Electrical, Morphological, and Compositional Characterization of Screen-Printed Al Contacts Annealed in Horizontal and Vertical Configurations

Samir Mahmmod Ahmad, Cheow Siu Leong, Richard W. Winder, Kamaruzzaman Sopian, Saleem H. Zaidi

Research output: Contribution to journalArticle

Abstract

The electrical, morphological, and compositional characteristics of screen-printed Al paste contacts on p-doped Si wafers have been investigated in horizontal and vertical thermal annealing configurations over a wide temperature range. The horizontal configuration refers to an industrial six-zone conveyor belt rapid thermal annealing furnace. The vertical configuration refers to a modified three-zone quartz tube furnace with vertically stacked wafers. The contact resistivity was measured by using the transmission line method. In the horizontal configuration, the resistivity exhibited a pronounced minimum at temperature of ∼ 870°C, while higher temperatures resulted in a rapid increase in the contact resistivity. In contrast, the resistivity variation in the vertical configuration was linear. The lowest contact resistivities measured were 136 mΩ cm2 in the horizontal and 103 mΩ cm2 in the vertical configuration, demonstrating a 24% reduction with the latter approach. The surface morphology and composition of the Al/Si contact interface were determined by field-emission scanning electron microscopy and energy-dispersive x-ray spectroscopy. The measured elemental concentrations were curve-fit to accurately measure the width of the interface regions. The Al/Si contact region was observed to consist of five parts: (a) a top sintered paste layer of Al/Si spheres, (b) voids between the Al/Si spheres, (c) an Al/Si eutectic region, (d) an epitaxially grown Al-doped Si layer, and (e) the lightly Al-diffused Si substrate. Sintered Al/Si spheres were observed to consist of a solid core of Al embedded in a thin shell of Al, Al2O3, SiO2, and Si. The rapid rise in resistivity at high temperatures is attributed to enhanced oxidation of Al and Si islands, resulting in thicker Al2O3/SiO2 films between metallic Al spheres. The lower resistivity observed in the vertical configuration was attributed to larger, more uniform Al–Si eutectic regions, higher density of Al/Si films within the paste region, and transformation of sintered Al spheres into larger pseudosquare islands. The proposed Al/Si interface model was further supported by the higher resistance measured for the pulsed laser-based Al/Si contact with high Si concentrations in the Al/Si eutectic region. An approximately linear reduction in resistivity as a function of time over a broad range varying from microseconds to seconds reinforced the proposed model and suggests that longer, steady-state annealing is the preferred approach to achieve the lowest contact resistivity.

Original languageEnglish
JournalJournal of Electronic Materials
DOIs
Publication statusPublished - 1 Jan 2019

Fingerprint

Ointments
electrical resistivity
Eutectics
configurations
electric contacts
Furnaces
eutectics
Annealing
Temperature
Metallic films
Quartz
Rapid thermal annealing
annealing
Pulsed lasers
furnaces
Surface structure
Field emission
Surface morphology
Electric lines
wafers

Keywords

  • Al contact
  • Contact resistivity
  • energy-dispersive x-ray
  • scanning electron microscopy
  • transmission line method

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Condensed Matter Physics
  • Electrical and Electronic Engineering
  • Materials Chemistry

Cite this

@article{f199080ad3214110870c0e200854c16f,
title = "Electrical, Morphological, and Compositional Characterization of Screen-Printed Al Contacts Annealed in Horizontal and Vertical Configurations",
abstract = "The electrical, morphological, and compositional characteristics of screen-printed Al paste contacts on p-doped Si wafers have been investigated in horizontal and vertical thermal annealing configurations over a wide temperature range. The horizontal configuration refers to an industrial six-zone conveyor belt rapid thermal annealing furnace. The vertical configuration refers to a modified three-zone quartz tube furnace with vertically stacked wafers. The contact resistivity was measured by using the transmission line method. In the horizontal configuration, the resistivity exhibited a pronounced minimum at temperature of ∼ 870°C, while higher temperatures resulted in a rapid increase in the contact resistivity. In contrast, the resistivity variation in the vertical configuration was linear. The lowest contact resistivities measured were 136 mΩ cm2 in the horizontal and 103 mΩ cm2 in the vertical configuration, demonstrating a 24{\%} reduction with the latter approach. The surface morphology and composition of the Al/Si contact interface were determined by field-emission scanning electron microscopy and energy-dispersive x-ray spectroscopy. The measured elemental concentrations were curve-fit to accurately measure the width of the interface regions. The Al/Si contact region was observed to consist of five parts: (a) a top sintered paste layer of Al/Si spheres, (b) voids between the Al/Si spheres, (c) an Al/Si eutectic region, (d) an epitaxially grown Al-doped Si layer, and (e) the lightly Al-diffused Si substrate. Sintered Al/Si spheres were observed to consist of a solid core of Al embedded in a thin shell of Al, Al2O3, SiO2, and Si. The rapid rise in resistivity at high temperatures is attributed to enhanced oxidation of Al and Si islands, resulting in thicker Al2O3/SiO2 films between metallic Al spheres. The lower resistivity observed in the vertical configuration was attributed to larger, more uniform Al–Si eutectic regions, higher density of Al/Si films within the paste region, and transformation of sintered Al spheres into larger pseudosquare islands. The proposed Al/Si interface model was further supported by the higher resistance measured for the pulsed laser-based Al/Si contact with high Si concentrations in the Al/Si eutectic region. An approximately linear reduction in resistivity as a function of time over a broad range varying from microseconds to seconds reinforced the proposed model and suggests that longer, steady-state annealing is the preferred approach to achieve the lowest contact resistivity.",
keywords = "Al contact, Contact resistivity, energy-dispersive x-ray, scanning electron microscopy, transmission line method",
author = "Ahmad, {Samir Mahmmod} and Leong, {Cheow Siu} and Winder, {Richard W.} and Kamaruzzaman Sopian and Zaidi, {Saleem H.}",
year = "2019",
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T1 - Electrical, Morphological, and Compositional Characterization of Screen-Printed Al Contacts Annealed in Horizontal and Vertical Configurations

AU - Ahmad, Samir Mahmmod

AU - Leong, Cheow Siu

AU - Winder, Richard W.

AU - Sopian, Kamaruzzaman

AU - Zaidi, Saleem H.

PY - 2019/1/1

Y1 - 2019/1/1

N2 - The electrical, morphological, and compositional characteristics of screen-printed Al paste contacts on p-doped Si wafers have been investigated in horizontal and vertical thermal annealing configurations over a wide temperature range. The horizontal configuration refers to an industrial six-zone conveyor belt rapid thermal annealing furnace. The vertical configuration refers to a modified three-zone quartz tube furnace with vertically stacked wafers. The contact resistivity was measured by using the transmission line method. In the horizontal configuration, the resistivity exhibited a pronounced minimum at temperature of ∼ 870°C, while higher temperatures resulted in a rapid increase in the contact resistivity. In contrast, the resistivity variation in the vertical configuration was linear. The lowest contact resistivities measured were 136 mΩ cm2 in the horizontal and 103 mΩ cm2 in the vertical configuration, demonstrating a 24% reduction with the latter approach. The surface morphology and composition of the Al/Si contact interface were determined by field-emission scanning electron microscopy and energy-dispersive x-ray spectroscopy. The measured elemental concentrations were curve-fit to accurately measure the width of the interface regions. The Al/Si contact region was observed to consist of five parts: (a) a top sintered paste layer of Al/Si spheres, (b) voids between the Al/Si spheres, (c) an Al/Si eutectic region, (d) an epitaxially grown Al-doped Si layer, and (e) the lightly Al-diffused Si substrate. Sintered Al/Si spheres were observed to consist of a solid core of Al embedded in a thin shell of Al, Al2O3, SiO2, and Si. The rapid rise in resistivity at high temperatures is attributed to enhanced oxidation of Al and Si islands, resulting in thicker Al2O3/SiO2 films between metallic Al spheres. The lower resistivity observed in the vertical configuration was attributed to larger, more uniform Al–Si eutectic regions, higher density of Al/Si films within the paste region, and transformation of sintered Al spheres into larger pseudosquare islands. The proposed Al/Si interface model was further supported by the higher resistance measured for the pulsed laser-based Al/Si contact with high Si concentrations in the Al/Si eutectic region. An approximately linear reduction in resistivity as a function of time over a broad range varying from microseconds to seconds reinforced the proposed model and suggests that longer, steady-state annealing is the preferred approach to achieve the lowest contact resistivity.

AB - The electrical, morphological, and compositional characteristics of screen-printed Al paste contacts on p-doped Si wafers have been investigated in horizontal and vertical thermal annealing configurations over a wide temperature range. The horizontal configuration refers to an industrial six-zone conveyor belt rapid thermal annealing furnace. The vertical configuration refers to a modified three-zone quartz tube furnace with vertically stacked wafers. The contact resistivity was measured by using the transmission line method. In the horizontal configuration, the resistivity exhibited a pronounced minimum at temperature of ∼ 870°C, while higher temperatures resulted in a rapid increase in the contact resistivity. In contrast, the resistivity variation in the vertical configuration was linear. The lowest contact resistivities measured were 136 mΩ cm2 in the horizontal and 103 mΩ cm2 in the vertical configuration, demonstrating a 24% reduction with the latter approach. The surface morphology and composition of the Al/Si contact interface were determined by field-emission scanning electron microscopy and energy-dispersive x-ray spectroscopy. The measured elemental concentrations were curve-fit to accurately measure the width of the interface regions. The Al/Si contact region was observed to consist of five parts: (a) a top sintered paste layer of Al/Si spheres, (b) voids between the Al/Si spheres, (c) an Al/Si eutectic region, (d) an epitaxially grown Al-doped Si layer, and (e) the lightly Al-diffused Si substrate. Sintered Al/Si spheres were observed to consist of a solid core of Al embedded in a thin shell of Al, Al2O3, SiO2, and Si. The rapid rise in resistivity at high temperatures is attributed to enhanced oxidation of Al and Si islands, resulting in thicker Al2O3/SiO2 films between metallic Al spheres. The lower resistivity observed in the vertical configuration was attributed to larger, more uniform Al–Si eutectic regions, higher density of Al/Si films within the paste region, and transformation of sintered Al spheres into larger pseudosquare islands. The proposed Al/Si interface model was further supported by the higher resistance measured for the pulsed laser-based Al/Si contact with high Si concentrations in the Al/Si eutectic region. An approximately linear reduction in resistivity as a function of time over a broad range varying from microseconds to seconds reinforced the proposed model and suggests that longer, steady-state annealing is the preferred approach to achieve the lowest contact resistivity.

KW - Al contact

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KW - energy-dispersive x-ray

KW - scanning electron microscopy

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