Browse Prior Art Database

Method for a taper-free micro-via

IP.com Disclosure Number: IPCOM000100602D
Publication Date: 2005-Mar-15
Document File: 6 page(s) / 71K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for a taper-free micro-via. Benefits include improved functionality, improved performance, improved reliability, improved design flexibility, and improved process simplicity.

This text was extracted from a Microsoft Word document.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 31% of the total text.

Method for a taper-free micro-via

Disclosed is a method for a taper-free micro-via. Benefits include improved functionality, improved performance, improved reliability, improved design flexibility, and improved process simplicity.

Background

      Conventionally, the CO2 laser-drilled micro-via exhibits tapering (such as, a difference between the top and the bottom diameter of ~10 micron for a typical 30-micron dielectric thickness). Via tapering increases the residual strains at the via bottom and decreases the via bottom diameter. The interfacial strength between the via bottom and the underlying capture pad decreases. Approaches to eliminate via tapering are very important because they enable an increased via bottom diameter at a given top diameter, increasing the interfacial fracture energy.

      Micro-vias are fabricated using the CO2 laser-based drilling technology. It utilizes either one or two shots of laser drilling with a Gaussian (TEM00circular) beam with one pulse or two identical pulses. Using a Gaussian beam profile means that the laser intensity at the dielectric surface is varying across the via diameter with the intensity at via circumference being approximately (1/e) times that at the via center. Variation in the laser beam intensity induces a parabolic thermal distribution filed within the dielectric layer. This variation changes the ablation rate across the via diameter, causing via tapering (see Figure 1).

      The ablation rate variation across the via diameter is dictated by the inter-isotherm spacing of the thermal field. The thermal field distribution is specific for a given dielectric material with certain laser beam characteristics. A critical isotherm exists beyond which the laser fluence is higher than the ablation threshold of the dielectric. The position of such a critical isotherm defines the diameter and shape of the drilled via. The depth of the critical isotherm is dictated by the pulse width. Its full-width-at-half-maximum (FWHM) value is governed by the laser beam spot size and shape for a stagnant work piece.

General description

      The disclosed method controls the shape of the laser drilled micro-via by engineering the laser pulse shape and the intensity profile in the laser beam. The method is a two-shot process (two pulses) splitting the original laser beam into two profiles, enabling two panels to be processed simultaneously. The first shot utilizes a top-hat beam (such as, TEM00Square profile) with a beam diameter smaller than the required micro-via diameter. The second shot utilizes a TEM01* beam (doughnut-shaped). It has a ring width equal to half the difference between the final via diameter and the laser beam diameter used in the first shot. Both pulses are square-shaped pulses with a pulse peak power higher than the ablation threshold of the dielectric material in hand.

      A square-shaped pulse minimizes the heat-affected zone around via walls, decreasing the wal...