During a solar eclipse The Moon covers The Sun on some places on the Earth. Now the focal point is a bit above the Earth so the shadow area is about 160 miles. In history The Moon was a bit closer to the Earth but could there have been a time that the shadow area was just a dot. (probably for just some places as the Earth is not really perfect a sphere and the orbit of the Moon isn't also a perfect circle).
What you are calling a focal point is the end of the umbra, the point at which the umbra changes to the antumbra. In a total solar eclipse, that point is below rather than above the surface of the Earth. An annular eclipse occurs when that point is above the surface of the Earth. The Moon appears to be smaller than the Sun, leaving an unbroken ring of the Sun that appears to surround the Moon.
There are some solar eclipses that transition from annular to total, and from total to annular. A point on the surface of the Earth is exactly at the end of the umbra at those times of transition points in time. At this point in space and time the Moon will appear to be exactly the same size as and exactly in line with the Sun.
As the Moon's eccentric orbit around the Earth brings it nearer and farther, current solar eclipses can be total or annular. A few in between are hybrid eclipses: total along the midday part of the path, and annular near the sunrise or sunset end. This happens mostly due to the Earth's surface curvature and partly due to the Moon's orbital eccentricity.
Solar eclipses in the 21st century are about 34% partial, 32% annular, 30% total, and 3% hybrid. As millennia of tidal effects enlarge the Moon's orbit, there will be more annular eclipses and fewer total eclipses. Hybrid eclipses will happen sporadically as long as total eclipses are possible.
Hybrid eclipses occurred on 2005-04-08 and 2013-11-03 and will occur on 2023-04-20 and 2031-11-14. On Espenak's maps, the central path is blue where total and red where annular. At an annular/total transition point, the eclipse magnitude is 1.00, and an observer would see a "diamond ring" flip from one side to the other.
Is it possible that the shadow of the moon is a single dot during solar eclipse?
No, because the Moon is not round. Yes. I have to modify my answer after reading @IlmariKaronen's comment(s) below. Mathematically the umbra could collapse to a line segment, arc, or other short 1D shape, or to two or more points, but these are not likely for a randomly cratered Moon.
Images below are screenshots from the very cool NASA Goddard video Tracing the 2017 Solar Eclipse. They were used in The Moon's shadow could not possibly look like this — could it?.
In this answer I calculate the GIF shown below, which illustrates that the non-circularity remains and doesn't shrink. If the non-circularity of the Moon is say 10 km, then the umbra can't be smaller than roughly that size before you start getting light from the valleys reaching the center of the spot.
However, even if it were irregular the last bit of umbra would likely end up as a point, though it may not fall at the exact center of the Sun-Earth line.