When optimizing low-temperature PECVD processes under thermal budget constraints, the critical question is not simply "how low the temperature can go." What truly matters in PECVD is how well dielectric film quality can be maintained as the deposition temperature decreases.

From a practical process development perspective, this article explores why thermal budget constraints are becoming increasingly stringent, what physical and chemical changes occur in films at lower temperatures, and what key criteria should be evaluated when selecting deposition equipment.

What is Thermal Budget? Understanding Thermal Exposure in Semiconductor Fabrication

Thermal budget refers to the total thermal exposure—the integral of temperature over time—that a semiconductor wafer undergoes throughout the entire manufacturing flow. In practice, the key point is that thermal budgets are managed as a cumulative sum across all process steps, rather than merely by the peak temperature of an isolated process. Even if the thermal load of an individual step appears within tolerance, the total cumulative load can easily exceed allowable limits when combined with preceding and subsequent steps. Therefore, evaluating deposition temperatures must not be treated as a single-step optimization, but rather as an allocation problem encompassing the entire thermal history until device completion.

While thermal budget management has long been critical for suppressing dopant diffusion, today's advanced devices are pushing these thermal limits to their physical and chemical boundaries. Driving this shift is the architectural transition toward 3D integrated circuits (3D-IC) and heterogeneous integration using chiplets, where fundamentally disparate materials are stacked and integrated together.

Why Thermal Budget Constraints Are Becoming More Stringent Today

To understand the source of these constraints, it is helpful to look not just at the device structures themselves, but at the co-existing materials present during processing.

First is the thermal limit of organic temporary bonding adhesives. In via-reveal processes after Through-Silicon Via (TSV) formation, as well as in hybrid bonding flows, temporary bonding adhesives are used to secure wafers to carrier substrates. Organic materials such as polyimide (PI) and benzocyclobutene (BCB) exhibit thermal limits—often governed by their glass transition temperatures—typically around 200°C to 250°C. Exceeding these thresholds not only causes thermal degradation of the adhesive layer but also triggers severe outgassing in the vacuum chamber, which can disrupt the entire deposition process （Source：3D InCites, Sept. 2012）

Second is the thermal vulnerability of specific substrates and active devices. Optical and piezoelectric single-crystal substrates such as LiTaO₃ and LiNbO₃ used in RF filters, MicroLEDs, OLEDs, and Josephson junctions in quantum processors are highly sensitive to thermal history. Even modest thermal exposure can induce substrate warpage/thermal strain or degrade optical and electrical properties.

Under these restrictions, PECVD processes for passivation layers and interlayer dielectrics (ILD) are increasingly required to operate well below the conventional 300°C–400°C range—moving down to 200°C or below, and in some cases around 100°C. Driving this trend is the surging demand for High Bandwidth Memory (HBM) in AI accelerators and high-speed communication devices for 5G/6G applications.

Market Data Highlights the Demand Structure for Low-Temperature Dielectric Films

This shift in demand is clearly reflected in market size projections. According to estimates by market research firm QYResearch, the global PECVD systems market is projected to grow from approximately US$3.5 billion in 2024 to US$3.7 billion in 2025. It is expected to reach approximately US$5.4 billion by 2031, expanding at a compound annual growth rate (CAGR) of 6.2% over the forecast period （Source：QYResearch "Plasma Enhanced Chemical Vapor Deposition (PECVD) Systems - Global Market Share and Ranking, Overall Sales and Demand Forecast 2026-2032" Dec 2025）.

Notably, the primary growth drivers identified by the firm all converge around low-temperature processing requirements:

• Wide-bandgap (third-generation) semiconductors (such as SiC and GaN) demand the formation of high-quality passivation and dielectric layers at reduced temperatures.

• Heterogeneous integration and wafer-level packaging increasingly require PECVD processes to deposit stress-buffer and insulating layers onto diverse substrates—including silicon, glass, and organic materials (Source: ibid). 

None of these applications are viable within conventional process temperature regimes of 300°C to 400°C. In short, the underlying market drivers directly coincide with the technical necessity for low-temperature processing.

What is PECVD? The Operating Principles and Limits of Lowering Deposition Temperatures

PECVD (Plasma Enhanced Chemical Vapor Deposition) is a thin-film deposition technique that supplies the energy required to dissociate precursor gases via plasma rather than thermal energy. This "energy source substitution" is the fundamental principle that enables film formation at significantly lower substrate temperatures compared to thermal CVD.

However, utilizing plasma does not mean deposition temperatures can be reduced arbitrarily. In the mechanisms governing vapor-phase deposition, lowering substrate temperatures impedes both the precursor gas dissociation efficiency and the surface migration of adatoms upon reaching the substrate. Plasma primarily compensates for the former; the latter—the rearrangement of atoms on the surface—remains heavily dependent on thermal energy from the substrate. The intrinsic trade-offs in low-temperature PECVD originate directly from this fundamental asymmetry.

Film Quality Trade-offs in Low-Temperature PECVD

In general, lowering the CVD process temperature suppresses both gas-phase reactions and surface adatom migration, leading to a degradation in the electrical and mechanical properties of the deposited films. Specifically, SiO₂ and SiNx films suffer from reduced film density and become susceptible to microvoid formation. Furthermore, hydrogen originating from precursor gases such as 
SiH₄ and NH₃ tends to remain trapped excessively within the film network.

The adverse effects of excessive hydrogen incorporation vary across different device types. In optical waveguide devices, it causes optical losses due to parasitic infrared absorption (e.g., Si-H and N-H vibrational bonds). In semiconductor devices, it manifests as shifts in electrical characteristics driven by hot-carrier injection and increases in leakage current（Source: SEMI Public Data）. Even for the same film type, the acceptable hydrogen content threshold must be evaluated on an application-by-application basis.

Stress Control, Film Cracking, and Wafer Warpage Issues

Another critical challenge associated with lower deposition temperatures is controlling thin-film residual stress. At low temperatures, the film tends to develop high tensile or compressive stress, making mechanical properties highly sensitive to deposition parameters（Source：Elsevier B.V.「Effect of deposition conditions on mechanical properties of low-temperature PECVD silicon nitrides」2006）.

This impact is particularly severe in devices featuring free-standing membranes, such as MEMS, and in optical devices requiring thick films of several micrometers. Stress-induced cracking, delamination, and pronounced wafer warpage are not only defects in themselves, but also severely degrade overall yield by causing lithography and alignment errors in downstream processes.

When handling thick films, finding the right trade-off between deposition rate and residual stress becomes the crucial practical design point. In this domain, process engineering must focus on minimizing accumulated internal stress, achieving high-throughput deposition while suppressing the formation of voids, cracks, and wafer warpage.

Evaluating Low-Temperature Deposited Dielectric Films

However, uniformly concluding that low-temperature deposition yields unusable film quality is practically inaccurate. Multiple studies have demonstrated that optimizing process parameters can secure film quality fully sufficient for practical deployment.

In a study on insulation liner films for TSV applications, a plasma TEOS process deposited below 200°C achieved more than 2.5 times higher sidewall coverage (step coverage) in medium-to-high aspect ratio vias compared to conventional silane (SiH₄)-based PECVD processes. Furthermore, this low-temperature dielectric film demonstrated an extremely low leakage current density of less than 1×10⁻⁷ A/cm² under an electric field of 2 MV/cm, along with a breakdown field strength exceeding 10 MV/cm（Source：IEEE／ECTC 2010 Proceedings「Low temperature PECVD of dielectric films for TSV applications」2010）.

Electrical properties are not the only evaluation criteria. In the surface passivation of crystalline silicon, it has been demonstrated that an a-Si/SiOx/SiNx triple-layer dielectric stack deposited by low-temperature PECVD below 250°C provides excellent chemical passivation, achieving an extremely low effective surface recombination velocity（Source：D-NB publication data）.

What is critical is that these achievements are not governed by deposition temperature in isolation, but are realized through the interplay of precursor gas systems, layer stacking engineering, and plasma processing conditions. Consequently, assessing low-temperature PECVD feasibility cannot rely merely on nominal deposition temperature ratings; rather, a comprehensive evaluation encompassing sidewall conformity, leakage current density, breakdown electric field, bonded hydrogen concentration, and intrinsic stress must be performed in relation to the specific requirements of the targeted device architecture.

Application-Driven Requirements for Low-Temperature Deposition

In practice, the requirements for low-temperature PECVD vary significantly depending on the application. Even when target temperatures are identical, the prioritized film properties often differ.

In advanced packaging, the thermal budget is strictly capped by the temperature limits of temporary bonding materials. For passivation in via-reveal processes and dielectric films for hybrid bonding, the primary requirement is to achieve both void-free bonding and stable dielectric isolation while remaining below an upper limit of around 200°C.

Conversely, in quantum devices and optical crystal substrates, the allowable upper temperature limit is set even lower. For materials where even minimal thermal budget exposure directly leads to performance degradation—such as Josephson junctions or LiTaO₃ substrates—deposition at around 100°C becomes a practical necessity. In this regime, beyond maintaining baseline film quality, minimizing the thermal damage imparted to the substrate by the deposition process itself is paramount.

Furthermore, in MEMS, optical waveguides, and power devices, residual stress and film thickness emerge as major constraints alongside thermal limits. Even within the same low-temperature regime, feasibility hinges on the ability to deposit several-micrometer-thick films without cracking.

The Philosophy of Low-Temperature PECVD: Not Merely "Lowering Temperature," but "Preserving Quality at Low Temperatures"

Lowering the deposition temperature itself has already been achieved through multiple technological approaches. Options are diverse and plentiful if one only compares the lowest achievable temperatures—ranging from high-density plasma (HDP) systems capable of deposition near room temperature, to configurations combining liquid precursors with cathode coupling, and specialized transfer systems equipped with low-temperature heaters. Consequently, the practical criterion for equipment selection has shifted from how low the temperature can go, to whether high-temperature-equivalent film quality can be secured at that reduced temperature.

SPP Technologies’ latest PECVD platform officially specifies the capability to deposit high-quality SiO₂ and SiN films at temperatures as low as 100°C, matching the film quality achieved at 300°C on our conventional systems. Beyond simply making low-temperature deposition "possible," this means critical parameters directly governing device performance—such as film density, low hydrogen concentration, and dielectric breakdown strength—can be reliably maintained without imposing an excessive thermal budget.

Even if the nominal deposition temperatures in the product specifications are identical, the optimal solution varies depending on device architecture, target film thickness, and allowable wafer warpage. When evaluating low-temperature PECVD, the practical starting point is not to focus solely on temperature as an isolated metric, but rather to work backwards from the total allowable thermal budget of your specific process combined with the essential film properties that must be guaranteed.

Inquiries Regarding Process Conditions

If you have specific requirements—such as deposition on heat-sensitive substrates, crack-free thick film deposition, or high-throughput processing for temporarily bonded wafers—we can propose tailored process solutions based on your device architecture and target specifications. For technical documentation requests and inquiries regarding process conditions, please contact SPP Technologies.