US-12619151-B2 - Monitor structure for photoresist thickness in trench

Abstract

A method of forming a microelectronic device includes forming positive tone photoresist on the microelectronic device, filling a trench, extending over a top surface adjacent to the trench, and covering a thickness monitor on a substrate containing the microelectronic device. The photoresist in and over the trench is exposed at a trench energy dose, and the photoresist in the monitor area is exposed at a monitor energy dose that is less than the trench energy dose. The photoresist is developed, leaving photoresist in the trench having an in-trench thickness less than the depth of the trench and leaving an in-monitor thickness of the photoresist on the monitor area less than an unexposed thickness. The in-monitor thickness of the photoresist on the monitor area may be measured and the measured thickness value may be used with a calibration chart to estimate the in-trench thickness of the photoresist.

Inventors

• Yunlong Liu
• Hong Yang
• Peng Li
• Yung Shan Chang
• Sheng Pin Yang
• YA PING CHEN

Assignees

• TEXAS INSTRUMENTS INCORPORATED

Dates

Publication Date

20260505

Application Date

20230428

Claims (20)

1. 1 . A method of forming a microelectronic device, comprising: exposing photoresist on a device substrate having a trench and a monitor area, the monitor area being planar, wherein the photoresist in the trench is exposed at a first energy dose and the photoresist on the monitor area is exposed at a second energy dose that is different than the first energy dose; and developing the photoresist, wherein an in-trench thickness of the photoresist is left in the trench and an in-monitor thickness of the photoresist is left on the monitor area.
2. 2 . The method of claim 1 , wherein the second energy dose is less than the first energy dose.
3. 3 . The method of claim 1 , including measuring the in-monitor thickness.
4. 4 . The method of claim 3 , wherein measuring the in-monitor thickness includes an ellipsometry process.
5. 5 . The method of claim 3 , wherein measuring the in-monitor thickness includes a contact profilometry process.
6. 6 . The method of claim 3 , wherein measuring the in-monitor thickness includes an optical spectroscopy process.
7. 7 . The method of claim 3 , wherein measuring the in-monitor thickness includes a non-contact profilometry process.
8. 8 . The method of claim 3 , including estimating the in-trench thickness using the measured in-monitor thickness.
9. 9 . The method of claim 8 , including: exposing a second photoresist on a calibration substrate having a calibration trench and a calibration monitor area, the calibration monitor area being planar, wherein the second photoresist in the calibration trench is exposed at the first energy dose and the second photoresist on the calibration monitor area is exposed at the second energy dose; developing the second photoresist on the calibration substrate, wherein a trench calibration thickness of the second photoresist is left in the calibration trench and a monitor calibration thickness of the second photoresist is left on the calibration monitor area; measuring the trench calibration thickness, prior to exposing the photoresist on the device substrate; and measuring the monitor calibration thickness, prior to exposing the photoresist on the device substrate.
10. 10 . The method of claim 1 , including removing material from the trench where exposed by the photoresist in the trench.
11. 11 . The method of claim 10 , wherein removing material from the trench includes removing silicon dioxide from a liner in the trench.
12. 12 . The method of claim 1 , including implanting dopants into the device substrate abutting the trench, where exposed by the photoresist in the trench.
13. 13 . The method of claim 1 , including: exposing a second photoresist on the device substrate, wherein the second photoresist in the trench is exposed at a third energy dose and the second photoresist on the monitor area is exposed at a fourth energy dose that is different than the third energy dose; and developing the second photoresist, wherein a third thickness of the second photoresist is left in the trench and a fourth thickness of the second photoresist is left on the monitor area, the third thickness being greater than the in-trench thickness and the fourth thickness being greater than the in-monitor thickness.
14. 14 . The method of claim 1 , wherein exposing the photoresist in the monitor area at the second energy dose is performed with a photomask having a pattern of light blocking features having dimensions less than twice a wavelength of light used for exposing the photoresist.
15. 15 . The method of claim 1 , wherein exposing the photoresist in the monitor area at the second energy dose is performed by a maskless photolithography process having a temporally modulated exposure dose in the monitor area.
16. 16 . The method of claim 1 , further including: measuring the in-monitor thickness; estimating the in-trench thickness using the measured in-monitor thickness using a calibration chart, wherein the calibration chart indicates the estimated in-trench thickness is out of specification; removing the photoresist; forming a second photoresist on the device substrate; exposing the second photoresist in the trench is at a third energy dose and exposing the second photoresist on the monitor area at a fourth energy dose that is different than the third energy dose; developing the second photoresist, wherein a second in-trench thickness of the second photoresist is left in the trench and a second in-monitor thickness of the second photoresist is left on the monitor area; measuring the second in-monitor thickness; and estimating the second in-trench thickness using the measured second in-monitor thickness using the calibration chart.
17. 17 . The method of claim 16 , wherein the fourth energy dose is less than the third energy dose.
18. 18 . The method of claim 16 , wherein the third and fourth energy doses are identical to the first and second energy doses, respectively.
19. 19 . The method of claim 1 , wherein the photoresist is a positive tone photoresist.
20. 20 . The method of claim 1 , wherein developing the photoresist includes dissolving an exposed portion of the photoresist in an aqueous alkaline solution.

Description

TECHNICAL FIELD This disclosure relates to the field of microelectronic devices. More particularly, but not exclusively, this disclosure relates to trenches in microelectronic devices. BACKGROUND Many microelectronic devices include trenches, for example, isolation trenches, gate trenches, or field plate trenches. During fabrication of some of these devices, photoresist is formed in the trenches, partially filling the trenches to a desired thickness. Attaining the desired thickness of the photoresist in the trenches is frequently challenging. SUMMARY A method of forming a microelectronic device includes forming photoresist on the microelectronic device, filling a trench of the microelectronic device and extending over a top surface of the microelectronic device adjacent to the trench. The photoresist also covers a thickness monitor on a substrate containing the microelectronic device. The photoresist is a positive tone photoresist. The photoresist in and over the trench is exposed at a trench energy dose, and the photoresist in the monitor area is exposed at a monitor energy dose that is less than the trench energy dose. The photoresist is developed, leaving an in-trench thickness of the photoresist in the trench and leaving an in-monitor thickness of the photoresist on the monitor area. The in-monitor thickness of the photoresist on the monitor area may be measured and the measured thickness value may be used to estimate the in-trench thickness of the photoresist in the trench. BRIEF DESCRIPTION OF THE FIGURES FIG. 1A through FIG. 1O are cross sections of an example method of forming a microelectronic device. FIG. 2A through FIG. 2D depict example photomask patterns with sub-resolution geometries. FIG. 3 is an example chart of photoresist thickness versus average photomask transmission for the thickness monitor. FIG. 4 is a top view of an example substrate that includes multiple instances of microelectronic devices with trenches, and includes thickness monitors, depicted in various locations. FIG. 5A through FIG. 5E depicts stages of a method of generating a calibration curve. FIG. 6A through FIG. 6J are cross sections of another example method of forming a microelectronic device. FIG. 7 depicts a graphical method for determining an appropriate value for the trench energy dose and the monitor energy dose. DETAILED DESCRIPTION The present disclosure is described with reference to the attached figures. The figures are not drawn to scale and they are provided merely to illustrate the disclosure. Several aspects of the disclosure are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide an understanding of the disclosure. The present disclosure is not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with the present disclosure. A microelectronic device includes a trench in a substrate. A method of forming the microelectronic device includes forming photoresist on the microelectronic device, filling the trench, and extending over a top surface of the microelectronic device adjacent to the trench. The photoresist also covers a thickness monitor, which is planar, on the substrate. The monitor area may be located within the microelectronic device, or may be located outside the microelectronic device. The photoresist is subsequently heated in a pre-bake operation to remove at least a portion of solvents from the photoresist. After the pre-bake operation, the photoresist may be rehydrated to attain a desired water content. After the photoresist has been rehydrated, the photoresist on the thickness monitor has a pre-exposure thickness, which may be 1 micron to 3 microns, by way of example. The photoresist is a positive tone photoresist, that is, exposing the photoresist to ultraviolet (UV) light in a specific wavelength band will increase solubility of the photoresist in a developer solution. Unexposed photoresist absorbs the UV light with an absorption length of 0.5 microns to 2 microns. Photoresist at an upper surface of the photoresist is exposed before underlying portions of the photoresist. As the photoresist at the upper surface is exposed, its absorption coefficient decreases, so that the underlying unexposed photoresist is subsequently exposed. Exposure by the UV light is commonly expressed as a dose, measured in units of energy per unit area, such as millijoules per square centimeter (mJ/cm2). The exposure dose is absorbed by the photoresist through the volume of the photoresist, at an energy density, expressed in units of energy per unit volume, such as joules per cubic centimeter (J/cm3). The photoresist absorbs the UV light at top levels of the photoresist downward. As more UV light is