2.1.

DCM Design Rule Compatible Targets

The first DCM component is to design a target that will behave similarly to actual devices. Since pitch is one of the major factors for both lithographic and nonlithographic behaviors, we designed the OVL target elements to be segmented with a pitch below 400 nm, to make sure it is not resolved by the tool optical system. There is a balance between having a large pitch which is more process robust, especially in the scribe line areas where the process is less controlled, to the fine pitch of the device which allows the target to behave more like the device. The new designed targets are design rule compatible in more than 90% of their area. Design rule compatibility, especially with advanced nodes, in conjunction with small target size, allows placing the targets within the die itself. In die, the targets are obviously useful for high order corrections of the scanner. Once the target is compatible with the device design rules, it should also behave similarly to the device in response to the etch process. The target design shown in Fig. 3 has several improvements over previous generation AIMid targets. The high density pattern makes it self-protected and hence more robust to the polish process. To improve the correlation of the edge of the measured pattern with the pitch behavior, a subresolution assist feature and other optical proximity corrections can be applied to the design. The previous layer segmentation is perpendicular to the current layer segmentation to avoid the influence of one layer’s signal on the other. One needs to take into account that target segmentation in most cases leads to lower light reflection and lower contrast, which can best be overcome with advanced metrology tools with more light output and sensitivity.

Fig. 3

Device-correlated metrology (DCM) target design example, from left to right: previous layer pattern graphic database system (GDS) layout, current layer pattern GDS layout, and optical image of the combined target.

2.2.

Accurate OVL Measurement

The second component of DCM is measuring the OVL target correctly. Even for the same OVL target, different OVL values can be reported by different measurement conditions or different targets.^1–6 For example, different color filter selection can report different OVL values, specifically when the OVL target exhibits asymmetry (for either current or previous layer). Therefore, measurement conditions which are defined in the measurement recipe need to be optimized to provide the correct OVL. The reader might want to refer to the elements of Fig. 2 in the following discussion. In the past, a low total measurement uncertainty (TMU) was one of the key elements for such selection, however, in advanced metrology tools, with a TMU performance below 0.5 nm, TMU is no longer a key indicator for accuracy or a component of inaccuracy. Model residuals are often used for determining the most accurate recipe or target. However, they also have systematic issues; for example, a measurement with a low sensitivity to OVL will produce low OVL values, potentially with low residuals. Another way to evaluate the accuracy is the Qmerit algorithm, described last year and this year in SPIE papers.^1–4 This algorithm produces a quantitative measure of the asymmetry impact. The Archer Self Calibration algorithm, described in a poster in this year’s SPIE,³ allows correcting for that asymmetry and improving the OVL accuracy. The OVL linearity arrays and programmed OVL shifts in different fields are used to verify the target sensitivity to OVL and have some correlation to accuracy, but they are not immune to systematic OVL shifts. Another way to select the optimal recipe condition is by ADI to AEI matching, however, in some cases this is disturbed by an OVL bias introduced by the etch process. In this case and others, the CD-SEM target embedded in the hybrid target allows independent verification of the measurement accuracy. We can see in this table that all accuracy measurement methods with an HVM acceptable ease of use (methods 1 to 6) are not accurate enough. Using a CD-SEM hybrid target is both accurate and acceptable for HVM recipe verification. Obviously, during the development stage, the ease of use is less a factor and then different methods (methods 7 to 10) come into play. They will be discussed in the next section.

2.3.

Calibration Map

The third component of DCM is to accurately measure, generate, and use a calibration map representing the delta between reference OVL measurement (at AEI) and OVL value of the OVL metrology target (at ADI). This calibration map can be used to calibrate the OVL before feeding the corrections to the scanner, so the scanner compensates for the difference and will print the device OVL more accurately on the next wafer. Reference OVL measurement can be done by CD-SEM result after de-cap, or an electrical test result or $x$-section result of the actual device, as captured in Fig. 2. These methods are commonly implemented at the development stage. The main disadvantages for these methods arise if the calibration map needs to be re-measured from time-to-time or when the process is changed. Re-measurement is required if stability issues are suspected, potentially resulting from process changes, metrology change and process or tool instability. Frequent re-measurement of the device OVL might result in significant effort and time delays, depending on the measurement method. Out of all the device correlation methods with excellent accuracy presented in this table, the one with the best ease of use is the insertion of CD-SEM targets in the hybrid OVL target, as introduced in this paper. In Fig. 3, such a CD-SEM target which resides in the center of the hybrid OVL target depicted in Fig. 3 can be seen. The CD-SEM image is superimposed on the schematic drawing of the OVL target. This type of CD-SEM target allows measuring the process induced effects (mostly litho and etch) that cause the shift between the target and device-like elements on the same layer. We denote this shift as “$\mathrm{\Delta }\mathrm{Layer}1$” for the delta between the target and device as measured by CD-SEM at the AEI of the previous layer. Also, similar to the current layer, we denote “$\mathrm{\Delta }\mathrm{Layer}2$.” These shifts, for both previous and current layers, can be introduced into a calibration map between the optical OVL measurement at ADI and the resulting device. Another element in the calibration map is obviously the difference between the optical AEI-ADI measurements. In the general case, the calibration map (${\mathrm{Bias}}_{\text{Target}\text{-Device}}$) will take the following form

A simple example for using Eq. (1) is the following: a systematic constant 1-nm shift between the target and device at AEI on layer 1 (previous): $\mathrm{\Delta }\mathrm{Layer}1=3\mathrm{nm}$. No shift at the current level at AEI $\mathrm{\Delta }\mathrm{Layer}2=0\mathrm{nm}$. While ADI to AEI, the OVL bias as measured by the Archer is also a constant at ${\mathrm{OVL}}_{\mathrm{Target}(\mathrm{ADI})}\text{-}{\mathrm{OVL}}_{\mathrm{Target}(\mathrm{ADI})}=2\mathrm{nm}$ in the opposite direction. The results of Eq. (1) will be $2\text{-}3\text{-}0=-1\mathrm{nm}$. Obviously, this should be done separately for $X$ and $Y$ directions. A more common case is when ${\mathrm{OVL}}_{\mathrm{Target}(\mathrm{ADI})}\text{-}{\mathrm{OVL}}_{\mathrm{Target}(\mathrm{ADI})}$ and also $\mathrm{\Delta }\mathrm{Layer}1/2$ can be represented by a radial function of the distance from the center of the wafer (like an expansion or rotation term).

The method described in Eq. (1) is general and can be used for most layers. The optical OVL of the ADI and AEI can be measured in most cases. The AEI shift as measured by CD-SEM requires measuring both elements (target and device) at the same layer, without the need to penetrate any layer, and can be done relatively easily in HVM. The downside of using this method by itself is that it does not verify the accuracy of the optical measurement by independent means. Therefore, a need for independent validation of the optical OVL arises. This can be done for the OVL target at ADI only for a very few cases since CD-SEM normal conditions do not penetrate the resist and other layers below it. Though, for some double-patterning layer cases it can be done at ADI, when both the current and previous layers are at the same level. Such results were already published elsewhere.^7–10 A more common case is to validate by CD-SEM the OVL at the AEI. This can be done in many cases, such as when the etch process removes and reveals a conducting element of the previous layer. Depending on the process flow, in some cases this measurement can be done a few steps after AEI. Two kinds of OVL can be measured with CD-SEM at AEI: validation of the optical OVL target (measuring target elements) and validating the actual device OVL. In Fig. 4, a CD-SEM measurement of a DCM hybrid target is shown. This measurement allowed verification of the actual OVL values and measurement conditions for the relevant layers.

Fig. 4

Magnification of Hitachi CD-SEM measurement of an OVL CD-SEM target embedded within another DCM OVL target. The measurement was performed after AEI at a process step where both previous and current layers are visible and measurable by CD-SEM. In this picture OVL between poly and isolation layers was measured.

3.1.

Results of Same Layer Shift: ΔLayer1 and ΔLayer2

Measurements of shift within the same layer done on the target seen in Figs. 3 and 5, resulted in significantly low values. This means that the segmentation chosen for the target does not introduce significant systematic or other shifts between the target and the device. Note that this statement is dependent on the specific layer measured, the scanner and other process conditions, so it may vary case by case. In Fig. 6, the CD-SEM picture of the CD-SEM target is shown with the lines used for the OVL measurement. The pink lines on the thinner, device like bars were used to determine the device’s center. The red lines on the wider bars were used to determine the target’s center. The difference between the two centers as seen in Fig. 7 is mostly less than 0.5 nm in both $X$ and $Y$ directions. As can be seen in Fig. 8, the $\mathrm{\Delta }\mathrm{Layer}1$ values measured at the four corners of each field do not show any wafer signature and are fairly constant. Since the shift is so small and does not exhibit a systematic wafer signature, we conclude that the shift is practically zero and is of the order of the CD-SEM measurement errors (0.2-nm reproducibility). In this case, Eq. (1) takes a simpler form as seen in Eq. (2)

Fig. 5

Magnified image of the center area in the DCM target from image 3, both GDS layout and critical dimension scanning electron microscope (CD-SEM) image. Measuring the $\mathrm{\Delta }\mathrm{Layer}1$ or $\mathrm{\Delta }\mathrm{Layer}2$ values is done on this pattern at after etch inspection (AEI). $\mathrm{\Delta }\mathrm{Layer}1$ or $\mathrm{\Delta }\mathrm{Layer}2$ is the relative shift between target features (thick lines) and device features (thin lines). Note that this measurement is done by CD-SEM at each layer (previous and current) separately.

Fig. 6

Hitachi CD-SEM image of a CD-SEM target intended to measure the shift between the target and device at the same layer: $\mathrm{\Delta }\mathrm{Layer}1$ and $\mathrm{\Delta }\mathrm{Layer}2$. The red lines on the wider bars were used to determine the target’s center. The pink lines on the thinner device like bars were used to determine the device’s center.

Fig. 7

Histogram of the measurement of $\mathrm{\Delta }\mathrm{Layer}1$ described in Fig. 5 for both $X$ and $Y$ directions.

Fig. 8

$\mathrm{\Delta }\mathrm{Layer}1$ element for the calibration map of the measurement described in Fig. 6 and shown in Fig. 7. CD-SEM has 0.2-nm reproducibility and in this experiment could measure a very small bias precisely without measurement error.

3.2.

Results of Direct OVL Measurement with CD-SEM

As mentioned earlier, direct CD-SEM OVL measurements were used to verify which measurement conditions reflect the most accurate OVL. When approaching the issue of verifying the optical OVL measurements, one needs to take into account that the CD-SEM has its own error budget. It also has specific best-known methods to calibrate it and improve the measurement accuracy. The co-operation with Hitachi was essential in those aspects. The optimized measurement condition is the one which has sufficient measurement performance but must be accurate as well. In the results shown in Fig. 9, a good correlation is clear between the optical OVL measurements done with Archer500 using a wide near infrared (WNIR) color filter and the Hitachi CD-SEM measurements. The measurements were done on the Hybrid target depicted in Fig. 5 at AEI.

Fig. 9

Correlation between Archer 500 wide-near infrared color filter X-OVL and Hitachi CD-SEM direct X-OVL measurements done on the structure in Fig. 4. The slope is 1.07 over a range of a few nanometers and the $R^2$ is 0.98.