As semiconductor technologies scaled down to the deep-submicron regime, a critical bottleneck in achieving higher operating frequencies became evident: the resistance-capacitance (RC) delay associated with Back-End-of-Line (BEOL) interconnects. The traditional SiO2/aluminum interconnect scheme with tungsten plugs was no longer suitable for the high-speed transistors being developed. The interconnects were lagging behind the transistor advancements.
Root Causes of the Problem
Capacitance: The capacitance between metal interconnects and the inherent resistance of metal wiring was identified as the main culprits causing RC delays. Interconnect capacitance contributes significantly to the delay.
Resistance: High resistance in aluminum interconnects was another part of the problem. Many metals with lower resistivity than aluminum had integration and electro-migration issues.
Low-k Dielectrics: To reduce interconnect capacitance and lower RC delay, modern BEOL processes incorporate interlayer dielectrics (ILD) with low permittivity or dielectric constant (k). These materials, often fluorinated oxides and exotic dielectrics, have tightly bound valence electrons or significant void space, reducing capacitance.
Copper Interconnects: Copper (Cu) is chosen as the metal of choice for advanced interconnects due to its lower resistivity than aluminum. However, Cu is difficult to etch directly. Therefore, a process called dual damascene is employed, where Chemical Mechanical Polishing (CMP) selectively removes excess copper.
Dual Damascene Process
ILD Deposition: A low-k interlayer dielectric is deposited and capped with a thin etch-stop layer, typically silicon nitride (Si3N4).
Patterning the Top ILD: Photolithography and etching are used to pattern the top ILD layer, terminating the etch on the etch-stop layer from the first ILD deposition.
Metal Wiring Layer Patterning: At this point, the metal wiring layer is patterned without the vias present.
Via Patterning: A photolithography/etch process is repeated to pattern the vias within the openings formed during the first etch process. This step presents challenges due to limited depth-of-focus optics.
Copper Filling: Copper is deposited, preceded by a thin barrier/liner layer of tantalum (Ta) and tantalum nitride (TaN) to aid in initiating the electroplating of copper.
Excess Copper Removal: A Cu electroplating process deposits a thickness greater than the combined ILD layer thicknesses. Excess copper is removed by CMP.
This process is repeated for multiple metallization layers required by the technology. These advancements in low-k dielectrics and copper interconnects have been instrumental in reducing RC delays and enabling the continued scaling of semiconductor devices
What was the primary issue that arose as planar technologies scaled down to the deep-submicron regime?
The primary issue that emerged was the resistance-capacitance (RC) delay associated with interconnects formed in the backend of the line (BEOL). Traditional SiO2/aluminum with tungsten plugs was no longer suitable for high-speed transistors, as interconnects lagged behind transistor advancements.
How does the choice of interlayer dielectric (ILD) material impact interconnect capacitance and the associated RC delay?
The choice of ILD material with a low dielectric constant (k) can significantly reduce interconnect capacitance, thus lowering the RC delay. Materials with tightly bound valence electrons or those with large void spaces are preferred for low-k dielectrics.
Why is electroplated copper (Cu) commonly used as the metal for advanced interconnects, and what challenges does it pose in the etching process?
Electroplated copper is preferred for advanced interconnects due to its lower resistivity compared to aluminum. However, Cu is challenging to etch directly. To address this, the dual damascene process is employed, using chemical mechanical polishing (CMP) to selectively remove excess copper.
Can you explain the basic process steps involved in the low-k/Cu dual damascene backend?
The process begins with the deposition of an interlayer dielectric (low-k) and a thin etch-stop layer (usually Si3N4). The second interlayer dielectric is deposited, and photolithography and etching are used to pattern the top ILD layer. The metal wiring layer is patterned next without vias present. A separate photolithography/etch process is used to pattern the vias within the openings formed from the first etch process. The combined openings are then filled with copper using electroplating. Finally, excess copper is removed by CMP.
How does the dual damascene process address the challenges associated with limited depth-of-focus optics in via patterning?
Dual damascene simplifies the via patterning process by separating it into two steps. First, the top ILD layer is patterned, and then vias are patterned within the openings from the first etch process. This approach mitigates the challenges associated with limited depth-of-focus optics in via patterning.
|Analog and Memory Layout Design Forum|
|Physical Layout Design Forum|
|RTL & Verilog Design Forum|
|Analog Layout Design Interview Questions||Memory Design Interview Questions|
|Physical Design Interview Questions||Verilog Interview Questions|
|Digital Design Interview Questions||STA Interview Questions|