Foundry CVD engineers must understand! 50 high-frequency professional terms, which can be interpreted in layman's terms

In the "thin film growth" process of wafer manufacturing, chemical gas deposition (CVD) is one of the core processes - CVD technology is inseparable from the insulating layer and conductive layer in the chip to the through-hole insulating film of 3D ICs. But newcomers who are new to contact are often dizzy by a bunch of "professional slang": What exactly is "step coverage"? What is the difference between "precursor" and "carrier gas"?

Basic concepts: first understand "what is CVD and what does it do"

1. CVD(Chemical Vapor Deposition)
Core principle: The process of passing the gaseous "raw material" into the reaction chamber and undergoing a chemical reaction on the wafer surface to "grow" a uniform film, such as plating the sidewall of the TSV with a SiO₂ insulating film.

2.Thin Film
The final product of CVD is usually in the nanometer (nm) to micrometer (μm) thickness (1μm=1000nm), for example, the CuSeed layer may be only 20-50nm thick, which is equivalent to 1/10,000th of the diameter of a hair.

3. Substrate
The "carrier" of the film usually refers to silicon wafers in wafer fabs, and occasionally special substrates such as sapphire and silicon carbide (such as when making power devices).

4. Reaction Chamber
The "core workshop" of CVD equipment is an enclosed space for gas reaction and film growth, and strict control of temperature, pressure, and gas concentration is required.

0040-09094 CHAMBER 200mm is a CVD cHAMBER

5. Precursor
The "feedstock gas" that forms the thin film, such as SiH₄ (silane), which is commonly used to deposit SiO₂.

6. Carrier Gas
The "tool gas" that helps the precursor to be "evenly transported" to the reaction chamber does not participate in the reaction, and inert gases (such as Ar, N₂) are commonly used to avoid contaminating the precursors.

7. Reaction Byproduct
The remaining "waste" after the precursor reaction, such as SiH₄ and O₂ reactions to produce SiO₂, produces H₂O (water), which needs to be discharged from the chamber in time, otherwise it will affect the quality of the film.

8. Deposition Rate
The "length and thickness" of the film per unit time is often expressed in nm/min (nanometers per minute), for example, the rate at which SiO₂ is deposited in PECVD may be 50nm/min, and in ALD, it may be only 1nm/min.

9. Film Thickness Uniformity
Differences in film thickness at different locations on the same wafer, such as "±3%" means that the difference between the thickest and thinnest points is no more than 3% of the average thickness, and poor uniformity can lead to inconsistent chip performance.

10. Film Purity
The "impurity content" in the film, such as the SiO₂ film, should not contain too many metal ions (such as Na⁺), which can lead to a decrease in insulation properties, and the purity of the ALD process is usually higher than that of PECVD.

II. Process parameters: the "core knob" that controls the quality of the film

11.Substrate Temperature
The "heating temperature" of the wafer determines whether the precursor can activate the reaction: if the temperature is too low, the reaction is insufficient, and the film is prone to peeling; Temperatures that are too high can cause wafer deformation (e.g., exceeding the melting point of silicon at 1414°C), typically in the range of 200-800°C.

12. Chamber Pressure
The gas pressure in the reaction chamber is divided into "normal pressure" (similar to the external atmospheric pressure), "low pressure" (a few tors, 1 torr≈ 133Pa), and "vacuum" (millirotter). Low pressure helps to spread the gas evenly, while high pressure increases the deposition rate.

13. Gas Flow Rate
The volume of gas entering the chamber per unit time is often expressed in sccm (standard cubic centimeters per minute), such as 50sccm for SiH₄ flow rate and 500sccm for Ar carrier gas – improper flow can lead to inadequate reaction or uneven films.

14.Partial Pressure
The "separate pressure" of a gas in the gas mixture, such as the total pressure of the chamber is 10 torr, of which the partial pressure of SiH₄ is 1 torr, the partial pressure of O₂ is 2 torr, and the partial pressure of Ar is 7 torr.

15. Plasma Power
For PECVD (plasma-enhanced CVD), the "electrical energy" added to the gas is used to excite the gas to form a plasma (a charged ion/electron hybrid) and reduce the temperature required for the reaction (e.g., PECVD can be deposited at 300°C, which is lower than 600°C for conventional CVD).

16. RF Frequency
The "current frequency" that generates the plasma, commonly 13.56MHz (high frequency) or 400kHz (low frequency): high-frequency plasma is more uniform and suitable for large wafer areas; Low-frequency plasma has higher energy and is suitable for high aspect ratio structure deposition.

17.Deposition Time
The time from "starting to vent" to "stopping the reaction", when the deposition rate is fixed, the longer the time, the thicker the film (thickness = rate × time), for example, the rate is 50nm/min, and the deposition is 100nm thick for 2 minutes.

18. Purge Time
After passing through the precursors, the time to "clean the chamber" with a carrier gas is to blow away unreacted precursors and by-products to avoid residual contamination of the next deposition (e.g., in the ALD process, purge after each precursor pulse).

19. Preheating Time
Before formal deposition, the wafer is given time to "warm up to the target temperature" to avoid uneven reactions caused by insufficient temperature during initial gassing, which usually takes 5-10 minutes.

20. Vacuum Level
The "degree of vacuum" of the reaction chamber, expressed in terms of "Pascal" or "Torr", is that the higher the vacuum degree, the fewer impurity gases (e.g., O₂, H₂O) in the chamber and the higher the film purity.

21. Gas Residence Time
The "length of stay" of the gas in the chamber is calculated as "chamber volume ÷ total gas flow", and the residence time is too short and the reaction is insufficient; If it is too long, the by-products will accumulate, usually controlled from a few seconds to tens of seconds

22. Wafer Rotation Speed
For "rotary CVD equipment", the wafer rotation speed (e.g. 500 rpm) is designed to make the gas more uniform on the wafer surface and reduce the difference in thickness between the edges and centers.

23. Showerhead Temperature
The temperature of the "gas spray device" at the top of the reaction chamber is usually close to the temperature of the substrate to prevent condensation of the precursor in the nozzle head (e.g., SiH₄ will condense into a liquid when cold, blocking the nozzle hole).

24. Cooling Time
After deposition, allow the wafer to "cool down to room temperature" to avoid oxidation of the high-temperature wafer in direct contact with the atmosphere, or scalding during subsequent operations, which usually takes 10-20 minutes.

25. Pressure Ramp Rate
Chamber pressure "from low to target" or "from target to low" (e.g., 1 Tor/sec) at a rate that is too fast can cause gas turbulence and affect film uniformity.

III. Film performance and testing: the standard for judging "whether the film is good or not"

26. Step Coverage
The uniformity of the film coverage on the surface of the wafer with a concave and convex structure (e.g., trenches, vias) is expressed as "sidewall/bottom film thickness÷ planar film thickness ×100%". For example, TSV vias need to ≥ 95% step coverage, otherwise the sidewall insulation is too thin and will leak electricity.

27. Conformality
Similar to step coverage, it emphasizes "uniform coverage of three-dimensional structures", such as micropores with a depth-width ratio of 10:1, ALD can achieve almost the same film thickness on the sidewall, bottom, and top, and the conformity is far better than PECVD.

28. Dielectric Constant,k Value
A parameter that measures the "ability to store charge" of an insulating film, and the lower the k value, the better the insulation performance (the less signal crosstalk). For example, the k value of traditional SiO₂ is about 3.9, and the k value of low-k media membranes such as SiOC can be as low as 2.5.

29. Breakdown Voltage
The minimum voltage of the insulating film is "broken down and loses insulation", such as the breakdown voltage of the SiO₂ film is usually ≥ 10V/μm, and a low breakdown voltage will cause the chip to leak and fail.

30. Film Stress
After the thin film is deposited, the internal stress caused by the "difference in thermal expansion coefficient" or "crystal structure mismatch" is divided into "tensile stress" (the film wants to shrink and pull the wafer) and "compressive stress" (the film wants to expand and press against the wafer). Too much stress can lead to wafer warping or film cracking, such as PECVD, which can be controlled to within ±50MPa by adjusting the power.

31. Adhesion
The "firmness of the film sticking to the substrate" is often tested by the "grid method" (using a knife to scratch the lattice on the film to see if it comes off), and poor adhesion will cause the film to peel off in the subsequent process.

32. Density
For example, the density of Al₂O₃ film deposited by ALD is ≈3.0g/cm³, which is denser than PECVD's 2.7g/cm³ and has a stronger ability to block water vapor.

33. Pinhole
"Tiny holes" in the film (which can be only a few nanometers in diameter) can cause leakage of the insulating film, poor contact of the conductive film, and ALD has far fewer pinholes than PECVD due to atomic growth.

34. Refractive Index
The ratio of the speed of light propagation in the film to the ratio in the vacuum can be used to indirectly determine the composition and density of the film (e.g., the refractive index of SiO₂ is about 1.46, if measured is 1.5, it may be doped with other elements).

35. Grain Size
In polycrystalline films, the larger the grain, the lower the resistivity (the better the conductivity), but the stress may also be greater, and the grain size can be controlled by adjusting the temperature.

36. Impurity Concentration
The "content of unwanted elements" in the film, such as O, C, N, etc., is often expressed as "atomic percentage (at%)", for example, the O content in the Cu film needs to be ≤0.1at%, otherwise CuO will be formed, increasing resistivity.

37. Surface Roughness,Ra
The "degree of concaveness and convexity" of the film surface is expressed as Ra (arithmetic mean deviation), for example, the surface of the film of Ra≤1nm is very flat, which is suitable for subsequent lithography processes; Too high a roughness can cause distortion of the lithography pattern.

38. Etch Rate
For example, if the etch rate of SiO₂ film suddenly becomes faster, it may be due to abnormal pressure during deposition and loosening of the film.

39. Thermal Stability
The film's ability to maintain stable performance in "subsequent high-temperature processes (such as annealing)", such as the resistivity change of the CuSeed layer after annealing at 400°C, needs to change ≤5%, otherwise it will affect chip reliability.

IV. Equipment and process types: CVD "tools and schools"

40. PECVD(Plasma-Enhanced CVD)

The most commonly used CVD type, with reduced reaction temperature (200-400°C) through plasma, fast deposition rate (50-200nm/min), suitable for large-area deposition (such as SiO₂ insulation on the wafer surface), but inferior to ALD in step coverage and purity.

41. ALD(Atomic Layer Deposition)
Grow thin films layer by layer, with a step coverage of ≥ of 98% and a thickness control accuracy of 0.1nm, suitable for high aspect ratio structures (e.g., TSV sidewalls, insulation layers for 3D NAND), but with slow deposition rates (1-5nm/min) and high cost.

42. LPCVD(Low-Pressure CVD)

Deposition at low pressure (0.1-10 Torr), uniform gas diffusion, high film purity (less impurities), suitable for depositing polysilicon (Poly-Si), but high reaction temperature (600-800°C), may affect other structures on the wafer.

APCVD(Atmospheric Pressure CVD)

Deposition at atmospheric pressure, simple equipment and low cost, but poor gas diffusion and poor film uniformity, are rarely used in advanced processes and are only used in some low-end devices (such as solar cells).

44. Showerhead
The "gas distribution device" at the top of the reaction chamber has many tiny nozzle holes (several tens of microns in diameter), which can evenly spray the precursor and carrier gas to the wafer surface, avoiding excessive local gas concentrations.

45. Heating Stage
For components that carry the wafer and heat the wafer, resistance heating or infrared heating is commonly used, and it is necessary to ensure uniform heating (temperature deviation ≤± 5°C), otherwise the thickness of the film will vary greatly between the edge and center of the wafer.

46. Vacuum Pump
The equipment for vacuuming the reaction chamber is divided into "mechanical pump" (pumping low vacuum, such as drawing from atmospheric pressure to 1 torr) and "molecular pump" (pumping high vacuum, such as pumping from 1 to 10⁻⁶ torr), and the performance of the vacuum pump directly affects the vacuum of the chamber.

47. MFC,Mass Flow Controller
For components that accurately control gas flow, the error is usually ≤± 1%, such as when the SiH₄ flow rate is set to 50sccm, and the MFC adjusts the valve in real time to ensure stable flow and avoid uneven film thickness caused by flow fluctuations.

48. In-situ Monitoring
Techniques for "real-time detection of thin film performance" during deposition, such as "optical ellipsometer" to measure film thickness in real time and "mass spectrometer" to monitor by-product composition, can detect process anomalies (such as flow deviations, pressure fluctuations) in time.

49. Process Window
The range of parameters that can prepare "qualified films" (e.g., temperature 250-300°C, pressure 5-10 torrs, flow rate 40-60sccm) is possible, and the wider the process window, the more stable the process, and the higher the fault tolerance rate

The above 49 terms basically cover the core scenarios of CVD engineers' daily communication, process debugging, and problem troubleshooting.

Newcomers can start with "basic concepts + process parameters" and understand them in combination with actual operations - for example, when debugging PECVD, try to adjust the pressure to see the change in step coverage, and slowly turn the "terminology" into "intuition"!