DC signalDC signalDC signalDC signal

(W(CO₆)(Ref)

(CO₂)(Free CO)

IR source Gas cell

IR filters

Thermopileelements

INTRODUCTION—ALD processing is an increasingly critical deposition method for conformally coating high-aspect ratio features in advanced logic and memory devices. Accurate, consistent and controllable delivery of precursor materials to the deposition system is a necessity. To achieve reliable, low-cost deposition processes, a sensitive and non-destructive real-time method for monitoring the precursor concentration is increasingly important. An IR method o�ers chemical specific information for both the reactant and the reaction by-products. An IR based system was developed and used to measure the ‘direct’ flux from a solid source in real time. The real-time mea-surement of precursor concentrations in the gas-phase can be applied to process monitoring, process control and towards the detailed characterization of key vari-ables in the precursor delivery. Further, this

method can be used to characterize am-poule performance under di�erent pres-sure, temperature and flow conditions for a specific chemical precursor.

A broadband IR source is collimated and illuminates a gas-cell containing the pre-cursor vapor. A 4 channel thermopile or pyroelectric detector is equipped with chemical specific optical filters to detect the specific precursor, co-reactants and/or by-products. The precursor needs to be IR ‘active’ to be detected. Using solid precur-sors, we can directly measure the gas-phase concentration delivered from the ampoule under specific deposition condi-tions. We can also examine thermal de-composition from the precursor during transport. For example, we can measure the gas-phase concentration of W(CO)

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and simultaneously examine the liberation of CO using this method, by choosing wavelength specific filters, associated with both chemical species.

EXPERIMENTAL TEST SYSTEM—Long optical path-lengths allow for lower detection limits, as required for some depo-sition precursors and applications. In our test configuration, we have used a 1 meter pathlength cell, as shown in Figure 2 (linear cell). An automated control and data col-lection system (LabVIEW) was assembled for real-time monitoring and actual experi-mental testing. The solid delivery ampoule

is uniformly heated during use and tem-perature control is critical for uniform delivery. The gas-cell is uniformly heated and pressure is monitored at several points. Valves are automated, flow is controlled by MFCs and pressure is controlled by an automated butterfly valve. Transported precursor can be collected and analyzed to confirm that no decomposition has occurred during transport testing.

REPEATABLE DELIVERY FOR ALD THIN-FILM CONTROL—Using a small form-factor prototype (Figure 11), we have demonstrated the ability to measure the repeatable delivery of W(CO)₆ under a wide range of process conditions. The repeatable delivery of the precursor for specific pulses of precursor, enables a greater understanding of the

process variables and optimization of the precursor delivery performance. Repeatable pulses of precursor eliminate variation that might be experienced in an ‘uncontrolled’ deposition process. Controlled delivery and uniform pulse concentrations are critical for ALD manu-facturing and high-volume IC fabrication. W(CO)₆ concentration is repeatable under di�erent flow conditions.

SUMMARY—In this work, we have demonstrated the ability to detect and monitor the precursor concentration in real-time for a wide vari-ety of solid precursors under various pro-cess conditions. Using a simplified IR method with four separate channels of detection, we can monitor the precursor, a reference and possible by-products of de-composition. This technique is not limited to solids but can be equally applied to liquid and gaseous precursors useful for ALD thin-films.

The IR method enables the ‘direct’ com-parison of ampoule delivery designs and a computational model has been developed and correlated to our experimental results.

Excellent correlation between the model and the experimental results allows further optimization of the delivery system and precursor delivery methodology under a wide set of temperature, pressure, flow conditions and for a wide array of precur-sor chemistry.

The IR test system was extensively studied for several solid precursors, including Cp₂Mg, W(CO)₆, PDMAT, and TMI. Liquids such as CCTBA and gases, such as WF₆ and B₂H₆ have been examined with equal success using this IR spectroscopic method. Real-time, gas-phase detection has enhanced our understanding of pre-cursor delivery and fluid dynamic model-ling was correlated to experimental data.

SOURCE UTILIZATION AND DEPLETION STUDIES—The IR measurement system was used to investigate the delivery performance and >90% utilization of a solid precursor in a ProE-Vap ampoule. Precursor utilization

was 300% higher in the ProE-Vap 100 when compared to a commercial flow-over ampoule for the identical solid pre-cursor. Carrier flow or ampoule tempera-ture can be adjusted to maintain constant flux over time of delivery. Below is an ex-ample of constant delivery (8 hours) after an initial ‘break-in’ period.

Entegris®, the Entegris Rings Design®, and other product names are trademarks of Entegris, Inc. as listed on entegris.com/trademarks.

©2019 Entegris, Inc. | All rights reserved.Entegris — ALD Conference — July 2019 www.entegris.com

In-Situ Process Monitoring of Precursor DeliveryUsing an IR Spectroscopic Method

Robert L. Wright and Thomas H. Baum – Entegris, Inc.

Figure 1. IR photometer basic operation

MFC

PT-2

AV1

AV5

AV3

NV1

MV1

MV2

MV3

Coldtrap

Pump

LinearIR Cell

Scrubber

AV4AV2

PT

-1

AV - Actuated valve

MV - Manual valve

NV - Needle valve

PT - Pressure transducer

Heat trace

Oven

Figure 2. Schematic of IR measurement and test system

Figure 8. Characterization of W(CO)6 pulses during

start-up” in a flow-over ampoule

Figure 11. Picture of multi-pass prototype

Figure 9. After ampoule stabilization, uniform pulse concentrations were observed under constant delivery conditions

Figure 4. Cp2Mg repeated 50 ppm pulses from a ProE-Vap

100 detected by the IR method described herein

Figure 3. Four channel thin-filmdetector with specified optical filters

Table 1. General IR sensor characteristics

Specifications

Cp2Mg, TMI, W(CO)₆, PDMAT, CCTBA, WF₆;

Precursor must absorb IR wavelengths

Cp2Mg <1 ppm, W(CO)₆ <5 ppm for 1 meter

optical pathlength

4 channels (3 active plus 1 reference)

Parameter

Precursors tested

Lower detection limit (LDL)

Multi-channel monitor

Data collection rate

Chemical differentiation

Variable, typically 1 – 10Hz

Can measure and differentiate between precursorand by-products at the same time

Figure 10. W(CO)₆ delivery repeatability under various flow rate conditions; the blue line is monitoring ‘free’ CO byproduct generation

Figure 12. High resolution monitoring of W(CO)6

using the multi-pass prototype

SOLID SOURCE DELIVERY AMPOULES AND PREDICTIVE MODELLING—

ProE-Vap® 100 Series

Original (2012)

ProE-Vap 200 Series (2012)

>5X larger than Series 100 volume

ProE-Vap 300 Series (2015)

+30% larger than Series 200 volume

ProE-Vap 500 Series (2016)

+50% larger than Series 300 volume

Figure 5. Modelling of a ProE-Vap 100 solid delivery ampoule. Computational fluid dynamic modeling reveals ‘saturated’ (red) and ‘unsaturated’ (blue) concentrations for a given solid precursor, under variable pressure and flow conditions, for one temperature. Correlation of the computational model to experimental data, collected by the IR method, was in good agreement

Surface: c/c_sat (1) Surface: c/c_sat (1) Surface: c/c_sat (1) Surface: c/c_sat (1)

ProE-Vap 600 Series (2017)

Double the PE-500 volume

Scaling-up ProE-Vap for Solid Delivery Applications to Support HVMHigh-temperature valves (>200°C)

High thermal mass of stainless steel with respect to solid load for uniform temperature distribution

High precision machining to tight tolerances for excellent thermal conductivity

Multiple trays to distribute solid and maximize gas/solid surface interaction

Gas flows bottom to top through vents in trays and radially across solid surface

Figure 6. Comparison of W(CO)₆ delivery in a flow-over ampoule to a ProE-Vap 100 ampoule under identical process condition

Figure 7. Loss of Cp₂Mg vapor “saturation” at one temperature with increasing flow rates

COMPARING SOLID DELIVERY AMPOULE PERFORMANCE

For W(CO)₆ at 55°C ampoule temperature, the ProE-Vap delivers a consistent and uni-form concentration (Figure 6) for a given

flow rate. In contrast, the flow-over ampoule displays an ‘unsaturated’ (initial spike, followed by a drop in concentration) for every flow rate tested (left). The pulse sequence was 3 minutes ON / 3 minutes OFF for both ampoule designs, under identical process parameters (temperature, pressure and flow rates) for one precursor.

GAS-PHASE MONITORING AND PRECURSOR CHARACTERIZATION—Each solid precursor has unique physi-co-chemical properties. Therefore, solid delivery of that precursor will strongly depend upon temperature, pressure and flow conditions. Under some conditions,

the precursor will display saturation in the gas-phase. Under other conditions, the precursor transport may not be saturated and will be lower than the calculated flux in the gas-phase. An ALD process engineer needs to understand precursor behavior under a wide range of process conditions to ensure the deposition of thin-films with the desired properties.

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