Bipolar Pulsed Sputtering for GST Target Thermal Management
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Solution Overview
Problem
DC magnetron sputtering of metal chalcogenides like germanium antimony telluride (GST) faces challenges due to their low thermal conductivity, leading to excessive heating and target degradation, which affects sputtering uniformity and requires frequent target cleaning, especially when depositing in high aspect-ratio holes.
Innovation Solution
The use of bipolar pulse sputtering with controlled temperature and chamber design, including a re-entrant target sidewall and peanut-shaped floating shield, along with pre-coating of shields to prevent amorphous deposition, allows for sustained plasma and reduced thermal load, enabling uniform deposition of GST in either crystalline or amorphous form.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If DC magnetron sputtering is used to deposit metal chalcogenides, then deposition speed and equipment cost are improved, but excessive heating occurs due to low thermal conductivity of the target material
Solution Approach 1:
The patent applies pulsed DC magnetron sputtering where the sputtering process is interrupted periodically by reversing the voltage polarity. During the reverse polarity phase, the target is heated uniformly without ion bombardment, allowing thermal equilibrium to be achieved. This periodic alternation between sputtering and heating phases enables continuous deposition while preventing excessive temperature buildup, resolving the contradiction between high deposition speed and target temperature control.
2Productivity
If high target power is applied to achieve high ionization fraction, then sputtering efficiency is improved, but target degradation including cracking occurs due to excessive heating
Solution Approach 1:
The pulsed sputtering process alternates between high-power sputtering phases that achieve high ionization fractions and deposition rates, and reverse-polarity heating phases that uniformly distribute thermal energy throughout the target. This periodic cycling prevents localized overheating and thermal stress accumulation that would otherwise cause target cracking and degradation, thereby maintaining target integrity while preserving sputtering efficiency.
Solution Approach 2:
The reverse polarity phase induces a phase transition in the heating mechanism: instead of ion bombardment heating (which causes localized stress), electron thermal conduction heating is utilized during the reverse phase. This different heating mode distributes energy more uniformly through the target bulk, preventing the thermal stress conditions that lead to cracking while still achieving the necessary thermal effects.
3Productivity
If continuous sputtering is performed, then deposition rate is maintained, but temperature profile across the target becomes non-uniform leading to degraded sputtering uniformity
Solution Approach 1:
The pulsed sputtering process periodically interrupts continuous deposition with reverse-polarity phases that allow thermal diffusion to equalize temperature gradients across the target. During the sputtering phase, deposition rate is maintained, while during the reverse phase, heat redistributes uniformly throughout the target material. This periodic thermal equilibration prevents the development of non-uniform temperature profiles that would otherwise cause variations in sputtering rate and film uniformity across the target surface.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach extends target lifetime, improves sputtering uniformity, and allows for precise control of GST phase-change properties, enhancing the performance of phase-change memories by maintaining the GST target at controlled temperatures and preventing particle generation.
Implementation Method 1
A magnetron positioned in back of the target projects a magnetic field adjacent the front face of the biased target
Implementation Method 2
The argon ions of the plasma efficiently sputter metal atoms from the target
Implementation Method 3
bipolar pulse sputtering with controlled temperature
Data Source
AI summary
A plasma sputtering method for metal chalcogenides, such as germanium antimony telluride (GST), useful in forming phase-change memories. The substrate is held at a selected temperature at which the material deposits in either an amorphous or crystalline form. GST has a low-temperature amorphous range and a high-temperature crystalline range separated by a transition band of 105-120° C. Bipolar pulsed sputtering with less than 50% positive pulses of less than 10:s pulse width cleans the target while maintain the sputtering plasma. The temperature of chamber shields is maintained at a temperature favoring crystalline deposition or they may be coated with arc-spray aluminum or with crystallographically aligned copper or aluminum.