As interconnect dimensions shrink, achieving the required accuracy becomes increasingly difficult for existing in-line metrology methods. Non-destructive, small spot X-ray fluorescence (µ-XRF) can accurately measure copper interconnect thickness on various line space test structures as well as in the open spaces. Back-end metallization stacks have become more and more complex. Today's state of the art copper back-end process requires deposition of a semi-metal adhesion layer (TaN) followed by a diffusion barrier (Ta), followed by a PVD copper seed, and then by electro-deposition of bulk copper. To minimize yield loss to oxidation, all deposition steps are closely spaced in time, thus delaying metrology control until after the complete stack deposition. Tightly integrated deposition operations have increased the requirements for thickness metrology control. To provide deposition feedback as well as CMP feed-forward control, a metrology technique must accurately measure the thick copper layer and be sensitive enough to measure sub 50 Å barrier layer as well.

Jordan Valley XRR/XRF technology provides a small spot capable of measuring the entire back-end stack in scribe lines, and without the resolution limitations associated with laser acoustic techniques. Measurements include thickness, density and phase control. With advanced XRF capability, Jordan Valley X-ray metrology answers the challenges of metal back-end process control at the 65nm node level and beyond.

In addition, the thinning of copper lines becomes a roadblock as design rules shrink and more accurate measurement resolution is needed.

Measurements on copper pads are no longer sufficient, as they don't represent the structures used in the device, so the JVX systems measure line/space structures for copper thickness and erosion.

Measuring copper thinning on different line/space structures on product is critical to controlling today's advanced interconnect processes as the erosion varies with line density.

Copper thickness, measured by Jordan Valley µ-spot XRF, correlates strongly with cross-sectional SEM data on a broad range of thicknesses and array structures.

Copper thickness, measured by Jordan Valley µ-spot XRF, correlates very strongly to sheet resistance on different patterned line/space structures.

Phase, Texture, Grain Size, Stress

Jordan Valley polycrystalline X-ray diffraction technology (XRD) provides an ideal metrology solution for thin film microstructure applications, including phase, texture, grain size and stress. Jordan Valley's XRD mode offers many advantages, including:

• Non- destructive method, no contact with samples to avoid contamination
• Unique information on the microstructure of polycrystalline materials
• Single and multiple layers can be investigated
• Penetration depth control (grazing incidence)

XRD Overview

Metal Barrier Phases: Ta or TaN

Requirement:

• Maintain low resistivity of the barrier layer
• Different Ta and TaN phases have different resistivity: Ta(α) = 35 Ω-cm and Ta(β) = 175 Ω-cm

In-line Monitor:

• Ratio of Ta(α) and Ta(β) phases

Taα also promotes growth of (111) texture in Cu
Phase issues with Ta/TaN barriers

PVD Ta Phase & Texture

Metal diffusion barrier properties depend on crystal structure and composition, not just thickness. Phase variations are a known problem in Ta/TaN barriers.

• Ta(α) (bcc) has a low resistivity so a good barrier choice - Ta(β) (tetragonal) is undesired
• Phase variation a root cause of some process problems
• XRD is already adopted at major semiconductor manufacturers as a means to identify phase variations

[1] C.C. Wang et al, Proc. Characterisation and Metrology for ULSI Tech. 2003, p.519
[2] D. Edelstein et al, Proc. IITC SF 2001, p.9-11
[3] H. Donohue et al, Proc. IITC, SF 2002 p.179

Mixed Ta/TaN Phases

Seed vs. electroplated Cu texture

Ta, TaN, TiN, Ti, Cu-Seed

With each technology node, diffusion barrier layers are becoming increasingly thinner. One of the processing challenges is to deposit thinner, conformal barrier and seed layers over high-aspect ratio structures. With today's fast XRR, diffusion barrier films can be characterized down to the sub nanometer level, which makes XRR uniquely qualified to characterize ultra thin Atomic Layer Deposition (ALD) barriers.

Copper interconnect structure

A single and extremely fast XRR measurement can deliver thickness, density and roughness information on a multi-layer stack of diffusion barrier and Cu seed, as well as their interfacial layers.

Ultra thin diffusion barrier materials like this 20 Å ALD TaN can be quickly and accurately measured by the Jordan Valley systems.