JP2000162783A - Method for forming resist pattern having reverse tapered profile - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for forming a reverse tapered resist pattern having excellent chemical stability, line width controllability, and easiness for process control and handling. SOLUTION: A naphthoquinone diazide novolac positive photoresist 12 is applied on the upper face of a substrate 11. The thickness of the photoresist 12 is controlled in such a manner that the thickness t of the photoresist 12 corresponds to the film thickness on the slope of the swing curve between its top and bottom. Then a mask 13 is disposed facing the photoresist 12, and the photoresist 12 is irradiated with UV rays through an opening 13a of the mask 13 to be exposed according to a specified pattern. Then by developing the photoresist 12, only the exposed region 12a is dissolved and a reverse tapered resist pattern 14 is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for forming an inversely tapered resist pattern. In particular, the present invention relates to a method for forming a resist pattern by photolithography such that the cross-sectional shape thereof has an inverted tapered shape.

[0002]

2. Description of the Related Art In recent years, various metals have been studied as electrode materials for next-generation SAW filters. As electrode materials other than the conventionally used Al-Cu (1.0%),
By using a pure metal such as Ag, Al, Au, and Cu, or an alloy obtained by adding various impurities to these pure metals, a multilayered metal film, or the like, the specific resistance and power resistance of the SAW filter electrode can be improved. It is possible to improve characteristics such as stress migration.

In order to form an electrode using these metal materials, the following two methods can be considered. After a metal material is formed, a resist pattern is formed on the metal film by photolithography, and the metal film is dry-etched (dry etching method). After a resist pattern is formed by photolithography, a metal material is vapor-deposited on the resist pattern, and an unnecessary metal material is removed by removing the resist pattern (lift-off method).

The above-described dry etching method has the advantages that the resolution of the resist pattern is high and the frequency of occurrence of foreign matter is relatively low. On the other hand, there is a problem that damage to the substrate is large and the material that can be etched is limited. On the other hand, in the method by lift-off, the sharpness of the image of the resist pattern is inferior to that of the dry etching method. The advantage is that it is wide.

Therefore, in order to form an electrode made of a metal material as described above or an electrode having a multi-layered structure, it is essential to apply a lift-off method. However, in order to apply the lift-off method, a reverse tapered or T-shaped resist pattern cross section is required to facilitate separation of the resist pattern from the electrode material.

The effect of the cross-sectional shape of the resist pattern on the lift-off process is shown in FIGS. FIG. 1 (a)
(B) and (c) show the case where the cross-sectional shape of the resist pattern is a forward tapered shape. In this method, a resist is coated on a substrate 1 as shown in FIG. 1A and patterned by photolithography to form a resist pattern 2 and then, as shown in FIG. An electrode material 3 is deposited on a substrate 1. At this time, since the substrate 1 is exposed in the opening 2a of the resist pattern 2, the electrode material 3 is deposited on the upper surface of the resist pattern 2 and the upper surface of the substrate 1 in the opening 2a. Next, as shown in FIG. 1C, the resist pattern 2 is peeled off to form a wiring pattern made of an electrode material 3 on the upper surface of the substrate 1.

However, if the cross-sectional shape of the resist pattern 2 is a forward tapered shape, the opening 2a of the resist pattern 2 is wider on the opening side, so that the electrode material 3 deposited in the opening 2a of the resist pattern 2 And the electrode material deposited on the resist pattern 2 is connected [the portion indicated by reference numeral 4 in FIG. 1C]. Therefore, even if the resist pattern 2 is removed as shown in FIG. The portion (the portion deposited on the upper surface of the resist pattern 2) is not removed even by lift-off.

FIGS. 2A, 2B and 2C show the case of a vertical resist pattern cross section. Also in this method, the electrode material 3 is patterned on the substrate 1 in the same manner as in the case of the forward tapered type shown in FIGS. 1 (a), 1 (b) and 1 (c). However, also in this case, as shown in FIG. 2B, the electrode material 3 deposited in the opening 2a of the resist pattern 2 cannot be reliably separated from the electrode material 3 deposited on the resist pattern 2. Therefore, the electrode material 3 on the substrate 1 and the electrode material 3 on the upper surface of the resist pattern 2 can be separated. However, even after the resist pattern 2 is peeled off and the electrode material 3 on the resist pattern 2 is removed by lift-off, As shown in FIG. 2C, there is a possibility that protrusions 5 are formed on both sides of the upper surface of the electrode material 3 formed on the substrate 1 to cause poor characteristics.

FIGS. 3A, 3B and 3C show the case where the cross-sectional shape of the resist pattern is an inversely tapered shape. Also in this method, the electrode material 3 is patterned on the substrate 1 in the same manner as in the case of the forward tapered type shown in FIGS. 1 (a), 1 (b) and 1 (c). However, in this case, the cross-sectional shape of the resist pattern 2 has an inverse tapered shape, and the opening width of the resist pattern 2 is narrowed as shown in FIG. A space is formed between the side surface of the electrode material 3 and the side surface of the
The electrode material 3 in FIG. 3A and the electrode material 3 deposited on the upper surface of the resist pattern 2 are also completely separated, and an electrode material 3 (electrode pattern) having a good cross-sectional shape is obtained as shown in FIG. As described above, in order to obtain a good electrode pattern by using the lift-off method, it is necessary to form a resist pattern having a reverse tapered or T-shaped cross-sectional shape.

Conventionally, the following two methods are mainly used as a method for forming a resist pattern having a reverse tapered or T-shaped cross section. (A) A method using a negative resist. (B) A method using a positive resist having an image reversal function.

First, an example of a method using a negative resist for lift-off is shown in FIGS. 4A, 4B, 4C and 4D, and this will be described. In this method, after a negative resist 6 containing a dye is applied on the substrate 1, an exposure mask 7 is overlaid on the negative resist 6, and the negative resist 6 is exposed to ultraviolet rays through the openings 7 a of the mask 7. The negative resist 6 is exposed in the exposed area 6a exposed to the ultraviolet rays by the ultraviolet irradiation, and becomes insoluble in the developing solution [FIG.
(A)]. This exposure process is the same as the normal exposure process, except that the amount of exposure of the dye-containing negative resist 6 by ultraviolet rays is set to be lower. However, this negative resist 6
Contains a dye that absorbs exposure light (ultraviolet light), so that the exposure intensity in the exposure region 6a of the negative resist 6 decreases nearer to the substrate 1 as shown in FIG.

Therefore, after exposure, the negative resist 6
In the initial stage of development, as shown in FIG. 4B, the non-exposed area 6b of the negative resist 6 is dissolved in the developing solution 8, and the non-exposed area 6b decreases in the vertical direction and finally disappears. Further, when over-development is performed, as shown in FIG. 4C, the negative resist 6 further dissolves, and the exposed area 6a of the negative resist 6 starts to decrease in the lateral direction. At this time, since there is a difference in exposure intensity between the vicinity of the surface of the exposure region 6a and the vicinity of the substrate 1, the dissolution rate is higher in the vicinity of the substrate 1 where the exposure intensity is relatively small than in the vicinity of the surface of the resist 1. As a result, as shown in FIG. 4D, a resist pattern 9 having a reverse tapered cross-sectional shape is obtained.

Next, a method of using a positive resist having the above-mentioned image reversal function will be described with reference to FIGS. 5 (a), 5 (b) and 5 (c).
This will be described with reference to FIG. In this method, an AZ5200E series manufactured by Hoechst is used as a resist [AZ5200E series catalog, M. Borsen,
"Submicron processing technology by image reversal of positive photoresist", Electronic Materials, 6, 1 (1986), and M.Sp
ac et al, "Mechanism and lithographic evaluation o
f image reversal in AZ5214 trin. ", Proc.of
conference on photopolymers principle processing a
nd materials., Ellenville (1985)]. This resist is obtained by adding a negative working agent such as a basic amine to a positive resist which is a mixture of an alkali-soluble phenol resin and naphthoquinonediazide, thereby imparting an image reversal function.

FIGS. 5A, 5B, 5C, and 5D show image formation using an image reversal positive resist. In this method, the resist 10 is applied on the substrate 1, an exposure mask 11 is superimposed on the resist 10, and the resist 10 is exposed through the light-transmitting portion 11a of the mask 11 [first exposure;
FIG. 5 (a)]. At this time, when the exposure amount is set to a relatively low value and ultraviolet exposure is performed, the exposure intensity distribution in the exposure region 10a of the resist 10 is reduced by the light absorption of the resist 10 itself.
As shown in FIG. 5A, the width of the exposure region 10a (the alkali-soluble region) decreases on the side closer to the substrate 1 as shown in FIG. Conversely, the non-exposed area 10b of the resist 10 becomes wider on the side closer to the substrate 1. Subsequently, after performing a reverse bake (reversal bake) on the resist 10a as shown in FIG. 5 (b), and exposing the entire surface of the resist 10 [2nd exposure; FIG. 5 (c)], the alkali of the resist 10 The soluble region and the alkali-insoluble region are inverted, so that the exposed region 10a becomes an alkali-insoluble region and the non-exposed region 10b becomes an alkali-soluble region. Therefore, when this resist 10 is immersed in a developing solution and subjected to a developing treatment, an inversely tapered resist pattern 12 as shown in FIG. 5D is obtained.

[0015]

However, in general, negative resists are inferior to novolak positive resists in chemical stability and line width controllability of the resist, and the negative resist of the above (A) is used. According to the method, process control in the resist pattern forming process is difficult.

In the method (B) using a positive resist having an image reversing function, the chemical stability is lower than that of a normal novolak-based positive resist, and the resist is easily affected by the atmosphere. Control was difficult. In addition, the number of steps was large and complicated.

The present invention has been made in order to solve the above-mentioned technical problems, and has as its object to improve chemical stability, line width controllability, process control and ease of handling. An object of the present invention is to provide an excellent method of forming a reverse tapered resist pattern.

[0018]

SUMMARY OF THE INVENTION A method for forming a reverse tapered resist pattern according to claim 1 is a method for forming a naphthoquinonediazide-novolak-based positive photoresist obtained by measuring the relationship between the resist film thickness and the resist pattern line width. When the resist film thickness changes in an increasing direction in a swing curve, a photoresist is applied with a film thickness of an inclined portion from a peak portion to a valley portion, and then the photoresist is exposed and developed.

In a naphthoquinonediazide-novolak positive photoresist, the cross-sectional shape of the resist pattern changes depending on the thickness of the resist due to the standing wave effect. When the film thickness of the inclined portion from the peak to the valley is changed when changing in the increasing direction, the exposure intensity near the resist surface decreases relatively, so that the dissolution rate in the developer near the resist surface decreases, Tapered shape (T
) Is obtained.

The naphthoquinonediazide-novolak-based positive photoresist used here is more excellent in chemical stability, line width controllability and handleability than a negative resist or a positive resist having an image reversal function. Because
It is possible to reduce the number of processes and stabilize pattern formation such as line width control.

According to a second aspect of the present invention, in the method for forming a reverse tapered pattern according to the first aspect, the exposure is performed by shifting the focus position of the exposure means toward the exposure means from the best focus position. It is characterized by:

As the focus position of the exposure means shifts from the best focus position to the side opposite to the exposure means side, the cross-sectional shape of the resist pattern deteriorates from the upper end portion of the resist pattern cross-section, while the focus position moves toward the exposure means side. In the case of deviation, the cross-sectional shape of the resist pattern deteriorates from the bottom of the resist pattern. Therefore, by shifting the focus position of the exposure unit to the exposure unit side,
A more stable resist pattern having a reverse tapered cross section can be formed.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Embodiment) FIGS.
(C) is a view for explaining a method of forming a resist pattern having a reverse tapered cross section using a naphthoquinonediazide-novolak-based positive photoresist. First, FIG.
As shown in (a), a naphthoquinonediazide-novolak-based positive photoresist 12 is applied on the upper surface of a substrate 11. At this time, the thickness t of the photoresist 12 becomes
The slope of the swing curve from the peak to the valley (Fig. 9
The thickness of the photoresist 12 is controlled so as to have a thickness (▽). Next, as shown in FIG. 6B, the mask 13 is made to face the photoresist 12, and the photoresist 12 is irradiated with ultraviolet rays through the openings 13a of the mask 13 to expose the photoresist 12 in a predetermined pattern.
Thereafter, when the photoresist 12 is developed, the exposed region 1 is exposed.
Since only 2a is dissolved, a resist pattern 14 having a reverse tapered cross section as shown in FIG. 6C is obtained.

After the resist pattern 14 having the inverse tapered cross section is formed in this manner, an electrode material is deposited on the substrate 11 through the opening 14a of the resist pattern 14, and the resist pattern 14 is peeled off. If an unnecessary electrode material on the resist pattern 14 is removed, a wiring pattern having a fine line width can be obtained (see FIG. 3).

Hereinafter, the background of the present invention will be described in detail and concretely. First, FIG. 7 shows a result of simulating a swing curve of a naphthoquinonediazide-novolak-based positive photoresist (PFI-26A: manufactured by Sumitomo Chemical) used in the method of forming a reverse tapered resist pattern of the present invention. Prolith 2 (Finley) was used for the simulation of the swing curve. The swing curve indicates the relationship between the resist film thickness and the line width of the resist pattern (line pattern) when a resist pattern is formed using a predetermined L / S (line and space) mask. is there. The swing curve in FIG. 7 is obtained when a mask having an L / S of 0.4 μm is used.

FIGS. 8A, 8B, 8C and 8D
(E) shows P1 (resist film thickness of 1.02 μm) of the swing curve of FIG. 7 when a mask having L / S = 0.4 μm is used.
m), P2 (resist film thickness 1.04 μm), P3 (resist film thickness 1.07 μm), P4 (resist film thickness 1.09 μm)
m) and P5 (resist film thickness: 1.12 μm) are diagrams showing the results of simulating the resist pattern cross-sectional shape at each point. For the simulation of the cross-sectional shape of the resist pattern, Prolith 2 (Finley)
Was used.

The simulation conditions for the swing curve in FIG. 7 and the cross-sectional shape of the resist pattern in FIG.
It is as follows. Resist PFI-26A (Sumitomo Chemical) Substrate 3 inch bare Si substrate Prebake condition 90 ° C, 90sec Resist thickness 1.00-1.15μm Exposure condition NA = 0.6, σ = 0.7 i-line (365nm), 240mJ Bake after exposure (PEB) Conditions 110 ° C, 90sec Development conditions TMAH 2.38wt%, no-surfactant, 23 ° C Puddle-method 60sec Pattern 0.4μm L / S

When the resist applied on the substrate is exposed while changing the resist film thickness at a constant exposure amount, the resist pattern line width after development changes in a sinusoidal manner as the resist film thickness increases. This is due to the effect of interference between the incidence and reflection of exposure light, the so-called standing wave effect. This sinusoidal change in the line width of the resist pattern
This is called a swing curve, and FIG. 7 shows this by simulation.

As shown in the simulation of the cross-sectional shape of the resist pattern in FIG. 8, when the thickness of the resist changes, not only does the line width of the resist pattern change, but also the cross-sectional shape of the resist pattern changes. I understand. In particular, the change in shape at the upper end of the cross section of the resist pattern is significant.

FIG. 9 schematically shows the simulation result of FIG. 7, and FIG. 10 schematically shows the simulation result of FIG. 9 and 10, the following correlation between the swing curve and the resist cross-sectional shape can be seen. (1) At the resist film thickness (indicated by □ in FIG. 9) located at the peak or the valley of the swing curve, the cross-sectional shape of the resist pattern 17 is rectangular (normal) (FIG. 10B). (2) At the resist film thickness (indicated by の in FIG. 9) located in the region where the swing curve climbs, the cross section of the resist pattern 16 has a shoulder-cut shape [FIG. 10 (a)]. (3) At the resist film thickness (indicated by Δ in FIG. 9) located in the downward slope region of the swing curve, the cross section of the resist pattern 18 has an inverted tapered shape (or T shape) [FIG.
(C)].

When a normal resist pattern is formed, a peak or a valley (square in FIG. 9) of the swing curve having the highest line width controllability is set. It can be seen that by setting (▽ in FIG. 9), an inverse tapered or T-shaped resist pattern sectional shape considered suitable for the lift-off process can be obtained.

The graph in FIG. 11 shows that a naphthoquinonediazide-novolak positive photoresist PFI-26A (manufactured by Sumitomo Chemical Co., Ltd.) is actually exposed using a 0.4 μm L / S mask and further developed. It shows a swing curve obtained by measuring the line width of the obtained resist pattern (line pattern). The result of FIG. 11 substantially agrees with the swing curve obtained from the simulation of FIG.

The conditions for measuring the swing curve shown in FIG. 11 are as follows. Resist PFI-26A (Sumitomo Chemical) Substrate 3 inch bare Si substrate HMDS treatment 2-stage treatment at 90 ° C and 120 ° C (90sec each) Prebake condition 90 ° C, 90sec Film thickness 1.02 to 1.13μm Exposure condition Canon FPA2000i3 (NA = 0.6, σ = 0.7) 170mJ Reticle HOYA PSM Test Reticle Post exposure bake condition 110 ℃, 90sec Development condition TMAH 2.38wt%, no-surfactant, 23 ℃ Puddle-method 60sec pattern 0.4μm L / S Line width measurement SEM (S- 4500) Observe the top surface and measure the bottom width

FIGS. 12 (a), 12 (b) and 12 (c) show the slope region (line width 1.06 μm) of the swing curve shown in FIG.
m), a simulated image of a resist cross-sectional shape in a peak portion (line width 1.07 μm), a downward slope region (line width 1.105 μm), a point Q1 in FIG. 11 (line width 1.06 μm),
Q2 point (line width 1.07 μm), Q3 point (line width 1.105 μm)
m) shows an SEM image.

12 (a), 12 (b) and 12 (c), when a simulation image and an SEM image having the same line width are compared with each other, they are very similar to each other. (Line width 1.1
05 μm), it can be seen that a reverse tapered resist pattern cross-sectional shape is formed.

As described above, according to the present invention, by setting the film thickness of the naphthoquinonediazide-novolak-based positive photoresist in the downward slope region of the swing curve, the reverse taper which is considered suitable for the lift-off process is provided. A T-shaped resist pattern can be formed.

This naphthoquinonediazide-novolak-based positive photoresist is superior in chemical stability, line width controllability and handleability to a negative type resist and a positive type resist provided with an image reversal function. In addition, it is possible to reduce the number of processes and stabilize pattern formation such as line width control.

(Second Embodiment) Next, a second embodiment of the present invention will be described. FIG. 14 shows a simulation result of the defocus characteristic of the resist pattern cross-sectional shape at the resist film thickness (1.10 μm) in the downward slope region of the swing curve (indicated by the triangle in the swing curve).
At this time, a mask having L / S = 0.4 μm was used.

The conditions for this simulation are as follows. Resist PFI-26A (Sumitomo Chemical) Substrate 3 inch bare Si substrate Pre-bake condition 90 ° C, 90sec Resist film thickness 1.10μm Exposure condition NA = 0.6, σ = 0.7, i-line (365nm) 240mJ Post-exposure bake condition 110 ° C 90sec Development condition TMAH 2.38wt%, no-surfactant, 23 ℃ Puddle-method 60sec pattern 0.4μm L / S

FIG. 13A shows the cross-sectional shape of the resist pattern when the focus of the exposure device is adjusted to the resist and the focus value is set to the best focus value (defocus value: 0 μm). FIG. 13 (b)
(C) and (d) show the cross-sectional shape of the resist pattern when the focus of the exposing machine is defocused in the minus direction, where the defocus values are -0.3 [mu] m and-.
0.6 μm and −0.9 μm. Here, the minus direction refers to a direction in which the substrate approaches the lens.
As can be seen from FIGS. 13B, 13C, and 13D, when the focus position shifts in the negative direction, the resist pattern gradually deteriorates from the upper end portion of the resist pattern cross-sectional shape.

On the other hand, FIGS. 13 (e), 13 (f) and 13 (g) show the cross-sectional shapes of the resist pattern when the focus of the exposing machine is defocused in the plus direction, and the defocus value is +0.3 μm. , +0.6 μm, +0.6.
9 μm. Here, the plus direction refers to a direction in which the substrate moves away from the lens. FIG. 13 (e)
(F) As can be seen from (g), when the focus position shifts in the plus direction, the resist pattern gradually deteriorates from the bottom of the cross-sectional shape of the resist pattern. In other words, it can be seen that a stable pattern can be formed by shifting the focus position to the plus side in order to obtain an inverse tapered (T-shaped) resist pattern cross-sectional shape.

FIGS. 14A to 14G are defocus characteristic SEM images of the resist cross-sectional shape at the resist film thickness (1.10 μm) in the downward slope region of the swing curve, where L / S = 0. The defocus values are 0 μm, −0.3 μm, and −0.6 using a mask having a pattern of 0.4 μm.
μm, +0.3 μm, +0.6 μm, +0.9 μm, +1.0.
2 shows an SEM image of a resist cross-sectional shape formed by exposing at 2 μm.

The operating conditions at this time are as follows. Resist PFI-26A (Sumitomo Chemical) Substrate Si substrate HMDS treatment Two-stage treatment at 90 ° C and 120 ° C (90 sec each) Pre-bake condition 90 ° C Exposure condition Canon FPA2000i3 (NA = 0.6, σ = 0.7) Exp = 170mJ Bake after exposure Conditions 110 ° C, 90sec Development conditions TMAH 2.38wt%, no-surfactant, 23 ° C Puddle-method 60sec Pattern 0.4μm L / S

FIG. 14 shows that a more stable inverse tapered (T-shaped) resist cross-sectional shape can be obtained at the plus focus position, as expected in the simulation of FIG.

Therefore, according to this embodiment, it is possible to form a more stable inversely tapered resist pattern in cross section.

In the first and second embodiments, the resist film thickness is set between 1.06 μm and 1.12 μm. However, the resist film thickness may be in the downward slope region of the swing curve. Any resist film thickness may be used.

[Brief description of the drawings]

FIGS. 1A, 1B, and 1C are cross-sectional views illustrating a process of forming a wiring pattern using a resist pattern having a forward tapered cross section.

FIGS. 2A, 2B, and 2C are cross-sectional views illustrating a process of forming a wiring pattern using a resist pattern having a rectangular cross section.

FIGS. 3A, 3B, and 3C are cross-sectional views illustrating a process of forming a wiring pattern using a resist pattern having a reverse tapered cross section.

FIGS. 4A, 4B, 4C, and 4D are cross-sectional views showing a method of forming a resist pattern using a negative resist.

FIGS. 5A, 5B, 5C, and 5D are diagrams showing a method of forming a resist pattern using a positive resist having an image reversal function.

FIGS. 6A, 6B, and 6C are schematic views illustrating a method for forming a resist pattern according to an embodiment of the present invention.

FIG. 7 is a view showing a result of simulating a swing curve of a naphthoquinonediazide-novolak-based positive photoresist.

FIGS. 8 (a), (b), (c), (d), and (e) are diagrams showing the results of simulating the correlation between the resist film thickness and the resist pattern cross-sectional shape.

FIG. 9 is a diagram conceptually showing the swing curve (simulation) of FIG. 6;

FIGS. 10A, 10B, and 10C are diagrams schematically showing a cross section (simulation) of the resist pattern of FIG. 7;

FIG. 11 is a view showing a result of actually measuring a swing curve of a naphthoquinonediazide-novolak-based positive photoresist.

12 (a), (b) and (c) show a simulated image of a resist pattern cross-sectional shape and S at a resist film thickness of 1.06 μm, 1.07 μm and 1.105 μm, respectively.
It is a figure which shows an EM image.

FIGS. 13A to 13G are explanatory views of another embodiment of the present invention, showing the result of simulating the relationship between the cross-sectional shape of the resist pattern and the defocus.

FIGS. 14A to 14G are explanatory views of another embodiment of the present invention, and are SEM images showing the results of actually measuring the relationship between the resist pattern cross-sectional shape and defocus.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Substrate 12 Photoresist 13 Mask 14 Resist pattern

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H01L 21/027 H01L 21/30 502R 514C

Claims (2)

[Claims] The present invention relates to a naphthoquinonediazide-novolak-based positive photoresist swing curve obtained by measuring a relationship between a resist film thickness and a resist pattern line width. A method of forming a reverse-tapered resist pattern, comprising: applying the photoresist with a thickness of an inclined portion from a valley to a valley, exposing and developing the photoresist. 2. The method according to claim 1, wherein the exposure is performed by shifting the focus position of the exposure unit toward the exposure unit from the best focus position.