Method of Forming a Resist Pattern

The present invention relates to a method of forming a resist pattern.

To improve obtainable complexity of a resist pattern, a conventional method of forming a resist pattern involves a reduction in the wavelength of a light and a change in exposure systems in a photolithographic process or employs a phase shift, a multi-layer resist, a surface pattern imaging or the like.

However, such attempts result in complication of fabrication steps and increased fabrication costs. Particularly, the phase shift requires the manufacture and maintenance of masks or even the manufacture of a special mask. The phase shift is also disadvantageous since it is difficult to form a resist pattern in a controllable manner due to diffraction.

It is therefore an object of the present invention to provide a method of forming a resist pattern which is able to form a resist pattern in a readily controllable manner and at low costs.

The foregoing object is met by providing a method of forming a resist pattern which comprises a first exposure step for selectively exposing a resist layer to light through a mask having a predetermined pattern, a first developing step for developing the resist layer after the first exposure step so as to form a stepped portion of a given thickness in the resist layer, a second exposure step for exposing the resist layer to a light after the first developing step to cause phase shift by 180.degree. in the stepped portion, and a second developing step for developing the resist layer so as to form a resist pattern along the step.

The given thickness is preferably such that the difference in optical path length between the interior and exterior of the resist layer at the given thickness is an odd number times a value of half of the wavelength of a light used in the second exposure step multiplied by a factor of odd numbers.

FIGS. 1A to 1D illustrate a sequence of fabrication steps involved in a method of forming a resist pattern according to one embodiment of the present invention; and

FIGS. 2A to 2C illustrate FIGS. 1A to 1D in more detail.

Reference will now be made to a method of forming a resist pattern according to one embodiment of the present invention. FIGS. 1A to 1D illustrate a sequence of fabrication steps involved in this embodiment. Referring to these drawing figures, the embodiment will be described in accordance with its fabrication order.

Firstly, a first exposure step takes place as shown in FIG. 1A. A resist layer 2 is formed on a wafer 1 and selectively exposed to light through a mask 3. The mask 3 has a predetermined pattern. The amount of the light is such that the exposed area of the resist layer 2 does not disappear during the following developing step.

Next, a first developing step takes place. The exposed area of the resist layer 2 is removed so as to form a stepped portion 4 in the resist layer 2 as shown in FIG. 1B. The next process is to effect baking by the use of a hot plate although it may not be necessary depending on the types of resist layers 2.

Next, a second exposure step takes place. In this step, the entire surface of the resist layer 2 is exposed to light with a single wavelength, such as g-ray or i-ray, without using the mask as shown in FIG. 1C. FIG. 2A schematically shows the phase of a light used in the second exposure step. There is a difference in phase between the interior and exterior of the resist layer 2 through the stepped portion 4. In this embodiment, the thickness of the stepped portion 4 is such that the difference in optical path length between the interior and exterior of the resist layer 2 at the thickness is an odd number times a value of half of the wavelength of a light used in the second exposure step multiplied by a factor of odd numbers. This is represented by the following relationship: 2d (n1-n2)=l(2n-1), where n1 is the refractive index of the light within the resist layer, n2 (<n1) is the refractive index of the light outside of the resist layer .lambda., is the wavelength of the light (in a vacuum) used in the second exposure step, d is the thickness of the stepped portion 4, and n is an integer and is at least one. In the stepped portion 4, the phase of a light passing through a thin portion 20 of the resist layer 2 is offset by 180.degree. from the phase of a light passing through a thick portion 21 of the resist layer 2. Where, for example, g-rays (.lambda.=0.436 .mu.m) are used to expose and in the air, the refractive index n2 of the exterior of the resist layer 2 is set at substantially 1 as in a vacuum. The refractive index n1 of the resist layer 2 is set at 1.6. The thickness, d, of the stepped portion 4 is set at approximately 0.4 .mu.m. With this, light interference occurs in the boundary between the thin portion 20 and the thick portion 21. FIGS. 2B and 2C show the relationship between the position of the light within the resist layer 2 and the amplitude and intensity of the light. Under the circumstances, some area of the resist layer 2 along the step is not subject to exposure. The range of the nonexposed area depends on the amount of light. Next, the baking again takes place by the use of a hot plate.

Fourthly, a second developing step takes place. A resist pattern appears as shown in FIG. 1D after the exposed area of the resist layer 2 has been removed.

As thus far described, the resist layer 2 functions as a phase shifter. Thus, this embodiment is less subject to diffraction than a conventional phase shifter and is able to form a resist pattern in a highly controllable manner. Also, this embodiment eliminates the need for a special phase shifter and thus reduces fabrication costs.

According to the present invention, a method of forming a resist pattern includes forming of a step in the resist layer so as to offset phases. This reduces light diffraction and eliminates the need for a phase mask. It is thus possible to form a resist pattern in a readily controllable manner and reduce the fabrication costs.