White Paper
®
Copper Interconnects in Altera Devices
Introduction
Semiconductor companies are constantly striving to improve device performance in order to meet the electronic and
communication industries’ high-bandwidth and high-performance requirements. Using copper instead of aluminum
as a metal interconnect for metal layers helps address these increased speed and density requirements.
In the past, most semiconductor manufacturers have used aluminum interconnects for their devices. Although
aluminum is a good conductor, it becomes less practical to use because of size limitations as semiconductor processes
shrink to 0.15 and 0.13 µm. Some semiconductor manufacturers use copper interconnects for the top two layers
containing the power distribution connections and other critical signals like clock and fast lines, and aluminum for the
remaining metal layers. However, since programmable logic device (PLD) system performance is driven by
interconnect delays, every interconnect is critical in the design. To gain the full advantages of copper interconnects,
all the metal layers must be copper.
APEXTM 20KC devices are all-layer copper devices that meet the high-performance requirements of complex
designs. Figure 1 shows the cross section of an APEX 20KC device.
Figure 1. APEX 20KC Device Cross Section
Power Plane
Interconnects
APEX 20KC Devices
Altera® APEX 20KC devices are the PLD industry’s first all-layer copper interconnect devices. These devices are
based on feature-rich APEX 20KE devices and are manufactured on a 0.15-µm all-layer copper interconnect process.
The copper interconnects allow APEX 20KC devices to offer performance improvements of approximately 25% over
equivalent 0.18-µm APEX 20KE devices. Visit the Altera web site (http://www.altera.com) for detailed information
on these devices. Figure 2 compares the performance of Altera devices.
M-WP-COPPER-01
April 2001, ver. 1.0
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Copper Interconnects in Altera Devices White Paper
Altera Corporation
Figure 2. Altera Device Performance
APEX 20KC
All-Layer
Copper
APEX 20KE
Aluminum
Performance
APEX 20K
Aluminum
FLEX 10KE
Aluminum
0.25-µm,
5-Layer Metal
0.22-µm,
7-Layer Metal
0.18-µm,
8-Layer Metal
0.15-µm,
8-Layer Metal
Process & Metal Layers
Copper Advantages
Copper has lower resistance and better electro-migration and scaling properties than aluminum. These properties are
discussed below.
Resistance
Copper has much less resistance than aluminum, resulting in a smaller time delay factor (τ) in the equation τ = R × C;
the lower resistance found in copper results in smaller delays. Typically, using copper instead of aluminum reduces
interconnect resistance by 40% (see Figure 3), resulting in better signal integrity, smaller propagation delays, lower
power dissipation, and higher performance. With shrinking device geometries, interconnect resistance becomes a
major factor in determining device performance.
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Altera Corporation
Copper Interconnects in Altera Devices White Paper
Figure 3. Aluminum & Copper Resistance
1.2
1.0
0.8
Relative
Delays
0.6
0.4
0.2
0
Aluminum
Copper
Interconnect Material
Electro-Migration Characteristics
The electro-migration properties of copper are also better than those of aluminum. Electro-migration is the currentinduced transportation in the conducting material. In the presence of high current stresses, electron momentum is
transferred to atoms in the metal, yielding a net atomic flux. This net flux causes the metal to deplete at some areas
and accumulate at others (see Figure 4).
Figure 4. Hills & Voids Caused by Net Flux
Unaffected Interconnect
Interconnect after Net Flux
Hills
Voids
Direction of
electron flow
Direction of
electron flow
Regions where the metal structure is depleted form a void, leading to interconnect failure due to the formation of an
open circuit. Likewise, interconnect material can accumulate and extrude to make electrical contact with neighboring
interconnect segments, potentially leading to circuit failure due to the formation of a short circuit. Over time, either
outcome can contribute to the gradual deterioration of a current-stressed interconnect, causing reliability concerns.
While aluminum is reliable down to 0.18 µm, these potential failures limit the minumum thickness and width of
aluminum interconnects, making copper a better metal choice at dimensions smaller than 0.18 µm.
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Copper Interconnects in Altera Devices White Paper
Altera Corporation
Scaling Properties
Copper is more suitable than aluminum for drawing or scaling into smaller dimensions. Copper wires can also be
packed closer together than aluminum wires on a device. Since the die size is greatly influenced by the interconnects
used, copper interconnects make smaller die sizes possible.
Conclusion
Copper offers various advantages such as lower interconnect delays and better electro-migration characteristics.
These properties make copper the metal of choice for layer interconnects at dimensions smaller than 0.18 µm. Altera
APEX 20KC devices feature an all-layer copper metal interconnect structure that offers all of these benefits.
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Copyright  2001 Altera Corporation. Altera, APEX, APEX 20KC, APEX 20KE, FLEX, FLEX 10K, and FLEX 10KE are trademarks and/or
service marks of Altera Corporation in the United States and other countries. Altera acknowledges the trademarks of other organizations for
their respective products or services mentioned in this document. Altera products are protected under numerous U.S. and foreign patents and
pending applications, maskwork rights, and copyrights. Altera warrants performance of its semiconductor products to current specifications
in accordance with Altera’s standard warranty, but reserves the right to make changes to any products and services at any time without notice.
Altera assumes no responsibility or liability arising out of the application or use of any information, product, or service described herein except
as expressly agreed to in writing by Altera Corporation. Altera customers are advised to obtain the latest version of device specifications before
relying on any published information and before placing orders for products or services. All rights reserved.