage after firing. Nowadays "Unconstrained Sintering" (UCS) method is the most pervading technology for firing of LTCC substrate. As against the many advantages of the technology the number of the layers is limited. As more layers are laminated and fired as more difficult is the positioning of the through holes. In contrast of Unconstrained Sintering method essentially almost eliminates the x/y-shrinkage.

There are different constrained sintering processes:

The sintering methods are summarized in the figure below.

Sintering technologies

The development of these technologies above are shown in the next figure.

The developing of sintering technologies and the results of them

Self-Constrained Pressureless Sintering (SCPLAS) is a new technology which works without external intervention. The mechanism of the process is founded on the content of glass-ceramic in substrate. It is a new technology where the variety of pastes which are compatible with it is limited. There is no resistive paste yet.

The main advanteges and disadvantages of sintering process

PAS method can be done in a special stove. A firing oven is a part of the laminator. The pressing force is between 0.6 kN and 35 kN, the temperature can be max. 1000 °C. The scheme of PAS can be seen on the figure below.

Scheme of PAS technology

The shrinkage of LTCC substrate is influenced by these factors besides the firing procedure:

• Content of glass in substrate
• Storage time
• Pretreatment of substrate
• Screen-printing (interaction between paste and surface)
• Drying after screen-printing
• Lamination
• Mechanical forces

PLAS process can be used to control x/y shrinkage of LTCC substrate to better than 0.3 %. The PAS process allows control of the shrinkage between 0.1 % and 0.3 %. The best result of experiments is 0.1 %. Figure shows the effect of several sintering pressure on x/y and z-shrinkage of substrate.

Shrinkage of LTCC substrate sintered with PAS technology in the function of pressure:
a) x/y shrinkage; b) z shrinkage

The sensitivity of PLAS technology is 0.1 % the sensitivity of PAS and SCS technology is 0.05 %. This enables high complexity circuits and microvias. This figure shows the difference between conventional sintering technology and PAS technology in the function of the position-ability of vias.

Position of vias on conventional and PAS technology sintered LTCC

Another special attendant of sintering is the shrinkage of the margins. Vias which are close to the margin are hollow if PLAS technology is used. PAS technology rids this problem.

Cross-section of vias sintered by:
a) PLAS technology b) PAS technology c) PAS technology.

This table shows the shrinkage of LTCC substrate in case of different thickness. Sintering is made by PAS technology. It can be seen that the shrinkage of inner layer is higher than the surface layers.

Shrinkage of substrates with different thickness in case of PAS technology

The conventional sintering process and PAS technology also affect the Roughness of LTCC substrate surface. Next figure shows this phenomena.

Roughness of LTCC substrate surface sintered by conventional and PAS technology

The well set pressure precludes the crystallizing of the surface and so it is possible to use thin film on the surface of the substrate.

PLAS and PAS technology enable the higher integrated wiring and via networks. With PAS technology 0.1 % shrinkage can be set (0.3 % - 0.5 %). That is why it is possible to make research to minimize the line width and the development and downsize of high frequency devices.

The main point of the technology is that the surface of LTCC is activated by laser for consecutive electroless chemical metal plating. The method enables high-conductance copper (Cu) networks, and thin seed layers of gold (Au) and silver (Ag) thin-film circuits with a lateral resolution of a few tens of micrometers. Several laser-assisted direct processes have been established in order to fabricate metallic micropatterns on various surfaces (polymers, ceramics, semiconductors. The most frequently utilized techniques are:

• Laser-assisted Chemical Vapor-phase Deposition, LCVD,
• Pulsed-Laser Deposition, PLD,
• Laser-Induced Forward Transfer, LIFT,
• Laser-assisted Pyrolytic Decomposition of Solids, LPDS,
• Laser-assisted Chemical Liquid-phase Deposition, LCLD.

In ceramics, the change in catalytic activity is explained by the generation of oxygen, nitrogen and carbon vacancies on the laser-treated surfaces of the metal oxides, nitrides and carbides, respectively. These vacancies result in the formation of Al and Si nanoclusters in Al2O3, AlN and SiC. The "electron-rich" environment formed promotes the reduction of the metal complexes during subsequent electroless plating.

The LTCC substrate are worked with Nd:YAG. The parameters are the following:

• Drift speed of the beam: 10- 5000 mm/s,
• Wavelength: 1064 nm,
• Repetition number of the impulse: 150 ns,
• Power: 0.1 - 3 mJ/impulse.

Depending on the laser power applied, localized annealing and/or ablation of the LTCC surface takes place. According to our observations, annealing is more favorable for subsequent plating than ablation. Analysis with X-ray diffraction did not reveal significant changes in the crystal structure, probably due to the rapid melting and resolidification, whereas the formed surface structures are mainly amorphous. Copper, silver and gold nano-seeds were obtained by electroless plating on laser-activated low-temperature cofired ceramic substrates. The plated LTCC surface are shown below.

Laser-activated pattern on LTCC:
a) using moderate laser power (f = 2 kHz, E = 0.35 mJ/pulse, v = 2000 m/s); b) Deposition of copper takes place all over the treated area (f = 1 kHz, E = 1 mJ/pulse, v = 20 mm/s); c) silver and d) gold plating.

Copper deposition was carried out on the activated surfaces from an alkaline tartrate-complex of Cu ions using formaldehyde as a reducing agent. The first metal seeds appeared on the surface within 4 h after immersing the laser-processed LTCC samples in the plating bath and a 150-200 nm thick Cu layer formed. The Cu coatings has high conductivity (2.5 µohm x cm).

This technology can be used to realize passive components like inductivities, resistors and capacitors. We show an example in the figure below.

Optical micrograph of planar coils, contact pads (on the surface) and a vertical capacitor (perpendicular to the surface in the bulk of LTCC).

The main point of the technology is the following:

1. LTCC substrates behave similarly to other oxide ceramics when they are illuminated with Nd:YAG laser pulses, viz. oxygen vacancies form, making the surface active for electroless metal plating.
2. The described laser processing increases the amount of glassy phase in LTCC.
3. The laser-treated surfaces enable growth of high-conductivity Cu films on LTCC, by which passive components for microelectronics can be fabricated.
4. The treated surfaces are suitable for depositing Ag and Au nanoparticles.

The name of the technology: Matrix Assisted Pulsed Laser Evaporation - Direct Write (MAPLE-DW). It was investigated for use with conventional thick-film inks. The applied linewidth can be as small as 10 µm. One of the advantages is that it does not need mask for patterning. The sctructure of MAPLE-DW equipment are shown on the figure below.

Schematic of setup for MAPLE-DW

The equipment is fixed on a vibration.free table and the whole process takes place on it. The MAPLE-DW has an Nd:YLF laser with the wavelength of 1047 nm. The length of the pulse is 20 ns and the repetition frequency can be varied between 1 Hz and 10 kHz. The diameter of the laser beam is increased with two expanders so a larger laser beam can be achieved on the substrate.

The optical x-y scanner consists of two mirrors, and they are controlled separately by servomotors. The diameter of lens is 100 mm, the focal length is 163 mm. The waist of focal plane is 16 µm. Shot-to-shot spacing is controlled by adjusting pulse repetition frequency and the speed of the X-Y scanner. The LTCC substrate is positioned by a CCD camera. Due to x-y scanner higher speed writing can be achieved than conventional thick-film process. The ribbon and the LTCC substrate are fixed too. A Fiber laser (l = 1100 nm) can be used to sinter the patterns deposited by Nd:YLF. The two lasers are aligned so that they have the same optical path.

QS300 is an Ag/Pt conductive ink manufactured by Dupont and specifically and it is applied to the experiment, the conventional conductive paste used for screenprining is mixed with 11% (by mass) a-terpineol. Three regions are also identified:

• protrusion,
• jetting,
• plume.

The first part is the protrusion of a bubble from the surface of the ink. The bubble protrusion regime is characterized by an expanding bubble that never fully detaches from the ink surface The bubble eventually collapses back into the ink surface because its kinetic energy is insufficient to overcome surface tension. A jet is formed when the energy is large enough to overcome surface tension and at least some of the ink is detached from the surface. Surface tension then causes this ink to collapse in the radial direction. The plume regime is similar to the jet except that the ink leaves the surface with a high enough velocity that it breaks into small droplets and continues to expand radially as well as normally to the surface.

Considerable instability was observed in the jets and the ink splatters when it comes in contact with the substrate due to the high velocity of the jet. These effects ultimately limit the feature size.

In another experiment dried ribbon was used in direct contact with the LTCC substrate. Because the ribbon is dried, it is more suitable for storage and more applicable for printing on conformal substrates. Since the ribbon is in contact with the substrate, radial spreading is minimized. In a similar experiment DuPont QS300 Ag/Pt conductive paste was used without the addition of any thinner. Paste was applied onto the glass surface.

These experiments showed that as laser fluence (J/cm2) is increased, the size of the bubble increases until it ruptures into a plume. The figure shows the maximum bubble displacement is graphed in the function of intensity.The range of laser fluences was selected to span the range of sub-threshold regime.

Maximum bubble displacement is graphed in the function of intensity

The size of the bubble increases in function of laser intensity. Wider laser beam causes larger shift. It also requires more laser fluence to displace a thicker ink layer a given distance than a thinner layer. Figure shows that a bubble with a larger base radius will have lower curvature for a given displacement than a bubble with a smaller radius. The stress in the bubble walls due to surface tension will be proportional to this curvature.

The best experimental results are obtained when the ribbon is positioned as close to the substrate as possible and the majority of the displaced ink is deposited. The interaction between the ink and the substrate is also important, and how well the ink wets the LTCC substrate can play a critical role in the morphology of the final pattern.

It is convenient to define a quality factor for the sub-threshold event to help quantify the shot to shot interference on the ribbon. The ratio of maximum displacement (Zmax) to the base radius (R0) is plotted in the function of laser intensity.

Ratio of maximum displacement (Zmax) to base radius (R0) as a function of laser fluence

The majority of the displaced ink is deposited when the displacement is maximal and the base radius is small.

This figure shows the deplaced ink in case of different laser intensity.

Deposition on substrate for various fluences

The lines in the figure were written at 7 cm/s, and the separation between the ink and the substrate is 12.5 µm. Figure shows a portion of a 20 µm wide, 5 mm long wire printed with 1.26 J/cm2. The conductivity is 1.6 x 107 1/ohm x m.

Conductive paste applied with MAPLE-DW technology

The fiber laser was also employed to sinter the ink, which was dried in a convection oven at 150 °C. The line with MAPLE-DW can be written as small as 10 µm.