One hot plate, several different jobs. This guide explains what the soft bake, rehydration, post-exposure bake, hard bake and film anneal each do, gives starting points from public application notes, and shows why the gap between plate and substrate can matter as much as the setpoint.
In one paragraph: Follow the material data sheet. The cited photoresist note provides empirical soft-bake starting points, not a universal time-temperature rule [2]. Film temperature can differ from the setpoint. PEB and hard-bake conditions are resist-specific [4][5]. Record contact or gap, ramp, cool-down and delay as well as temperature and time.
1. Which Bake Does What: Mapping the Steps
A coated film usually sees a hot plate more than once. Each bake has a different job, and some process problems that get blamed on “the bake” come from running one step with the logic of another. In photolithography the names are standardised; in functional thin films (oxides, perovskites, polymers) the final heating step is usually called an anneal. The table maps the steps covered in this guide.
| Step | When | Main job | Typical starting point in the cited sources |
|---|---|---|---|
| Dehydration bake | Before coating | Drive adsorbed water off the substrate | About 120 °C for a few minutes; above 140 °C for oxidised surfaces [1] |
| Soft bake (prebake) | After coating, before exposure | Lower the residual solvent in the film | 100 °C for 1 min per µm of film thickness, for the resists covered in the cited note [2] |
| Rehydration (a wait, not a bake) | After soft bake, before exposure | Let water diffuse back into DNQ-based resists | Seconds for a 1 µm film; hours for several tens of µm [3] |
| Post-exposure bake (PEB) | After exposure, before development | Complete or smooth the photoreaction | About 100–130 °C for a few minutes, depending on the resist type [4] |
| Hard bake | After development | Stabilise the resist mask for etching or plating | Optional; use resist- and process-specific conditions [5] |
| Anneal | After drying a functional film | Convert, crystallise or densify the film | Set by the material; see Section 7 |
Starting points only; the resist or material data sheet takes precedence.
Two ideas run through all of them. For a thermally activated step, temperature sensitivity depends on its activation energy and process conditions [6]. And what matters is the temperature of the film, not the number on the controller [2].
2. Soft Bake: Removing Solvent Without Damaging the Film
A freshly spun film from a liquid resist still holds a lot of solvent: typically 10–35 %, depending on film thickness and solvent [2]. MicroChemicals lists why it has to come down before exposure: to avoid the mask sticking to or being contaminated by the resist, to prevent nitrogen popping or foaming in DNQ-based resists during exposure, to improve adhesion, to reduce dark erosion of positive resists in the developer, to allow a second coat without dissolving the first, and to prevent bubbling during later thermal steps such as metallisation or dry etching [2].
What actually happens in the film
A solvent molecule has to diffuse to the film surface, evaporate, cross a diffusion boundary layer in the gas above the film and be carried away by the air flow [2]. Diffusion in the film and evaporation are thermally activated; removal through the boundary layer depends on air movement as well as temperature [2]. The diffusion constant also falls as the film dries, so the dried surface layer slows further drying. In MicroChemicals’ measurements the mean solvent content first drops quickly and then saturates; at higher temperatures it saturates earlier but at a lower level [2].
Two practical consequences follow. First, the film develops a solvent gradient: the surface dries first, the region near the substrate last [2]. A dry surface stops mask sticking, while a dry interface helps adhesion, so both ends matter. Second, drying harder is not automatically better. MicroChemicals notes that baking to very low solvent content is often detrimental because it increases mechanical stress in thick films [2].
A starting recipe and how to adjust it
Unless the data sheet says otherwise, MicroChemicals gives 100 °C for 1 minute per µm of film thickness as an empirical starting point for the resist families covered in that note [2]. Between 80 and 110 °C, the time can be halved for each 10 °C increase, and doubled for each 10 °C decrease [2]. That is a convenient lever for temperature-sensitive substrates.
These values refer to the temperature in the resist film. The setpoint of the heater and the film temperature over time can differ, depending on the substrate and the equipment [2].
Too cool, too hot and thick films
| Condition | What the cited source reports |
|---|---|
| Too cool or too short | More dark erosion in the developer; poorer adhesion and stability in wet-chemical media [2] |
| Too hot or too long, positive resists | Partial decomposition of the photoinitiator; dark erosion can rise again [2] |
| Too hot or too long, negative resists | Thermal cross-linking of unexposed resist; slower or impossible development, especially of small openings [2] |
| Thick films | Required time rises roughly with the square of thickness; the time and temperature window narrows; stress cracks become a risk [2] |
Summarised from the MicroChemicals softbake note [2].
For very thick resist films, MicroChemicals describes two workarounds: multiple coats with a moderate bake after each (for example 80 °C for 1 minute per µm after each intermediate coat, then 100 °C for the same time after the last), or drying under vacuum at moderate temperature [2]. The vacuum route has a catch: any air bubbles trapped during coating expand strongly in vacuum, so a short conventional bake first makes the film more resistant [2]. Vacuum drying requires a suitable chamber; vacuum hold-down beneath a substrate is not vacuum drying of the film.
3. Hot Plate vs Oven, Contact vs Proximity
A hot plate heats mainly by conduction through the substrate; an oven heats by convection through air. Conduction through a solid in good contact is far more effective than convection from gas, which is why a film on a hot plate reaches temperature faster [7]. MicroChemicals measured this directly and recommends adding roughly 2–10 minutes, depending on the substrate, when a soft bake is moved from a hot plate to a convection oven [2]. They also note that temperature differences between locations in an oven, and overshoot after the door has been opened and the oven reheats, make temperature-critical results harder to reproduce [2].
Contact, proximity and the air gap
Between the plate and the substrate there is almost always a gap, intended or not. MicroChemicals measured the surface temperature of a 575 µm silicon wafer and a 3.3 mm glass blank on a hot plate set to 100 °C, with different gaps [2]:
- With small gaps, the silicon wafer heated quickly, thanks to its good thermal conductivity and low heat capacity; the thick glass took noticeably longer [2].
- With larger gaps, the gap itself dominated the heating curve and the substrate type mattered less [2].
- Below about 100 µm, the result depended on whether the wafer back side was polished or rough, and a true zero-gap contact was hard to achieve without pressing the substrate down [2].
Two practical points follow. Vacuum hold-down can improve contact for a compatible, adequately sealed substrate. A deliberate proximity gap changes the heating history; qualify it rather than assuming that slower heating is better. Either way, the gap is a process parameter: write it down with the temperature and time.
Glass, ceramics and thick substrates
For massive, curved or poorly conducting substrates such as glasses, ceramics and polymers, the substrate takes time to heat through, and the film may end up significantly below the plate temperature [2]. Such substrates can need longer soft bake times or higher temperatures [2]. For thin, flat, well-conducting substrates on a hot plate, the film follows the plate temperature closely after a short delay [2].
The cited note describes film-temperature measurement difficulties, including thermocouple response and radiative effects through silicon [2]. Validate the measurement arrangement for the substrate and process. Development, adhesion and thickness measurements qualify bake performance; they are not direct temperature measurements.
4. Rehydration: The Wait Between Bake and Exposure
DNQ-based positive and image reversal resists need a small amount of water in the film, a few parts per thousand, for the photoreaction during exposure [3]. The soft bake removes most of it, so the film has to take water back up from the room air before exposure. This is rehydration, and it is a wait, not a bake [3].
- Time depends on thickness. Water diffuses in from the surface, so the time rises roughly with the square of film thickness: a few seconds for a 1 µm film, about a minute for 7 µm, and hours for films several tens of micrometres thick [3].
- Humidity sets the level. MicroChemicals recommends 40–50 % relative humidity. Much lower humidity reduces the development rate; much higher can hurt adhesion by adsorbing water on substrates before they are coated [3].
- Never rehydrate in water or saturated vapour. An over-saturated film can bubble badly during exposure [3].
In practice, this means a thick film taken straight from the hot plate to the aligner can develop very slowly near the substrate. MicroChemicals shows a 22 µm film that needed over two hours to develop with too short a delay, against a short development time after a 10 minute wait [3]. Chemically amplified resists are an exception to this water requirement [3].
5. Post-Exposure Bake: Three Different Purposes
A post-exposure bake is done after exposure and before development. Some qualified PEB processes exceed the resist’s softening temperature, but an undeveloped film is not by itself permission to exceed the specified bake conditions [4]. Depending on the resist, a PEB serves one of three purposes:
| Purpose | Resist type | What the bake does | Typical conditions [4] |
|---|---|---|---|
| Complete the photoreaction | Chemically amplified positive resists | Acid formed during exposure catalyses the reaction during the bake [8]; without it, the film hardly develops | About 100–110 °C, a few minutes |
| Image reversal or cross-linking | Image reversal resists; cross-linking negative resists | Inverts or cross-links the exposed areas so they stay in the developer | About 110–130 °C, a few minutes |
| Smooth standing waves | Positive resists on highly reflective substrates | Diffusion smooths the interference pattern of the photoproduct, giving smoother sidewalls | About 110–120 °C, a few minutes |
These examples concern the resist systems in the cited note; use the actual material’s specified PEB schedule [4].
Two details are easy to miss. First, use a resist-specific PEB schedule rather than transferring the soft-bake time-per-thickness heuristic [4]. Second, the PEB is a balance. For image reversal resists, a hotter bake makes the structures more stable in the developer but reduces photosensitivity, and for negative resists a hotter bake cross-links more but also starts to cross-link unexposed areas, which then develop poorly [4]. Not every negative resist needs a PEB at all [4].
Because acid diffusion during the PEB affects the final pattern in chemically amplified resists, bake temperature and time uniformity across the substrate matter for linewidth control [8][9]. This is one reason specifications such as temperature uniformity and repeatability are worth reading carefully; see Reading Hot Plate Specs: Temperature Uniformity, Accuracy and What ±1% Means.
6. Hard Bake: Stabilising the Mask, and Its Side Effects
A hard bake is an optional bake after development that makes the resist mask more stable for wet or dry etching or electroplating [5]. It is worth asking whether one is needed at all: MicroChemicals notes that with good substrate preparation many processes do without it, which also simplifies the process [5]. When it is used, several things happen at different temperatures:
| Approximate temperature | What happens in the developed resist [5] |
|---|---|
| From about 100–130 °C | Non-cross-linked positive and image reversal resist structures soften and their top edges round (reflow) |
| Below the softening point | Residual solvent near the substrate falls, stabilising the sidewalls of thick films |
| From about 120 °C | A significant fraction of the photoinitiator in positive resists decomposes within minutes; resist begins to embrittle from about 120–130 °C |
| From about 130–140 °C | Positive resist resin cross-links increasingly, improving resistance to alkaline media and solvents |
| From about 150–160 °C | Positive resists become hard to strip with conventional removers |
Approximate ranges for common resists; check the resist data sheet.
Brittleness matters for thick films: different thermal expansion of resist and substrate can cause stress cracks invisible to the eye. MicroChemicals suggests cooling slowly, for example by leaving the substrate on the switched-off hot plate, and never putting hot substrates straight onto a room-temperature surface [5]. Whatever method you use, make the cool-down a written part of the recipe.
Reflow is not always a defect: it is used deliberately to round resist structures, for example to form microlens shapes that are then transferred into the substrate by dry etching [5].
7. Annealing Functional Films: Why Temperature Dominates
Outside lithography, the last heating step on a hot plate is often an anneal that turns a dried film into a functional one. What happens depends on the material:
- Sol-gel oxides. A gel film loses solvent and water, then organic groups and residues burn out, and the oxide network densifies; these stages occur over different temperature ranges and shrink the film [10]. Calcination and densification at higher temperatures are the subject of High-Temperature Hot Plates to 600 °C: Calcination, Oxide Films and Uniformity Limits.
- Air-sensitive films. Some films degrade when annealed in air; oxygen and moisture can change conjugated polymers and halide perovskites. Annealing under nitrogen is covered in Annealing Under Nitrogen: Inert Hot Plates for Air-Sensitive Films.
- Polymer films. Heating a polymer film above its glass transition lets chains rearrange; on a substrate it does not wet, a very thin film can also dewet, as described in Why Your Ink Dewets: Surface Energy, Contact Angle and Wetting Before You Coat.
Most of these processes are thermally activated. The Arrhenius equation, k = A exp(−Ea/RT), describes how a rate constant changes with absolute temperature [6]. Temperature sensitivity depends on the activation energy and controlling mechanism; there is no universal exchange between temperature and time. The soft-bake halving rule is an empirical guideline within its stated range [2], not a result that follows from the Arrhenius equation alone.
When a functional film is coated on a heated substrate, part of the drying happens during spinning; see Heated Spin Coating: How Substrate Temperature Changes Evaporation and Thickness.
8. Writing a Bake Recipe You Can Repeat
Whatever the step, a bake is defined by more than a setpoint and a time. Record these, and change one at a time:
| Parameter | Why it matters | What to record |
|---|---|---|
| Setpoint and dwell time | Thermally activated rates depend on temperature and activation energy [6] | Setpoint, time from placement to removal |
| Contact or gap | The air gap dominates heating with larger gaps [2] | Contact, vacuum, or lift-pin height |
| Substrate | Glass, ceramics and thick substrates heat slower and may stay below the setpoint [2] | Material, thickness, back-side finish |
| Ramp | A gentler heat-up changes how fast solvent leaves the film [2] | Ramp rate or staged setpoints |
| Cool-down | Fast cooling can crack thick, baked resist films [5] | Where the substrate cools and for how long |
| Delay before the next step | Rehydration and queue times change results [3] | Time and room humidity |
| Atmosphere | Air, nitrogen or a glovebox changes oxidation and moisture uptake | Gas, flow, and where the plate sits |
A checklist for writing bake recipes; values come from your own process qualification.
A practical way to qualify a bake is to vary one parameter at a time and watch a result you can measure: thickness loss in the developer, development time, adhesion in the etch, or the final film thickness after anneal. The cited note discusses empirical bake optimisation where thermometry is difficult [2]; process outcomes validate performance, not temperature.
Where the substrate sits on the plate also matters. A hot plate is not perfectly uniform; edges usually lose more heat than the centre. Keep substrates within the area your process was qualified on, and read the uniformity specification and its conditions; see Reading Hot Plate Specs: Temperature Uniformity, Accuracy and What ±1% Means.
9. Levon™ Hot Plates for Baking and Annealing
The tables quote the Levon™ product pages; “Not stated” means the page does not give the value.
Compact and vacuum hot plates
| Specification | HP5 | HP10 | HP6 |
|---|---|---|---|
| Heating plate | 150 × 150 mm square, anodized aluminum | 220 × 220 mm square, anodized aluminum | Ø170 mm circular, hard-anodized aluminum; wafers up to 6 inches |
| Temperature range | RT to 300 °C | 300 °C configuration: RT to 300 °C; 400 °C configuration: RT to 400 °C | RT to 250 °C |
| Resolution | 0.1 °C | 0.1 °C | 0.1 °C |
| Control accuracy / precision | Accuracy ≤±1 °C | 300 °C: precision ±1 °C; 400 °C: precision ≤±1 °C | Precision ±0.2 °C (after the temperature is stable) |
| Temperature uniformity | ≤±2% | 300 °C: <±2%; 400 °C: ≤±2% | ≤1% |
| Controller | Precision digital temperature controller | Digital temperature controller | Digital temperature controller with optional linear control function |
| Substrate hold-down | Not stated | Not stated | Manually activated vacuum chuck; close-contact resist baking mode; vacuum input 0.04–0.09 MPa |
| Glovebox | Recessed heating structure for glovebox applications | Recessed heating structure for use inside a glovebox | Recessed heating element design for glovebox integration |
| Lid | Not stated | Not stated | Thermal-radiation-sealed lid |
| Power input | AC 220 V, 800 W | 300 °C: AC 220 V, 1,000 W; 400 °C: AC 220 V, 2,000 W | 220 V, 1,000 W |
Specifications as published on the Levon™ hot plate product pages on 22 October 2026. The supplied configuration is confirmed in the quotation.
Programmable hot plates
| Specification | HP100 | HP200 |
|---|---|---|
| Configurations | HP100-SE (300 °C or 400 °C); HP100-PE | HP200-SE; HP200-PE |
| Heating plate | Hard-anodized aluminum; HP100-PE: 220 × 220 mm square; HP100-SE size not stated | 340 × 340 mm square, hard-anodized aluminum |
| Temperature range | SE: RT to 300 °C or RT to 400 °C; PE: RT to 300 °C | RT to 300 °C |
| Control accuracy | SE: ≤±0.2 °C; PE: ±0.2 °C | SE: ±0.2 °C; PE: ≤±0.2 °C |
| Temperature uniformity | SE: ≤±1.5%; PE: ≤1% (RT to 300 °C) | RT to 200 °C: ≤±1%; 200 °C to 300 °C: ≤±1.5% |
| Control | 7-inch touchscreen, PLC; PE: linear control, 0.5–10 °C/min, 100 recipes × up to 30 stages | 7-inch touchscreen, PLC; 100 recipes × up to 30 stages; PE: linear control |
| Proximity baking | Auto-lift; SE pins 0–33 mm, resolution not stated; PE pins 0–30 mm, 0.1 mm resolution | Auto-lift; pins 0–30 mm (0.1 mm resolution), for substrates larger than 120 mm in diameter |
| Vacuum adsorption | PE only; vacuum input 0.04–0.09 MPa | PE only; vacuum input 0.04–0.09 MPa |
| Power | SE: heating plate 1,400 W; PE: 220 V, 2,800 W (110 V optional) | 220 V, 3,500 W (110 V optional) |
| Dimensions (W × D × H) | 350 × 460 × 320 mm | 450 × 580 × 320 mm |
Specifications as published on the Levon™ hot plate product pages on 22 October 2026. The supplied configuration is confirmed in the quotation.
How this connects to the guide: the HP6 has a manually activated vacuum chuck and a close-contact resist baking mode, which is relevant to the substrate-contact discussion in Section 3. The HP100 and HP200 have automatic lift pins for proximity baking. HP100-PE specifies 0.1 mm pin-height resolution, 100 recipes of up to 30 stages and 0.5–10 °C/min linear heating. Both HP200 configurations specify that recipe capacity and pin resolution; HP200-PE adds linear control without a published heating-rate range. Vacuum adsorption is listed for the PE versions. The HP5 and HP10 are compact plates with a recessed heating structure for glovebox use. The uniformity values on the pages are quoted as published; the pages do not state the measurement conditions, so ask the supplier how the specification was obtained before relying on it near the edge of the plate.
10. Keep Exploring the Bake and Anneal Knowledge Hub
Start here
- Spin Coating Explained: Stages, Physics and Film Thickness — how the film is formed before it is baked
- Solution-Processed Thin Films: Choosing Spin, Dip, Slot-Die or Blade Coating — coating methods that all end on a hot plate
- Heated Spin Coating: How Substrate Temperature Changes Evaporation and Thickness — drying that starts during spinning
Process guides
- Reading Hot Plate Specs: Temperature Uniformity, Accuracy and What ±1% Means — what a uniformity specification does and does not tell you
- Annealing Under Nitrogen: Inert Hot Plates for Air-Sensitive Films — annealing air-sensitive films under nitrogen
- High-Temperature Hot Plates to 600 °C: Calcination, Oxide Films and Uniformity Limits — calcination and oxide films at higher temperature
- Photoresist Spin Coating: From Viscosity to Target Thickness and Uniformity — coating the resist that you bake
- Sol-Gel Dip Coating: From Sol to Dense Oxide Film (SiO2, TiO2) — sol-gel films that need an anneal
Links will be added as each article is published.
Equipment
- HP6 Precision Vacuum Hot Plate — vacuum chuck, close-contact resist baking
- HP100 Programmable Hot Plate — auto-lift proximity baking; recipe capacity specified for HP100-PE
- HP200 Programmable Hot Plate — 340 × 340 mm plate, lift pins
- Levon™ Hot Plates — all hot plates
Questions about a specific hot plate can be sent through the contact form.
11. FAQ
What temperature and time should I use for a soft bake?
Follow the resist data sheet. The cited note gives 100 °C for 1 min per µm as an empirical starting point for the resist families it covers [2]. Qualify the actual material, thickness and heating arrangement rather than treating this as a universal recipe.
Why does the same recipe behave differently on glass?
Glass and other poorly conducting or thick substrates heat slowly and may stay below the plate temperature [2]. Lengthen the bake or qualify a separate recipe.
Is an oven as good as a hot plate for soft bake?
It heats more slowly; MicroChemicals suggests adding about 2–10 min, and notes oven location differences and overshoot reduce reproducibility [2].
Does PEB time scale with film thickness?
Do not apply a general time-per-thickness rule. Use the resist’s specified PEB schedule for the intended thickness and process; the temperatures and times quoted here are examples from the cited note [4].
Why did my thick resist crack after a hard bake?
Resist embrittles from about 120–130 °C, and different expansion of resist and substrate causes stress. Cool slowly on the switched-off plate [5].
Can I measure the film temperature directly?
Film-temperature measurements require a validated method. The cited note identifies thermocouple response and radiative effects as potential sources of error [2]. Use suitable thermometry and process-outcome tests as complementary checks.