**2.1 Experimental setup**

434 Recent Advances in Nanofabrication Techniques and Applications

technique allowed printing 2D logic cells and dense poly lines with two lithography steps,

The next alternative process is Self-Aligned Double Patterning (SADP, Fig. 1c), in which a spacer film is formed on the sidewalls of pre-patterned features. Etching removes all the material of the original pattern, leaving only the spacer material. Since there are two spacers for every line, the line density has now doubled. The spacer approach is unique in that with one lithographic exposure the pitch can be halved indefinitely with a succession of spacer formation and pattern transfer processes. The spacer film deposition process is very uniform and results in extremely good SADP CDU of less than 1nm. The spacer lithography technique has most frequently been applied in patterning fins for FinFETs

(a) (b) (c) Fig. 1. Various double patterning schemes: (a) Litho-etch-litho-etch (LELE), (b) Litho-freeze-

These pitch splitting double patterning techniques not only involve more demanding process steps, they also require tighter overlay control than conventional single patterning [2]. Therefore measurement of overlay with much higher certainty is a necessity. As technology transitions toward the 22nm and 16nm nodes using these methods there is serious concern about the capability of the available metrology solutions, both in process

High TIS and tool-to-tool matching errors make it difficult to meet the measurement uncertainty requirements using the traditional Image-Based Overlay method (IBO), even though most advanced IBO tools are operating at TMU levels under 1nm. Diffraction-based (scatterometry) overlay (DBO) measurement is an alternative optical measurement technique that has been reported to offer better precision than IBO and near zero TIS [6, 12, 14-15], and is therefore a possible solution to the measurement uncertainty budget. Bischoff *et al.* proposed measuring overlay using the diffraction efficiencies of the first diffracted orders [7]. Chun-Hung Ko used angular scatterometry combined with an experimental library to determine the overlay error on ADI stacks with intermediate poly-silicon lines [8]. H.-T. Huang *et al.* used spectra from reflection symmetry gratings and a rigorous coupled-

litho-etch (LFLE), and (c) Self-aligned double patterning (SADP)

development and production control.

illustrating good resolution and process margin [4].

and metal layers [5].

Spectroscopic scatterometry is used to measure overlay errors between stacked periodic structures (*e.g.,* gratings). In this technique, broadband linearly polarized light is incident perpendicular to the wafer surface and the zero-order diffracted signal (spectrum) is measured as a function of wavelength. Fig. 2 shows a typical experimental configuration. At normal incidence, different reflectance spectra are obtained for various angles of polarization with respect to that of the periodic structure. Typical data collection involved both TE and TM spectra. A specific advantage of using polarized light is that it provides enhanced sensitivity as both the amplitude and phase differences between the TE and TM spectra can be measured.

Fig. 2. The figure shows a typical hardware set up for a normal incidence scatterometer (a) spectroscopic reflectometry, (b) normal incidence reflection, and (c) TE TM data acquisition modes.

### **2.2 Theory**

Spectra are obtained from pads, each of which has gratings patterned in both layers between which the overlay error is being measured (Fig. 3). The gratings in each pad are overlaid but by design shifted with respect to each other. Spectra from pads with shifts of equal magnitude but opposite direction are identical due to symmetry:

Diffraction Based Overlay Metrology for Double Patterning Technologies 437

From equation (4), the ratio *ΔR(λ)/ΔR'(λ)* must be independent of *λ*. This arises from our assumption that ε is small and only applies in the linear-response range where this

Equation (4) shows that the overlay can be measured in one direction using a minimum of three pads with suitably defined offsets. In practice four pads are often used (Fig. 4a), with an additional calibration pad with offset –*x0*-*δ*, because the additional data improves precision and provides a check that the overlay error is within the linear response range (Fig. 4b). To measure overlay in two directions two sets of gratings are required. The second set with the gratings rotated by 90° from the first. As overlay is a vector quantity, it is usually measured in both X&Y directions. The nomenclature "2x3 target" and "2x4 target"

The nomenclature CD/pitch is also used to indicate the designed CD and pitch of the gratings in each target. For example 65/390 means CD=65nm and pitch=390nm. All targets

(a) (b)

Fig. 4. (a) 2x4 target used for measurement of signal by a normal incidence scatterometer

Fig. 5(a) and (b) show the spectral response to overlay in the difference spectra from programmed reflection-symmetry gratings (Equation 2). The black line in the figure is the

excellent agreement with the measured response, in accordance with equation 4. The maximum response increases from ~15x (Noise) to ~45x (Noise) for a ~3x change in overlay error. As expected, the spectral response also changes in sign with the measurement. The maximum spectral response at any wavelength is about 4x (Noise) per 1 nm overlay error. Measurement uncertainty much better than 0.25nm is possible because the data is summed

*δ*). The scaled signal shows

and (b) linear dependency of overlay shift as a function of spectral response.

average *ΔR'(λ*) from both pairs of calibration pads, scaled by (2

indicates whether three or four pads are used to measure in each axis.

use the same CD and pitch at both layers.

**2.3 Spectral response to overlay** 

over all available wavelengths.

assumption holds.

$$R(+\mathbf{x}\_0, \mathcal{A}) = R(-\mathbf{x}\_0, \mathcal{A}) \tag{1}$$

Here *R(x0)* is the reflectance spectrum from one pad as a function of wavelength and shift +*x0*. The difference spectra *(R(λ)=R(*+*x0,λ) - R(*-*x0,λ)*) from two pads with shifts +*x0* and -*x0* is zero in the absence of noise in the measuring tool. A small overlay error shifts both upper gratings in the same direction and breaks the symmetry. The resulting differential spectrum is proportional to the direction and magnitude of the overlay error:

$$\Delta R(\mathcal{A}) = R(\mathbf{x}\_0 + \varepsilon, \mathcal{A}) - R(-\mathbf{x}\_0 + \varepsilon, \mathcal{A}) = R(\mathbf{x}\_0 + \varepsilon, \mathcal{A}) - R(\mathbf{x}\_0 - \varepsilon, \mathcal{A}) \equiv 2\varepsilon \left. \frac{\partial R}{\partial \mathbf{x}} \right|\_{\mathbf{x}\_0} \tag{2}$$

Here, is the overlay error, *x0* is the offset bias, and a Taylor expansion of the reflectance around *x0* has been applied. The overlay error can now be calculated by comparing the measured differential spectrum, *R*, to a second differential spectrum, *R'*, acquired from a pair of test pads having a known relative offset. If, for example, a shift of *x0*+ is designed into a third pad, then within the linear-response range the difference between its spectrum and that from the +*x0* pad is:

$$
\Delta R'(\mathcal{A}) = R(\mathbf{x}\_0 + \mathcal{S} + \mathcal{E}, \mathcal{A}) - R(\mathbf{x}\_0 + \mathcal{E}, \mathcal{A}) \equiv \delta \frac{\partial R}{\partial \mathbf{x}}\bigg|\_{\mathbf{x}\_0} \tag{3}
$$

Equation (3) provides the calibration required to calculate the overlay error,

)(2 )( *R R* (4)

Fig. 3. Illustration of DBO targets design: (a) reflection symmetry with +*x0* and –*x0* shift (no spectral difference), (b) reflection symmetry broken due to overlay error with *+ x0+* and *– x0+* shift (*R12* spectral difference between pads 1 and 2).

0 0 *Rx Rx* ( ,) ( ,) 

and -*x0* is zero in the absence of noise in the measuring tool. A small overlay error shifts both upper gratings in the same direction and breaks the symmetry. The resulting differential

0 000 ( , , ( , ( - , 2

Here, is the overlay error, *x0* is the offset bias, and a Taylor expansion of the reflectance around *x0* has been applied. The overlay error can now be calculated by comparing the

into a third pad, then within the linear-response range the difference between its spectrum

0 0 ( , ,

(a) (b) Fig. 3. Illustration of DBO targets design: (a) reflection symmetry with +*x0* and –*x0* shift (no

spectral difference), (b) reflection symmetry broken due to overlay error with *+ x0+*

*R12* spectral difference between pads 1 and 2).

)(2 )( 

*R R*  *x x <sup>R</sup> RR R*

(2)

*x xxx <sup>R</sup> RR R R R*

 

pair of test pads having a known relative offset. If, for example, a shift of *x0*+

 

Equation (3) provides the calibration required to calculate the overlay error,

spectrum is proportional to the direction and magnitude of the overlay error:

*)* is the reflectance spectrum from one pad as a function of wavelength

Here *R(x0*

*x0+* shift (

measured differential spectrum,

and that from the +*x0* pad is:

shift +*x0*. The difference spectra *(*

 

 

*R*, to a second differential spectrum,

 

(3)

*R(λ)=R(*+*x0,λ) - R(*-*x0,λ)*) from two pads with shifts +*x0*

 

0

*x*

*x*

(4)

 

(1)

and

0

*x*

*R'*, acquired from a

and *–*

is designed

*x*

  From equation (4), the ratio *ΔR(λ)/ΔR'(λ)* must be independent of *λ*. This arises from our assumption that ε is small and only applies in the linear-response range where this assumption holds.

Equation (4) shows that the overlay can be measured in one direction using a minimum of three pads with suitably defined offsets. In practice four pads are often used (Fig. 4a), with an additional calibration pad with offset –*x0*-*δ*, because the additional data improves precision and provides a check that the overlay error is within the linear response range (Fig. 4b). To measure overlay in two directions two sets of gratings are required. The second set with the gratings rotated by 90° from the first. As overlay is a vector quantity, it is usually measured in both X&Y directions. The nomenclature "2x3 target" and "2x4 target" indicates whether three or four pads are used to measure in each axis.

The nomenclature CD/pitch is also used to indicate the designed CD and pitch of the gratings in each target. For example 65/390 means CD=65nm and pitch=390nm. All targets use the same CD and pitch at both layers.

Fig. 4. (a) 2x4 target used for measurement of signal by a normal incidence scatterometer and (b) linear dependency of overlay shift as a function of spectral response.

#### **2.3 Spectral response to overlay**

Fig. 5(a) and (b) show the spectral response to overlay in the difference spectra from programmed reflection-symmetry gratings (Equation 2). The black line in the figure is the average *ΔR'(λ*) from both pairs of calibration pads, scaled by (2*δ*). The scaled signal shows excellent agreement with the measured response, in accordance with equation 4. The maximum response increases from ~15x (Noise) to ~45x (Noise) for a ~3x change in overlay error. As expected, the spectral response also changes in sign with the measurement. The maximum spectral response at any wavelength is about 4x (Noise) per 1 nm overlay error. Measurement uncertainty much better than 0.25nm is possible because the data is summed over all available wavelengths.

Diffraction Based Overlay Metrology for Double Patterning Technologies 439

The linearity range is tested by printing a sequence of pads with varying overlay shifts. Pads in the right half of the sequence (pads 1-15) have increasing positive overlay shifts (Fig. 6(a)) in 15 nm steps. Pads (-1) to (-15) have increasing negative overlay shifts in 15 nm steps. Fig. 6b and 6c shows raw TE spectra collected from these pads. The difference signal is calculated by subtracting the pad 1 spectrum for pads 2-15, and the pad -1 spectrum for pads -2 to -15. The range of linearity observed (~±70 nm) is significant for this process as shown in Fig. 6(d). A similar linearity range is observed for the horizontal

In the absence of a EUV processing solution below 80nm pitch, DPT using the litho-etch-

The first DPT test structure is on a silicon substrate (fig 7a). The structure consists of ~120 nm photoresist lines and ~40 nm nitride lines with silicon over etch. The second DPT stack

Fig. 7. (a) DPT test structure with ~120 nm photoresist lines and ~40 nm nitride lines with silicon over etch on a silicon substrate. (b) Gate level (bitline) patterning step in a NOR flash

2 2 Dielectric layers

Silicon

**PR Poly** (a) (b)

DBO measurement accuracy is assessed by comparing the results against IBO and CD-SEM

Fig. 8(a) shows the comparison between DBO and IBO measurements on all 143 fields on the wafer. There is a very good correlation between DBO and IBO measurements (R2=0.99) with an offset of ~7 nm. The correlation is good for the subset of measurements less than ±3 nm (Fig. 8b). The inset histogram in Fig. 8a shows the difference in IBO and DBO measurements after removing the ~7 nm constant offset. The distribution is approximately normal with

**3.2 DBO Measurement accuracy: correlation with IBO and CD-SEM** 

(Fig. 7b) comes from a gate (bitline)-level patterning step in a NOR flash process.

litho-etch dual step became an attractive solution for the low k1 regime.

**PR**

~120nm

Si over etch

**2.4 Range of linearity** 

**3. Litho-etch-litho-etch (LELE)** 

**3.1 DPT structures for testing DBO** 

2

**Nit**

gratings.

process.

~40nm

data.

**3.2.1 Correlation with IBO data** 

standard deviation of 1.8nm.

Fig. 5. Spectral response (signal to noise ratio) for corresponding overlay errors (shown in the legend box). The plots (a) and (b) also show response (black line) calculated from the calibration pad spectra.

Fig. 6. (a) shows Pads 1-15 with increasing programmed positive overlay shifts and Pads (-1)-(-15) with increasing negative overlay shifts, (b) and (c) show TE spectra from corresponding vertical and horizontal gratings pad sequence respectively, and (d) shows measured spectral response in arbitrary units.
