Etching is a microfabrication process during which one or multiple layers of materials are being removed. A specific pattern can be used as a mask to etch only definite parts of the wafer.
For ease of understanding, we here split etching in two main categories : wet etching and dry etching.
Linked to the current practices at CMi, we also list and quickly explain terms and acronyms that are part of written guidelines and of standard vocabulary employed during training sessions in our cleanroom.
As its name suggests, the wet etching process uses a liquid as an etchant. The sample is usually completely emerged in a bath in order to attack, or etch, a target layer of material(s). This type of etching has the advantage of being highly selective across materials. It will mostly result in an isotropic etching.
Due to the high selectivity across materials, only one layer is reactive in contact with the liquid etchant.
The isotropy of the etch is highlighted with the under-etch below the mask.
Dry etching, on the other hand, uses ions, or gas, or plasma to remove layers of materials.
We consider here three different types of dry etching:
- Ion beam etching (sputtering) : ions are accelerated towards the sample with high enough energy to trigger physical etching, or sputtering, of the exposed materials. This technique can etch any types of material in an anisotropic manner, with limited etch selectivity between materials.
Below, the schematic illustrates the etch mechanism.
- Gas etching : the etchant is a reactive gas. This process is using the chemical affinity of the substrate and the gas to trigger specific reaction. This technique is usually highly selective among several materials. It is an isotropic etch. This type of etching is often used to clear full surfaces or to release channels and suspended structures by etching away a wisely selected sacrificial material.
- Plasma etching : reactive plasma is used as an etchant. Both chemical and physical reactions can occur. We speak about reactive ion etching (RIE). It offers great selectivity control over different materials and a precise anisotropic pattern transfer. A well known example is deep etching of Silicon, a well-established plasma etching process that achieves high aspect ratio patterns.
| Term | Quick definition |
| Anisotropic | Etch rate is not equal in all directions. |
| Aspect-ratio | Ratio between height and lateral dimensions of a feature. |
| By-products | Species/molecules created as a result of a chemical reaction. |
| DC-bias | Or self-bias. In a plasma etch system, RF power can be applied to the lower electrode the sample sits onto. Electrons from the plasma show high mobility to the applied RF power and they quickly accumulate in a blocking capacitor present below the electrode (negative DC potential). This time-averaged negative voltage accelerates the ions towards the sample (physical etching, or sputtering). |
| Edge-exclusion | Periphery of a sample that we do not consider for process. |
| End-point | Sensing/vision of when etching of a layer is fully completed. |
| Ions | Positively or negatively charged species (atoms or molecules). Important for physical sputtering with DC-bias. |
| Isotropic | Etch rate is equal in all directions. |
| Landing | Usually referring to a landing layer, or to a landing step within an etch recipe, after etching of a target layer is fully completed. |
| Macro-loading | Or loading effect. In plasma etch, is the decrease of etch rate as the total area of exposed target material increases. |
| Micro-loading | In plasma etch, local etch rate depends on local features pattern density and size. |
| Over-etch | After etch end-point is detected, etching is kept active long enough to counterbalance effects such as e.g. sample-scale non-uniformity, local micro-loading… |
| Passivation | Part of an etching process that selectively protects surfaces from being etched (usually the sidewalls to allow for anisotropic etching). |
| Radicals | Neutral atoms or molecules formed in plasma and highly chemically reactive because of unpaired electron. |
| Selectivity | Ratio between etch rate of a target material and etch rate of a mask or of a landing material. |
| Stop-layer | Layer that does not get attacked and that appears below a fully etched target layer. |
| Strike | Step in plasma etch systems that allows proper ignition of the plasma source. |
| Undercut | Lateral etching below a protective mask layer. |
| Uniformity | Usually etch-rate uniformity of a target material within the total exposed area. |
| Acronym | Meaning | Quick definition |
| AMU | Automatic Matching Unit | System placed after an RF power supply to ensure correct coupling of the electromagnetic power into the target system (e.g. ICP antenna for efficient plasma striking, or bottom electrode the sample sits onto). |
| APC | Automatic Pressure Controller | Feedback looped system that stabilizes pressure to setpoint within a process chamber. |
| ARDE | Aspect Ratio Dependant Etching | Etch rate variation of a feature with respect to the etched depth. |
| CD | Critical Dimension | Smallest dimension of interest on a design. Either a positive (e.g. line) or a negative (e.g. space) feature. |
| DRIE | Deep Reactive Ion Etching | Etching tens or hundreds of microns deep, or through a wafer. Term employed mostly for Silicon etching with pulsed and cyclic process steps. |
| EPD | End-Point Detection | System that allows live sensing/vision of when etching of a layer is fully completed. |
| ESC | Electro-Static Clamping or Chuck | Way of gripping wafers in place inside CMi’s ICP etchers. |
| IBE | Ion Beam Etching or Etcher | Etching occurs only through ions impacting the surface to sputter it away. |
| ICP | Inductively Coupled Plasma | Plasma ignition is ensured by inductive coupling an electromagnetic energy into a gas. |
| LER | Line Edge Roughness | Roughness present on the edge of a masking layer that tends to transfer on sidewalls of etched features. |
| LUR | Leak-Up Rate | Term used to quantify pressure rise of a vacuum chamber isolated from its pumping system. Either from air inlet, or from sample backside cooling gas escape. |
| MFC | Mass Flow Controller | Part of a tool that real-time accurately measures and controls the flow rate of a gas. |
| OES | Optical Emission Spectroscopy | Used as an end-point system that analyses characteristic optical emission that appears during chemical reactions between etchant and target species. |
| RF | Radio-Frequency | Band of frequencies the electromagnetic sources used in plasma etchers at CMi belong to. Frequencies used in tools at CMi are 1.8MHz, 13.56MHZ and 2.45GHz. |
| RIE | Reactive Ion Etching | Plasma dry etching that combines physical sputtering and chemical reactions. |
| SCCM | Standard Cubic Centimeters per Minute | Unit used for gas flow rate, essentially normalizing the number of gas molecules flowing per minute in fixed standard conditions. |